hydrationtherapy[1]

8/2/2019 hydrationtherapy[1]

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BY SUBSCRIPTION ONLY

In this paper1. Fluid balance: whensports drinks arebetter than water

6. Individualstrategies: what are

your personal fluidneeds?

8. Diuretics: howcaffeine and alcohol

affect fluid balance11. What The PapersSay:l Hydration and

crampsl Gastric emptying

12. Hydration: did USmarathoner DeenaKastor owe herOlympic bronze to

glycerol?

FLUID BALANCE

Sports drinks or water: what is thebest choice for sportsperformers?What does the term sports nutrition conjure up

in your mind? Carbohydrate and protein?Vitamins and minerals? Or maybe the moreexotic ergogenic aids like creatine? Whateversprings to mind, I bet it isnt water. Yet water isof supreme, overriding importance to both yourhealth and performance.

Your body might appear solid, but itsactually much more like a bag of salty water,containing a few bones to maintain its shape.Water accounts for around 70% of your body weight thats eight stone of water in an 11-stone adult! However, the loss of even a tiny

fraction of this water can significantly reduce your performance, which is why maintaininggood hydration is vital for all serious athletes.

Water is the medium in which thebiochemistry of the body takes place. Every oneof our trillions of cells both contains and isbathed in a watery medium. Its hardlysurprising, therefore, that we have developedmechanisms for keeping the water content of the body pretty constant. Because some water iscontinually being lost in urine (in the process of excreting waste products), a constantthroughput of water is required to maintainfluid balance. This balance is controlledprincipally by the kidneys and the thirstmechanism. When total body water drops,hormonal messages are sent to the brain tocreate thirst. Excessive water intake, on theother hand, stimulates an increase in urineproduction.

As well as providing the perfect chemicalenvironment for our bodies, water has anotherextraordinary property the ability to stop ourbodies overheating by evaporating via the skinin the form of sweat. This is particularly

important during exercise, when heat outputrises dramatically. At rest, the average 70kg adult consumes

around 0.25 litres of oxygen per minute, which

equates to about 70 watts of heat output. But when running at six-minute-mile pace, oxygenconsumption rises 16-fold to over four litres perminute and heat output rises to over 1,100

watts! Unless the ambient temperature issufficiently low, this extra heat cannot beradiated or carried away through convectionquickly enough to prevent heat build-up, soheat loss via evaporative cooling ( ie sweating)has to occur.

For a 70kg runner running at this pace, theapproximate energy burn rate is around1,000kcal per hour. In warm conditions it wouldtake over 1.5 litres per hour of sweatevaporation to remove the extra heatgenerated. When you take into account the factthat some sweat will drip off the skin withoutcontributing to evaporative cooling, it is easy tosee how runners can lose two litres of fluid perhour or more in hot conditions. And since fluidlosses of just 2% of body weight (thats 1.5 litresfrom our 70kg runner) can cause a significant

drop in performance, our mythical runner couldbe in trouble in less than an hour without takingextra fluid on board!

Because even small losses of water can causea drop in performance, optimum hydration isextremely important to athletes. However,replacing fluid lost in sweat and urine is not theonly justification for boosting fluid intake.Glycogen (stored muscle carbohydrate) is thebodys principle fuel for high intensity activities,and replenishing glycogen stores with dietarycarbohydrate is vital to continuing highperformance.

But the process of fixing carbohydrate intomuscles in the form of glycogen also requires

water; each gram of glycogen fixed into musclefibres requires around 3g of water, which is why

you often feel thirsty after a high-carbohydratepost-training meal. If you dont drink to aid thisprocess, water is simply drawn out of thebloodstream, leading to dehydration.

Fluid, then, is vital for adequate recovery not just to replace water lost through sweating,but also to help replenish lost glycogen.

A comprehensive hydration strategy involves

ensuring good hydration before training/competition, maintaining it during exercise andthen replacing any shortfall as soon as possibleafterwards. However, hydration isnt just about

HYDRATIONTHERAPY


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water: fluid loss via urine and, especially,sweating involves the loss of electrolyteminerals calcium, magnesium, sodium,potassium and chloride. Although thecomposition varies from person to person(partly as a function of acclimatisation) a litreof sweat typically contains the following (1,2):l Calcium 0.02gl Magnesium 0.05gl Sodium 1.15gl Potassium 0.23gl Chloride 1.48g

There are three reasons why replacing theseminerals by means of an electrolyte mineral-containing drink may be better then drinkingpure water alone:

1. Although the amounts lost in sweat aregenerally small in proportion to total bodystores, prolonged heavy sweating can lead to

significant mineral losses (particularly of sodium). Drinking pure water effectivelydilutes the concentration of electrolyteminerals in the blood, which can impair anumber of normal physiological processes. Anextreme example of such an impairment ishyponatraemia, when low plasma sodiumlevels can be literally life threatening ( see box,

below ).2. Drinks containing electrolyte minerals

particularly sodium are known to promotethirst, thereby stimulating a greater voluntary

intake of fluid(3)

. There is also evidence thatdrinks containing sodium enhance the rate andcompleteness of rehydration after a bout of exercise (4).

3. When the electrolyte minerals againparticularly sodium are present in appropriateconcentrations, the rate of fluid absorption

from the small intestine into the rest of thebody appears to be enhanced, especially inconjunction with small amounts of glucose (5).This is particularly important when rapiduptake of fluid is required, such as duringstrenuous exercise in the heat.

Pre-exercise hydration is principally afunction of your fluid intake patterns and yourdiet, which is discussed more fully elsewhere inthis issue ( see page 8 ). The use of glycerol toinduce a state of hyper-hydration before longevents in very hot conditions has also beencovered in Peak Performance ( PP205, November

2004 see box opposite for summary ), so we wont dwell on it here. Suffice it to say that if the fundamental dietary and normal fluidintake patterns are right, good pre-training/competition hydration will be the norm not something that requires special attention

a few hours before the event! As for post-t raining/competition

rehydration, the most reliable indicator is body weight, and your fluid replacement needs areconsidered in detail on page 6 of this issue.Research evidence suggests that fluidscontaining significant amounts of electrolytes(especially sodium) have a slightly greaterimpact in restoring hydration than fluids withlittle or no electrolytes/sodium (6).

However, the amount of sodium in the drinkis critical. American scientists compared

rehydration efficiency using each of thefollowing (7):l a 6% carbohydrate solution with no addedsodium;l a 6% carbohydrate solution with 25mEq(0.58g) of sodium per litre;l a 6% carbohydrate solution with 50mEq

Fluid is vital for adequate recovery not just to replace water lost in sweat but alsoto help

replenish glycogen

Hyponatraemia the dangers of fluid overloadHyponatraemia is a disorder in fluid-electrolyte balance that results in an abnormally low plasmasodium concentration (less than 135mmol per litre compared with a normal range of 138-

142mmol/L. A sustained decrease in plasma sodium concentration disrupts the dynamics of water exchange (osmotic balance) across the blood-brain barrier, resulting in a rapid influx of water intothe brain. This can cause swelling in the brain, leading to a series of increasingly severe neurologicalresponses, such as confusion, seizure, coma even death.

The lower the blood sodium and the faster it falls, the greater the risk of life-threatening consequences. A drop in plasma sodium concentration to 125-135mmol/L often results in littlemore than gastrointestinal symptoms, such as bloating and nausea. Below 125mmol/L, thesymptoms become more severe and can include confusion, throbbing headache, wheezy breathing,swollen hands and feet, unusual fatigue and reduced coordination. Below 120mmol/L, the risk of seizure, coma and death is increased.

Hyponatraemia in athletes is often, although not always, caused by excessive drinking. During

exercise, urine production is decreased, reducing the bodys ability to excrete excess water, while atthe same time sodium losses are increased through sweating. The combined effect makes it muchmore likely that the bodys sodium content will be significantly diluted.


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Hydrating an exercising human body

is not as simple as

topping up a leaking

bucket!

(1.16g) of sodium/L.The subjects dehydrated by 3% of body

weight during 90 minutes of exercise and drankas much as they wanted of one of the abovebeverages during a three-hour recovery period.The researchers found that the beverage with25mEq of sodium per litre stimulated thegreatest fluid intake, while the high sodiumdrink either suppressed thirst or diminished thepalatability of the fluid.

Although many athletes fail to get it right,maintaining optimum hydration before andafter exercise is a relatively straightforwardprocess. Staying hydrated on the move,however, is a different story. When fluid lossesare rapid ( ie in hot, humid conditions), largeamounts of fluids need to be absorbed quicklyto maintain hydration status. But hydrating anexercising human body is not as simple astopping up a leaking bucket! The rate of fluidabsorption in the body is determined by a two-stage process:l Gastric emptying how quickly ingested fluidleaves the stomach. In more dilute solutions,this is often the key step that determines theoverall rate of fluid absorption;l Intestinal absorption the rate of absorptionacross the intestinal wall.

Optimal fluid absorption requires rapidgastric emptying and efficient uptake in theintestine.

Contrary to what you might expect, fluidabsorption tends to take place in the smallintestine rather than the stomach. Studies haveshown that the larger the volume of fluid in the

stomach, the more rapid the emptying into thesmall intestine, which means that maintaining alarge fluid volume in the stomach by repeateddrinking will maximise the rate of fluid (and

Glycerol myths and realityFor most people, taking a glycerol/water solution before an event produces an increase in total bodywater (hyper-hydration). The question, however, is whether this extra water in the body actuallyenhances performance, and to date there is no clear-cut evidence to suggest that it does. It is truethat after ingestion glycerol stays in the body and holds water with it, but the unanswered question

is whether this extra water increases hydration within the cells or simply increases the amount of water swilling around in general circulation?

Overall, the current weight of evidence is tilted slightly in favour of a glycerol hyper-hydrationprotocol, but only in events where substantial dehydration is likely to be a problem. Moreover, thereis still no agreement about the best way to take glycerol solution, or about whether certain kinds of plain water hyper-hydration protocols might offer similar benefits.

Unless your event is long and taking place in hot/humid conditions, resulting in unavoidabledehydration, there is probably little point in using glycerol. Not only are there unlikely to be anyperformance benefits, but glycerol ingestion can cause stomach upsets, together with headachesand blurred vision at higher doses. If you are tempted to try glycerol, make sure youve tried other hydration methods first. Glycerol should be considered only as a last resort.

nutrient) delivery to the small intestine (8,9).Gastric emptying rate is also influenced by

fluid composition. Early studies showed that,regardless of their electrolyte or glucosecontent, solutions with a lower overallconcentration (or osmolality) than body fluids

were emptied as rapidly as plain water (10,11).With glucose solutions, for example, this wouldallow for a concentration of up to 2.5% (2.5gper litre of water). At the time it seemed that

concentrations above this threshold would slowgastric emptying. But more recent work hasestablished that drinks containing glucoseconcentrations of up to around 4-5% areemptied as rapidly as water (12) .

Beyond a concentration of 5%, glucosesolutions are emptied more slowly from thestomach, but they can nevertheless result in afaster delivery of glucose overall (13). This isbecause the increase in glucose per unit volumedelivered by these more concentrated drinksmore than makes up for the reduced volumeabsorbed; where fluid replacement is of a lesserimportance than energy replacement, moreconcentrated drinks may be preferable.

In recent years, there has been a growingtrend towards the use of short chain glucosepolymers, such as maltodextrins, in fluid/energyreplacement drinks. The theory is that glucosepolymers are emptied more rapidly from thestomach than pure glucose.

However, the evidence is far from conclusiveand the various studies that have been carriedout have reached conflicting conclusions (14-17).This may be because concentrated beverages

are known to increase the volume of gastric andintestinal secretions. Its possible, therefore,that the total volume of stomach contents mayhave been greater when solutions containing


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fructose much less desirable as an energycomponent in sports drinks than glucose.

A study on cyclists compared the effects of glucose and fructose in a 6% solution during a1hr 45min bout of cycling (20) . By comparison

with glucose, fructose was associated with moregastrointestinal distress, a greater loss of plasma volume, higher levels of stress hormoneand substantially poorer exercise performance!

Properly formulated carbohydrate/electrolytedrinks can and do increase hydration (and, as abonus, supply extra carbohydrate to workingmuscles), so its hardly surprising that they reallydo enhance performance when fluid loss is anissue (21-29). But whats the best strategy forindividual athletes? And how do you decide on thebest drinks for you? Here are some simpleguidelines derived from the evidence referred toin this article:

Pre-exercisel Make sure your normal diet contains plentyof water and a minimum of other substancesknown to impair hydration ( see article on page8);l Drink ample (but not excessive) water in therun-up to a training session or event;l Consider using glycerol for hyper-hydrationonly if you are an elite athletes undertakinglong endurance events in extremely hotconditions. Even then it has its drawbacks.

Post-exercisel Follow Ron Maughans advice on replacinglost fluid in terms of volume ( see page 6 );l Drinks containing electrolytes (especiallysodium) stimulate the desire to drink and maytherefore be preferable to plain water. Theresalso evidence that these drinks are absorbedmore efficiently from the small intestine,especially when carbohydrate is present;l Remember that youll need to absorb extrafluid for glycogen replenishment about 300mlfor every 100g of carbohydrate consumed.

Mid-exercisel For events lasting less than 30 minutes, mid-exercise fluid replacement isnt necessary, sinceits not possible to lose enough fluid to affectperformance in such a short space of time;l Weather and exercise intensity affect fluidneeds; the higher the temperature, humidityand exercise intensity, the greater the rate of fluid replacement required;l Gastric emptying is most efficient when thereis a high fluid volume in the stomach, so your

best strategy is to start exercise with fluid onboard and drink little and often to keep ittopped up;l Gastric emptying is also affected by the

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glucose rather than polymers were drunk, eventhough the amount of the ingested drinkremaining in the stomach was the same. This

would affect gastric emptying rates(remembering that gastric emptying is morerapid with high volumes of fluid in thestomach).

However, while the evidence that glucosepolymers can offer a significant advantage overpure glucose is thin on the ground, theresalmost no evidence to suggest that the emptyingrate of polymer solutions is slower than that of free glucose solutions with the same energycontent. Indeed, most studies have reportedthat polymer solutions are generally emptiedfaster, if not significantly so.

After gastric emptying, ingested fluids areabsorbed in the small intestine. Pure water, or

very dilute solutions, diffuse readily across the

intestine. However, research has shown thatdilute glucose/electrolyte solutions with aconcentration that is slightly less than that of plasma maximise the rate of waterabsorption (18) . The researchers found thatoptimum hydration from the intestine wasobtained with a solution containing 60mEq(1.38g) of sodium and 111mmols (20.0g)glucose per litre of water.

Where energy ( ie glucose) replacement is themain goal, studies have shown that uptake fromthe small intestine into the body rises as the

concentration of glucose rises in the intestine.This is simply because there is more glucoseavailable per unit volume for absorption.

However, very concentrated solutions of glucose (more than 6%) can have an adverseeffect on fluid balance. This is due to theprocess known as osmosis, whereby waterseparated by a permeable membrane (in thiscase the intestinal wall) passes from a moredilute to a more concentrated solution. When

you ingest a drink with a very highconcentration of glucose, the fluid in thebloodstream (on the other side of the intestinal

wall) will be relatively dilute by comparison. And the osmotic pressure exerted by the veryconcentrated glucose solution will actually draw

water out of the bloodstream and into theintestine. This results in a loss of available body

water, effectively increasing dehydration. Although it has a chemical structure similar

to glucose, the fruit sugar fructose diffusespassively across the intestinal wall. Studies haveshown that fructose is absorbed more slowlythan glucose and that it promotes less wateruptake (19) . Moreover, fructose is known to exert

a greater osmotic pressure, which means that,for a given concentration, it is more likely todraw water into the intestine, which can causeabdominal distress. These properties make

References1. Geigy ScientificTables, 8th Ed, Ciba-Geigy Ltd, 19812. HumanPhysiology, 2nd Ed,

Springer-Verlag,Berlin, 19893. Appl Physiol80:1112-1117,19964. Int J Sports Nutr 7:104-116, 19975. Amer J Physiol258 (Gastrointest.Liver Physiol)21:G216-G222,19906. Medicine and

Science in Sportsand Exercise,28:1260-1271,19967. International

Journal of SportsNutrition, 15:329,19978. Medicine and

Science in Sportsand Exercise23:307-313, 19909. Medicine and

Science in Sportsand Exercise,23:314-319, 199010. J Physiol154:254-269, 196011. J Physiol245:209-225,197512. Medicine and

Science in Sportsand Exercise,24:S70, 199213. Gastroenterology 89:1326-1330,198514. Res Quart 51:299-305, 198015. Eur J ApplPhysiol OccupPhysiol, 58(6):605-12, 198916. Medicine and

Science in Sportsand Exercise,

18:568-575, 198617. J Appl Physiol,72(2):468-75,1992


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18. J Pediatr,106(3):383-9,198519. J Clin Invest,55(4):728-37197520. Medicine and

Science in Sportsand Exercise,21:275-282, 198921. Medicine and

Science in Sportsand Exercise,27:S223,199522. European

Journal of AppliedPhysiology, 70:154-160, 1995.23. Medicine and

Science in Sportsand Exercise: Vol.27, No. 2:200-210,199524. American

Journal of ClinicalNutrition, 48:1023-1033, 198825. Medicine and

Science in Sportsand Exercise,20:110-115, 1988.26. Medicine and

Science in Sportsand Exercise,31:S123, 199927. Journal of

Applied Physiology,71(6):2518-2527,199128. American

Journal of Physiology,258:G216-G222,199029. International

Journal of SportsMedicine, 13:399-406, 1992

concentration of a drink. As a rule, the moreconcentrated the drink, the slower it empties;l Plain water empties rapidly as do lowconcentration (hypotonic) drinks and isotonicdrinks. More recent research also suggests thatenergy drinks containing up to 4-5% glucosealso empty as rapidly as water. However, drinkscontaining glucose and sodium are absorbedfrom the intestine more rapidly than plain water;l More concentrated drinks (more than 6%)leave the stomach more slowly, but still manageto deliver more carbohydrate. Where energyreplacement is the priority, these drinks arerecommended, although they are less efficientfor hydration;l Where hydration is the priority, water,isotonic or low concentration glucose drinks will all suffice, though hypotonic/isotonic

electrolyte/glucose containing drinks may beabsorbed more rapidly from the intestine;l Whatever sports drink you choose, ensure itcontains electrolyte minerals;

l Where hydration is your goal, water is okaybut high volumes of plain water are notrecommended where profuse and prolongedsweating occurs (more than 3-4 litres lost)because of the risk of sodium dilution. If wateris your preferred drink, consider using salttablets in these circumstances;l The evidence in favour of glucose polymerdrinks is mixed. Overall, they may confer aslight advantage in terms of gastric emptying,but be prepared to pay more!l Fructose or pure fruit juice drinks are notabsorbed rapidly and may cause abdominaldistress;l Never experiment with a new drink duringcompetition. Try it in training first to see how

your body tolerates it!l Choose a drink you find palatable. If itdoesnt taste nice, you wont drink it, no matter

how advanced the formula!

Andrew Hamilton

Sports drinks jargon busterWith so many sports drinks on the market, its easy to become confused about which type is best suitedto your needs. Isotonic, energy and recovery drinks can all be used to promote hydration, but tend tohave slightly differing effects, which are explained below. Its important to understand, though, thatthese categories can overlap eg energy drinks containing relatively small amounts of carbohydratecan be almost isotonic so the distinctions here should serve as a guide only.

* Isotonic drinks provide the body with water, energy and electrolytes in a form enabling the water tobe absorbed as rapidly as possible. Studies have shown that fluid is rapidly emptied from the stomachwhen it contains roughly the same concentration of dissolved substances as that of blood serum a

value of 280 milli-osmoles/kg for you technophiles out there! At this concentration, a drink is said tobe isotonic or at the same concentration as your body fluids. During exercise, energy in the form of carbohydrate, and electrolyte minerals, such as sodium, potassium, calcium and magnesium, are lostalong with water. When these substances are dissolved in water at an isotonic concentration they notonly help replace lost fluid more rapidly than even plain water but also help replace some of the lostenergy and minerals. However, research has demonstrated that drinks containing dissolved glucose athigher than isotonic concentrations (up to 5%) can be emptied from the stomach just as rapidly, andcan therefore replace lost energy more rapidly. Although not strictly isotonic, these drinks offer all the

fluid replacement benefits of isotonic drinks and are often marketed as such.* Energy drinks are less about replacing lost fluid and more about keeping the working musclessupplied with energy during very long and sustained workouts. Energy drinks need to contain muchhigher concentrations of soluble carbohydrates than isotonic drinks, because an isotonic solution of carbohydrate struggles to provide energy at a sufficient rate to replace what is lost during intenseexercise. The disadvantage of energy drinks is that their high carbohydrate concentrations tend to slowdown the rate of water absorption, particularly during hard exercise. They are therefore best reservedfor longer endurance events performed in more temperate conditions, where a very high rate of fluidreplacement is not quite so critical.

* Recovery drinks , as the name suggests, are taken after training to supply the muscles witheverything they need for recovery, including water, carbohydrate and amino acids. These drinks often

contain such additional nutrients as electrolyte minerals, vitamins needed to aid metabolism of theingested carbohydrate, and protein and more exotic co-factors designed to accelerate recovery.Because theyre taken after training, rapid gastric emptying and absorption is not a priority.


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can: this may be more than is necessary and canlead to problems of water overload and evenhyponatraemia (a potentially dangerous fall in theblood sodium level see box on page 2). Even atthe narrower range set by the IMMDA, 400ml willnot be enough for a heavy runner on a hot day,

while 800ml is likely to be too much for a lightrunner taking five hours to complete a marathonon a cold day.

The only sensible advice is for individualathletes to take personal responsibility fordeveloping their own hydration plan, and this isimplied in the first ACSM recommendation,encouraging runners to drink just enough toreplace fluid lost in sweat. That means looking at

what you currently do, assessing that against what you should be doing and seeing if any changesneed to be made. There are now many surveys of how much people drink in endurance races, and of

how much these same people sweat.If we look at what people actually do, we can

use as an example a paper published by TimNoakes and his colleagues in the British Journal of Sports Medicine last year (3). They weighed 258competitors before and after an Ironman distancetriathlon held in South Africa in 2000 and 2001.

Analysis of the results showed that weight changeranged from a loss of 8.0kg (10.7% of the pre-race

weight) to a gain of 3.0kg (3.7%). In other words,some people finished the race severely dehydrated

while others drank so much that they gained

weight. There was no measure of how much fluidcompetitors consumed during the race, but we canguess that those who lost most weight probablydrank a lot less than those who gained weight.

Footballers fluid intakeI and my colleagues have measured the fluidintakes of many top football players in training;its a simple matter of weighing their drinksbottles before and after training, having madesure that they drink only from their own bottles and that no-one else does! We saw that someplayers would put away almost two litres of fluidin a 90-minute training session while others drankalmost nothing. They all did the same training andthey all had access to the same drinks, so why thislarge difference?

At present, there are no good explanations for why some people sweat copiously while othershardly sweat at all. If youre a sweaty person, you

just have to learn to cope with it. What we dotend to find, though, is that those who drink mostare not necessarily those who sweat most; andthis is consistent with the belief that thirst is notclosely related to sweat loss at least not when

this amount is relatively small and therefore nota reliable guide to fluid needs.There is some evidence that the people who

drink most are those who begin a training session

Some top football players put away almosttwo litres of

fluid in a90-minutetraining

session while

others drank almost nothing!

INDIVIDUAL STRATEGIES

How to calculate yourpersonal fluid needsIn an issue of Peak Performance , I wrote aboutsome of the work on testing of fluid balance Ihave been engaged in with professional footballclubs ( PP196, April 2004 ). This work hasprovoked much interest in the football world andcarries some important lessons for athletes inother sports too. The key finding to emerge fromthose studies is that there is no standard answerto the following questions commonly asked byathletes:l What is the best drink?l How much of it should I consume?

Many factors will combine and interact to

determine the answers to those questions.However, the answers will vary according to thenature of the event and the weather conditions atthe time, so there is not even a standard answerfor any given individual. It is important thatathletes recognise this fact and stop searching fora single solution to all their needs.

Many expert committees have toiled long andhard over the questions, but none has as yet hadthe courage to admit that they cannot come up

with an answer. This has led to the publication of numerous sets of guidelines that appear, at first

sight, to provide answers.The American College of Sports Medicine(ACSM), for example, has published guidelinesfor endurance athletes recommending thatDuring exercise, athletes should start drinkingearly and at regular intervals in an attempt toconsume fluids at a rate sufficient to replace allthe water lost through sweating ( ie body weightloss), or consume the maximal amount that canbe tolerated (1).

The ACSM also advises: During intenseexercise lasting longer than 1h, it isrecommended that carbohydrates be ingested ata rate of 30-60 [grams per hour] to maintainoxidation of carbohydrates and delay fatigue.This rate of carbohydrate intake can be achieved

without compromising fluid delivery by drinking600-1,200 [ml per hour] of solutions containing4%-8% carbohydrates ( ie 40-80g per 100ml).

The International Marathon Medical Directors Association (IMMDA) has recommended a fluidintake of something between 400 and 800 ml perhour, with the higher rates being appropriate forfaster or heavier runners and the lower rates forslower runners and walkers (2).

The problem with both sets of recommendations is that they are too inflexible.The ACSM guidelines can be interpreted asencouraging runners to drink as much as they


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References1. www.acsm-msse.org/pt/pt-core/template-

journal/msse/media/ 0196.htm2. Clin J Sports Med2003 Sep:309-183. Br J Sports Med2004, 38; 718-724

already dehydrated: we dont yet know if thisapplies to a race situation, but it seems reasonableto suspect that it does.

The fact that fluid gains and losses could bemeasured in so many competitors in a racesituation and in top professional footballers intraining shows its not hard to do. In fact, it can beachieved by anyone with access to a set of scales.Just weigh yourself before and after a long run and youll have a good idea of whether you are asweaty runner or not. Sweaty runners shouldprobably be encouraged to drink more than theircounterparts who merely glow while exercising.

How much should you drink?There has been a tendency to move away fromthe ACSM suggestion that everyone should aimto replace as much fluid as they lose in sweat withthe aim of finishing the race weighing the same as

when they started. There is probably no realdanger to either performance or health frommild levels of dehydration. Yes, severedehydration will certainly result in a loss of performance capacity, but a little dehydrationmeans less weight to carry over those last fewmiles, and that may confer some benefits.

As a rule of thumb, during an endurance event you should aim to drink just enough to be sure you lose no more than about 1-3% of your pre-race weight. That may seem a difficult strategy toput in place, but all you need are a set of scales,

some common sense and a sense of personalresponsibility.First, you need to record your body weight

before and after as many of your long runs as youcan. Weigh yourself at the last minute before goingout, after that last-minute visit to the toilet. It isbest to do this without any clothes, as even a vestand shorts can soak up a lot of sweat and make youseem heavier after the run than you really are.

Weigh yourself as soon as you get back againand note the two measurements in your trainingdiary. (You do keep a training log, dont you? Atany rate you should if you are at all serious abouttraining.) You should also log the approximatedistance and duration of the run, and perhapsalso how you felt while running. Make a note,too, of whether you were wearing T-shirt andshorts or full tracksuit, hat and gloves.

If you have had anything to drink during therun, you need to know how much and add it tothe amount of weight lost. Kitchen scales areperfectly good for weighing your drinks bottle.However, if you use a bottle with calibrationmarks on the side, you dont need to weigh it atall. Youll soon get the hang of this weighing and

measuring, and once it is part of your routine itshould be no inconvenience at all.The sums are much easier to calculate if you

work in kilograms and litres rather than pounds

and pints, since 1kg of weight loss is roughlyequal to one litre of sweat, while 1lb of weightloss is four fifths of a (standard British) pint.There are some errors in these calculations, as

you will also be using up some stored fuels in theform of carbohydrate and fat, but these can beignored with no great loss of accuracy.

Another thing to note is the weatherconditions. Your local paper will give you thetemperature for the previous day, so you can lookthis up and add it to your other measurements.Ideally, you should also note the humidity(information available from your nearest weatherstation and on the internet). How much effort youput into gathering this data will depend on howserious you are about going for that performanceon race day.

After a few weeks, you should begin to seesome patterns emerging. You will probably lose

more weight (sweat) on longer runs, when yourun faster, or when the weather is warmer ormore humid. You can get rid of the first of thesesources of variability by calculating your sweatrate per hour. This may be as little as 200-300mlor as much as 2-3 litres, depending on yourphysiology, your running speed, clothing andconditions. If you collect enough measurements,

you should be able to allow for each of thesefactors and get an idea of how much you shouldbe drinking in any given set of conditions.

Once you know what your sweat losses are

likely to be in a particular set of conditions, youcan plan for race day. You know the distance, youknow how fast you plan to run and you have seenthe weather forecast. Thats all you need to knowin order to plan your drinking strategy.

Two hypothetical examplesLets take a look at how this might work in practice,using two different hypothetical examples:

1. You weigh 70kg and you plan to run for threehours at a temperature of 20C. You know that

you sweat about 1 litre per hour at this pace andtemperature, so you can expect to lose about threelitres of sweat. Because you want to lose no morethan about 2-3% of your body weight thats 1.4-2.1kg, or 1.4-2.1 litres of fluid you should aim todrink something between 900ml and 1.6 litresduring the race. Thats about 300-500 ml per hour,rather less than most of the recommendationssuggest and probably a lot more comfortable.

2. You weigh 60kg and plan to run 10 miles inabout 62 minutes on a hot day (about 25C). You

will sweat about 2.2 litres in that hour. A 2-3% lossin body weight represents 1.2-1.8 litres of fluid, so

you should plan to drink about 400-1,000ml. This

is a wide range, and you can probably opt forsomething near the lower end of it.See, its not really so difficult after all!

Ron Maughan


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100mg of caffeine in just 50ml of water, but youdhave to drink around a litre of weak black tea toobtain the same amount of caffeine, significantlyhydrating your body in the process!

How detrimental is caffeine to hydration inathletes? An early study on the effects of high-dose pre-exercise caffeine ingestion found nosignificant impact on hydration (3). In a double-blind trial, trained runners exercised to exhaustionon a treadmill at 70-75% of VO 2max on twoseparate occasions under the following conditions:1. After taking high-dose caffeine tablets,administered at the rate of 5mg per kg of body

weight, two hours before exercise, followed by 2.5mgper kg 30 minutes before;2. After taking placebo tablets.

Venous blood samples were taken every 15minutes during exercise, and afterwards water lossand sweat rates were calculated from the

difference between pre- and post-exercise body weight. The researchers found that, by comparison with placebo, caffeine ingestion produced nosignificant difference in plasma volume, sweat rateor overall water loss.

Conflicting findingsThese findings appear to conflict with those of amore recent study on coffee and hydration (4). Twelvehealthy volunteers were asked to abstain fromcaffeine for five days and then follow a standardiseddiet for the next two days. On day one of the

standardised diet, their fluid requirement was metpurely by mineral water; the next day they receivedthe same total amount of fluid, but with the mineral

water partly replaced by six cups of coffeecontaining 642mg of caffeine. This regime led to anincrease in 24-hour urine excretion, leading to anegative fluid balance and a concomitant decreasein body weight of around 0.7kg.

The researchers also measured total body water(using a technique called bioelectrical impedanceanalysis) and found it had decreased by 2.7%,

while urinary excretion of sodium and potassiumincreased by 66% and 28% respectively.

The results of this study should be interpretedcautiously by athletes, for the following reasons:l The five-day caffeine washout period was anartificial device which regular caffeine users

would not normally use;l There was no control group, so the fluid lossescould have arisen from other factors, such as thestandardised diet;l The study included no exercise component.

A similar study considered the effects on hydrationof different levels of caffeine consumption (5). Eighteenhealthy men consumed each of four different drinks:

pure water, water plus cola, water plus diet cola and water plus coffee. Body weight, urine output andblood samples were measured before and after eachtreatment, and there were no significant differences

Like it or not, alcohol and caffeine are drugs that most of us consume regularly

DIURETICS

Caffeine and alcohol just how dehydrating

are they?Do you do drugs? Think long and hard before you answer, because the answer is, very probably, yes! Like it or not, alcohol and caffeine are drugsthat most of us consume regularly as part of ourdiet. Like all drugs, they have side effects, one of

which is common to both a diuretic (water-loss) effect. But how strong is this effect, and is adiet containing these drugs detrimental to thegoal of optimum hydration?

Trimethyl xanthine (more commonly knownas caffeine) belongs to a family of naturally

occurring compounds found in a number of plants. The most common sources of caffeine in

western diets include coffee, tea, cocoa and itsderivatives (such as chocolate), and kola nuts.Caffeine is also added to a number of carbonated beverages, particularly cola drinks.

Part of the reason for the popularity of caffeine-containing beverages is that caffeine is acentral nervous stimulant. Caffeine blocks thebinding to nerve cells of a substance calledadenosine, which normally acts to slow downnerve cell activity. The resulting increased nerve

activity stimulates the release of the hormoneepinephrine (adrenaline) which, in turn, leads tosuch effects as increased heart rate and bloodpressure, increased blood flow to muscles andthe release of glucose by the liver. Caffeine alsoincreases brain levels of the neurotransmitterdopamine, which is involved in cognitive(thinking) processes, alertness and memory.

Caffeine is popular with athletes for a verydifferent reason it appears to enhanceperformance (especially endurance performance).The exact mechanism remains unclear, but itsergogenic properties may be linked to a reducedrate of muscle glycogen consumption during theinitial stages of exercise, a lower perceived rate of exertion (making strenuous efforts feel easier), orits adrenaline-like effects, which stimulate morecalcium to enter muscle cells during contractions,thus boosting the potential power of muscularactivity. ( For more detail on the ergogenic effects of

caffeine, see PP206, December 2004 ).However, a possible downside to caffeine use is

its diuretic effect, because at higher doses caffeineis known to promote some water loss, due partly toincreased blood flow to the kidneys and partly to

reduced reabsorption of sodium by the body.However, this diuretic effect is also governed by theconcentration of caffeine in any given drink. Forexample, an espresso coffee provides around


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By contrast with caffeine,the evidence on alcohol

and hydrationis quite

damning

in the effect of the various combinations of beverageson hydration status.

The researchers concluded that their resultsdid not allow them to support the standard adviceto disregard caffeinated beverages whencalculating daily fluid intake. To put it another way, cutting out caffeinated drinks in order toimprove hydration may well be counterproductiveunless such drinks are replaced by an equivalent volume of non-caffeinated drinks.

Further evidence that caffeinated drinks arentthe villains they are sometimes made out to be comesfrom two studies carried out last year. The firststudied the effect of tea on hydration in 13 membersof expeditions based at Mount Everest base camp at an altitude of 5,345m (6). Hydration is a particularproblem at such altitudes because of a phenomenoncalled altitude diuresis, whereby large amounts of water are lost via the lungs because of the very low

air pressure and temperatures.Two 24-hour trials were conducted: a tea

condition, in which hot brewed tea formed a majorpart of fluid intake, and a no-tea condition, wheretea was excluded from the diet. All subjects weredenied other caffeinated beverages, foods andalcoholic drinks. Analysis of the results showed thattea made no difference to urine output or othermarkers of hydration status. In fact, the onlyreported difference was in mood, with tea-drinkingsubjects reporting significantly reduced fatigue!

The second study looked at the effects on

rehydration of caffeinated beverages consumedduring the recovery period after strenuous workoutsin the heat (7). Ten partially heat-acclimatised athletescompleted three successive days of twice-daily two-hour practices in warm conditions (23C). In adouble-blind cross-over design, the athletesrehydrated either with Coca-Cola (containing100mg of caffeine per litre) or the decaffeinated variety. Although urine was a darker colour at onepoint with the caffeinated cola, all other measuresof hydration status, including urine output, body weight and total body water were similar. Theresearchers found little evidence to support the ideathat caffeinated drinks have an adverse effect onhydration during post-exercise recovery.

Alcohol and hydration Alcohol, technically known as ethyl alcohol orethanol, has been around for a long, long time.Prehistoric man discovered that the fermentation of fruit and vegetables produced alcohol and believedit had therapeutic properties. As civilisation spread,so did the use of alcohol. But it was onlycomparatively recently that the side effects andtoxicity of alcohol were fully understood.

Although small amounts of alcohol are known toreduce the risk of coronary heart disease andfacilitate social interaction by promoting relaxationand lowered inhibition, the possible effects of

excessive intakes are now known to include cancersof the mouth, larynx and oesophagus, liver damage,high blood pressure, diabetes, stroke, mentaldisorders, lowered immunity, reduced fertility andsexual performance, social and domestic problems,poor work performance and an increased risk of accidents and injuries.

Problems connected with alcohol use nowaffect as many as eight million people in the UK alone and this number is rising. Almost one-in-three adult men and nearly one-in-five womennow drink more than the recommended limits of 14-21 weekly units for women and 21-28 for men.While it is unlikely that health-conscious athletesor fitness enthusiasts would exceed these limits,there is still a performance issue with lower dosesof alcohol: hydration.

How alcohol dehydrates Alcohol promotes water loss by depressingproduction of the antidiuretic hormone called

vasopressin, which acts on the kidneys,concentrating the urine by promoting thereabsorption of water and salt into the body.Vasopressin helps to regulate the concentration of fluids in the body, and interference with its actionleads to an increased loss of body fluid fromurination, which can contribute towardsdehydration. To make matters worse, alcohol-induced water loss can lead to the additional loss of such minerals as magnesium, potassium, calcium

and zinc, which are not only important nutrients butare also involved in the maintenance of fluid balanceand nerve and muscle action.

Although alcohol use beyond therecommended limits is known to be detrimentalto health (and thereby to athletic performance),

what evidence is there that sensible use has any illeffects? In particular, can alcohol use impairhydration status? By contrast with caffeine, theevidence is quite damning.

A UK study examined the effect of alcoholconsumption on the restoration of fluid andelectrolyte balance after exercise-induceddehydration (8). In four separate trial conditions,six subjects consumed drinks containing 0, 1, 2, and4% alcohol over 60-minute period, beginning 30minutes after the end of a dehydrating exercisesession. Although a different beverage wasconsumed in each condition, the volume remainedconstant at just over two litres (equivalent to 150%of body mass loss during exercise the amountrecommended for efficient hydration).

The researchers found that the total volume of urine produced in the six hours after rehydrationtended to increase in line with the alcohol content of

drinks. Furthermore, the increase in blood andplasma volume during this rehydration period wasmarkedly slower with the 4% beverage and did notultimately rise significantly above the dehydrated


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level! The researchers concluded that while alcoholhas a negligible diuretic effect when consumed indilute solution (2% or less), drinks containing 4% ormore of alcohol tend to delay the recovery process.

Another study investigated the effects of alcoholon pulse rate and blood volume during orthostatictilting (changing from lying down to an uprightposture) (9). Ten men and as many women drank non-alcoholic beer mixed with either alcohol (1.1g per kgof body weight) or an equivalent volume of water.Pulse rate, blood volume and blood alcoholconcentration were monitored for the next eighthours, and the alcohol group were found to developa significant relative fluid deficit, beginning two hoursafter ingestion and averaging 0.5 litres by three hours.

Further evidence of the detrimental effects of alcohol on hydration comes from a more recentstudy examining the effects of alcohol after a cycleergometer test (10). Eleven active men cycled on the

ergo and one hour later drank either alcohol (0.7gper kg of body weight) mixed with flavoured wateror an equivalent volume of flavoured water alone(placebo). Measurements of a range of bloodmarkers were taken at one, five and 22 hours.

As expected, both groups showed a drop inplasma volume after training, but this had beenrestored in both groups an hour after the drinks

were consumed. The alcohol group (unsurprisingly)experienced a big rise in blood alcohol after theirdrink, but after five hours, their blood alcohol levelshad fallen back to zero.

However both groups were also tested for blood viscosity (stickiness). After cycling, viscosity wasraised in both groups (probably throughdehydration), but while the viscosity steadilyreturned to normal levels in the placebo group,

levels in the alcohol group remained raised, even at22 hours, despite the fact that their plasma volume

values had returned to normal. In other words,although the alcohol had left the bloodstream andhydration appeared normal in terms of bloodplasma volume, the blood of those who hadconsumed alcohol during the recovery phases wasstill behaving as though the body was dehydrated!The implication is that alcohol during post-exerciserecovery may have longer lasting effects than simplyimpairing rehydration.

In conclusionCaffeine and alcohol are widely-used drugs, andits not hard to understand why. They arepleasurable to consume, and not at all bad forhealth in moderate doses. Tea and cocoa beansare stuffed to the brim with health-protectingantioxidants, as is red wine! Moreover, small

amounts of alcohol are believed to reduce therisk of coronary heart disease.

But what of their effects on hydration? When itcomes to caffeine, the oft-repeated advice to cut outcaffeinated drinks to boost hydration seems well

wide of the mark. Not only is there scant evidencethat drinks containing even moderate levels of caffeine exert a diuretic effect, but cutting out thesedrinks and not replacing them with at least anequivalent volume of non-caffeinated beverages

would actually lead to poorer hydration.Normal-strength tea and coffee therefore

appear to be perfectly acceptable either before orafter exercise, although you should rememberthat sodium/electrolyte drinks may be a betterstrategy after strenuous exercise in the heat. Theonly caveat is that if you habitually abstain from

Alcohol during post- exercise recovery may have longer lasting effectsthan simplyimpairing

hydration

Typical caffeine content of common consumables(Approximate figures, depending on preparation methods) (1,2)

Item Caffeine content

ChocolateBittersweet 25mg/oz (875mg/kg)Milk 3-6mg/oz (100-210mg/kg)

Cocoa 0.5mg/fl oz (17mg/litre)CoffeeBrewed (drip) 4-20mg/fl oz (130-680mg/litre; 40-170mg/5fl oz cup)Decaffeinated 0.4-0.6mg/fl oz (13-20mg/litre)Instant 4-12mg/fl oz (130-400mg/litre)Espresso 100mg/fl oz (3,400mg/litre)Cola/carbonated drinksCoca Cola classic 34mg/12fl oz can (100mg/litre)Diet Pepsi 36mg/12fl oz can (105mg/litre)Red Bull 80mg/8.3fl oz can (337mg/litre)

Teas/other infusionsBlack tea, brewed (USA) 2.5-11mg/fl oz (85-370mg/litre)Black tea, brewed (other) 3-14mg/fl oz (100-470mg/litre)Black tea, canned iced 2-3mg/fl oz (70-100mg/litre)Black tea, instant 3.5mg/fl oz (120mg/litre)Green tea 2.5mg/fl oz (85mg/litre; 8-30mg per tea bag, ie one serving)Decaffeinated tea 0.5mg/fl oz (17mg/litre; 1-4mg per tea bag, ie one serving)


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References1. InternationalFood InformationCouncil, U.S. Foodand Drug

Administration, TheCoca-Cola Company 2. J Am Diet 74:28-32, 19793. Can J PhysiolPharmacol, 68(7):889-92, 19904. Ann Nutr Metab,41(1): 29-36, 19975. J Am Coll Nutr,19(5): 591-600,20006. Eur J ApplPhysiol, 91(4): 493-

8, 20047. Int J Sport Nutr Exerc Metab, 14(4):419-29, 20048. J Appl Physiol,83(4): 1152-8,19979. Ann Emerg Med,25(5): 636-41,199510. Clin HemorheolMicrocirc, 24(4):227-32, 2001

WHAT THE PAPERS SAY

Reports by Isabel Walker

No link betweenhydration and cramps

The popular theory that exercise-induced musclecramping (EAMC) is caused by fluid imbalances,particularly dehydration and abnormalities inblood electrolyte levels, has been overturned bya South African study of ultra-distance runners.

Electrolyte and fluid disturbances have beenassociated with muscle cramps in certain clinicalconditions, explain the researchers, and it istherefore often assumed that EAMC has thesame cause despite a lack of evidence to thateffect.

They set out to determine whether acuteEAMC in distance runners is related to changesin serum electrolyte concentrations andhydration status. A cohort of 72 male runnersparticipating in the Two Oceans Ultra-marathon,a 56k road race held annually in Cape Town,were asked about their history of EAMC and thenfollowed up for the development of the conditionduring the race.

All subjects were weighed before andimmediately after the race to assess changes inhydration status. Blood samples were takenbefore, immediately after and 60 minutes after the race and analysed for glucose, protein,sodium, potassium, calcium and magnesiumconcentrations, as well as various markers of hydration status.

Of the 72 runners in the study, 45 had a historyof EAMC, while 27 had no previous experience of muscle cramping. In the event, 21 of the 45runners with a history of cramping suffered acuteEAMC either during the race or within 60 minutes

of completing it, while 22 of the 27 runners withno history of cramping formed a control group for comparison purposes.

Key findings were as follows:

l All episodes of cramping occurred in the latter half of the race or immediately afterwards, withmost affected runners reporting three or moreepisodes. Most commonly affected muscleswere hamstrings (48%) and quadriceps (38%).Most cramps were moderate-to-severe inintensity and best relieved by slowing the pace or passive stretching;l There were no significant differences betweenthe groups for pre- or post-race body weight, per cent change in body weight, blood volume,plasma volume, or red cell volume, indicating nodifference in hydration status;l Immediate post-race serum sodiumconcentration was significantly lower in the crampgroup, while serum magnesium concentrationwas significantly higher. However, thesedifferences were considered to be too small to beof clinical significance.

Furthermore, report the researchers, thedecrease in serum sodium concentration following the race in the cramp group is probably related toan increased fluid intake during the race in thisgroup. Although drinking patterns were notmeasured directly, increased drinking in the crampgroup is likely because of the well publicised belief that cramping is caused by dehydration.

This supposition was supported by the finding that runners with EAMC were less dehydratedthan non-cramping runners immediately after therace, with per cent decreases in body weight (pre-to post-race) of 2.9% and 3.6% respectively.

The results of our study, conclude theresearchers, do not support the commonhypotheses that EAMC is associated with either changes in serum electrolyte concentrations or changes in hydration status following ultra-distancerunning. An alternative hypothesis to explain the[cause] of EAMC must therefore be sought.Br J Sports Med 2004; 38:488-492

How variable-intensityexercise slows gastric

emptying There is more to effective hydration than simplyingesting fluid, as anyone who has read AndrewHamiltons lead article in this issue should now beaware. Little benefit can be derived from aningested drink until its components have beenincorporated into the relevant body pool. And this,in turn, depends on the rate at which the ingesteddrink is emptied from the stomach, absorbed bythe small intestine and transported onwards.

The rate of gastric emptying may be affectedby a number of factors, including the composition

of the drink and the activity being performed. Anda new study from the UK and New Zealand wasset up to investigate two key questions:

1. Whether gastric emptying was slowed

caffeine, suddenly introducing it in the form of strong drinks may exert a mild diuretic effect.

The post-exercise use of alcohol is moreproblematical, and the evidence suggests that,unless you consume very weak alcoholic drinks(2% by volume or less), alcohol should becompletely avoided until you are fully hydrated.

And because alcohol takes 36 hours to completelyclear the system, athletes should refrain from itsuse for at least 48 hours before an event.

If you do find yourself in a situation wheredrinks are flowing, drink weak beers forpreference, avoid wine and spirits and aim toconsume one soft drink for every alcoholic one.

Andrew Hamilton


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during a standardised variable-intensity shuttlerunning test designed to simulate the activitypattern of football.

2. Whether low-energy fluids were emptiedmore effectively than carbohydrate solutionsduring this kind of intermittent exercise.

In a study on eight healthy males, theresearchers compared the volume of test drinkemptied during two 15-minute periods of walking exercise with the volume emptied during two 15-minute periods of the Loughborough IntermittentShuttle Test (LIST). For each type of exercise, thesubjects completed one bout after ingesting aset amount of a carbohydrate-free placebo andthe other after ingesting the identical amount of a 6.4% carbohydrate-electrolyte sports drink.

Gastric emptying was measured before andafter each of the four trials using a techniquethat required them to swallow a piece of

equipment known as a gastric aspiration tube. The researchers found that gastric emptying

was slowed during the LIST trials by comparisonwith the walking trials, although the overallexercise intensity would not normally beconsidered sufficient to affect gastric emptying had it been carried out at a constant power output. In fact, the subjects were exercising at or above 70% of their V0 2max for only about threeminutes, 21 seconds during each 15-minuteperiod of the LIST, and for the rest of the time ator below 50% V0 2max similar to the intensitiesmeasured in professional footballers competing

While early research favours glycerol, more recent findings are rather less clear-cut

in outdoor matches.This finding suggests, say the researchers, that

even a small amount of high-intensity sprinting canslow gastric emptying during variable-intensityexercise such as occurs during many sports.

Most studies have found that solutions with acarbohydrate content of 6% or more empty moreslowly than energy-free drinks when ingested atrest or during low-to-moderate intensity constantoutput exercise. And, indeed, in this study the

volume of placebo drink emptied over 30minutes during the walking trials was greater than that of the carbohydrate drink.

However, during the LIST trials both drinksemptied at the same slowed-down rate, whichindicates, say the researchers, that exercise hasa greater effect on gastric emptying thancarbohydrate content at this intensity.

This present study, they point out, is the first to

conclusively show that variable-intensity exercise,which simulates the activity pattern of soccer, slowsthe rate of gastric emptying of energy-free drinkssuch as water to the same extent as that of carbohydrate-containing sports drinks.

They conclude that, given the potential benefitof ingested carbohydrate in delaying the fatigueprocess, and also in promoting intestinal water absorption, there seems little reason to advocatedrinking water rather than a suitable sports drinkduring a football match.Med Sci Sports Exerc, vol 37, no 2, pp240-247,2005

HYDRATION

Glycerol could it bethe secret of OlympicMarathoner DeenaKastors success?One of the surprise results of the recentOlympic Games was American Deena Kastorsbronze in the womens marathon. Afterwards it

was revealed that she had imbibed a glycerolsolution as part of her pre-race preparation ina bid to enhance and maintain hydration in thescorching heat of Athens. Could this have beena factor in her success? Can it really helpathletes to keep hydrated? How does it work and are there any downsides to its use?

Glycerol is a 3-carbon molecule, which isproduced naturally in the body as a result of normal metabolism. Although classed as analcohol, glycerol plays a number of important

roles in the body. For example,phosphoglycerides, which consist of a glycerolbackbone bonded to two fatty acid chains andanother alcohol, are an important component of

cell membranes. Glycerol is also used to storefatty acids in the body; in this process, threefatty acid chains are chemically bonded to aglycerol molecule hence the termtriglyceride.

Pure glycerol is a sweet-tasting clear syrupyliquid which, when mixed with water solutions,is able to increase their concentration or, moretechnically, their osmolarity . Because thehuman body requires the osmolarity of bodyfluids to remain fairly constant, ingestingglycerol stimulates the absorption andretention of water in order to counter theincrease in osmolarity that would otherwiseoccur.

To put it another way, ingesting a solution of glycerol and water allows the ingested water tobe retained by the body and excreted only whenthe extra glycerol is either removed by thekidneys or broken down by the body (1).

Endurance athletes competing in hot andhumid conditions need to maintain maximumhydration, since fluid losses of as little as 1.5

litres can significantly impair performance.Moreover, studies have shown that manyathletes do not drink enough to offset


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Who would have thoughtthat a

tendency to over-pronation

could be

responsible for sports injuries ranging from runners knee

to sciatica?

dehydration during competition, even withunlimited access to fluid (2).

A temporary state of hyper-hydration can beachieved by drinking lots of water in excess of the bodys needs. However this situation is verytransitory because the consequent fall inosmolarity stimulates the kidneys to removemost of the excess water within an hour,requiring frequent trips to the loo, which arenot exactly conducive to fast race times!

However, adding glycerol to the waterprevents this drop in osmolarity and can prolongthe period of hyper-hydration for up to fourhours, which explains its use by athletes seekingto enhance endurance performance in hot weather conditions (3).

On the face of it, increasing and maintaininghydration levels in endurance athletes seems asure-fire way of enhancing hot weather

performance. And there is no doubt thatingesting glycerol solutions does increase waterretention by anything up to a litre (4,5).

The question is, however, whether thisincreased hydration translates directly intosuperior hot-weather performances. And this is where things start to get a bit less clear.

Glycerol v orange juice A study carried out in 1990 investigated whether glycerol hyper-hydration alteredsweating, regulation of body temperature and

cardiovascular function during exercise in a hotenvironment (42C and 25% relativehumidity) (3) . Six averagely fit people completedthree 90-minute runs at around 60% of theirVO 2max after drinking either orange juice,diluted orange juice or glycerol solution.

After glycerol ingestion, subjects produced,on average, 500ml less urine and retained700ml more total body water than those in theno-glycerol groups. Glycerol-treated subjectsalso sweated more and had smaller increases incore temperature throughout the 90 minutes of exercise. However, the small sample size andthe relatively low work rate used in the trialmeans its results should be interpreted withcaution.

Two subsequent studies examined the effectsof glycerol ingestion in 11 subjects of moderate-to-high endurance fitness (6). Over a 90-minuteperiod, the subjects consumed either a glycerolsolution or a placebo drink; then, an hour later,they cycled at 74% of their VO 2max until theycould not maintain their pedalling cadenceabove 60rpm (revolutions per minute).

As expected, glycerol intake increased pre-

exercise body water by 730ml and decreasedexcreted urine volume by 670ml. But, moreimportantly, subjects who had taken glycerolexercised significantly longer to fatigue,

averaging around 94 minutes compared with just 73 minutes for those on placebo.

The researchers then went on to look at whether these positive effects were still evident when carbohydrate was ingested at the sametime, as would be the case for most athletesduring prolonged endurance events. Sevenhighly trained endurance athletes completedthe same trial described above, but this time thesubjects in both groups also consumed a 5%glucose solution at the rate of 3ml per kg of body weight every 20 minutes.

Analysis of the results showed that, while theglycerol solution still led to the retention of more body water, it was now just 100ml morethan for those on placebo. Similarly, thedifference in excreted urine volume wasreduced to 92ml. Nevertheless, glycerol stillprolonged the time taken to reach fatigue (123

minutes compared with 99 for those onplacebo!).

Other studies have cast doubt on the efficacyof glycerol, with two subsequent investigationsfailing to find any significant benefits (4,7) .However, both of these studies used very gentleexercise intensities (around 50% VO 2max),

which makes their results less relevant toathletes. An earlier study also showed nobenefits, but as well as using a low exerciseintensity (50% VO 2max), this one also lacked apre-exercise hyper-hydration procedure, which

makes its results fairly meaningless(8)

.On balance, this early research comes downfirmly in favour of glycerol. More recentresearch, however, is rather less clear-cut.Benefits were observed in a study of sixendurance-trained cyclists, who ingested eithera glycerol solution or the same volume of placebo two hours before undertaking 90minutes of steady state cycling at 98% of lactatethreshold in dry heat (35C, 30% relativehumidity) (9). The cyclists were also allowed toingest a carbohydrate drink (6% solution) at 15-minute intervals during the ride. Afterwards,they cycled for a further 15 minutes while theirpower outputs were assessed.

As expected, pre-exercise urine volume waslower when taking glycerol solution and heartrate was also significantly reduced. And,although the researchers failed to find anysignificant metabolic differences (such as lactateaccumulation) between the glycerol and placebogroups, the work performed in the 15-minuteassessment period was 5% higher in those takingglycerol.

Another, arguably more relevant, study on

triathletes also found benefits with glyceroluse (10) . Seven male and three female triathletescompleted two Olympic-distance triathlons two

weeks apart, one on a hot day (30.5C) and the


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other on a warm day (25 C). The triathletes were randomly assigned to consume either aglycerol solution or placebo, plus acarbohydrate solution in both cases , over a 60-minute period, two hours before each triathlon.

Although there were no significantdifferences in sweat loss between the glyceroland placebo conditions, glycerol-supplementedtriathletes excreted a smaller volume of urineand subsequently retained more fluid thanthose on placebo.

More importantly, however, athletes onplacebo performed significantly worse underhot conditions than those on glycerol bycomparison with their performances under

warm conditions. The average extra time takenby placebo triathletes in hot weather was 11mins 40 secs, compared with just 1 min 47 secsextra for those on glycerol.

Protection from the heatThe researchers also discovered that most of this performance improvement occurred duringthe final 10k run leg of the triathlon on the hotday. And they concluded that glycerol hyper-hydration may provide some protection againstthe negative effects of competing in the heat.

However, two studies on glycerol andperformance published last year came to ratherless positive conclusions. The first comparedglycerol and water hydration regimens on

tennis performance(11)

. Eleven male subjectscompleted two trials, each consisting of threephases:1. Hyper-hydration with or without glycerolover 2.5 hours;2. Two hours of exercise-induced dehydration;3. Rehydration with or without glycerol over 90minutes.

In the second trial, those who had takenglycerol reverted to water alone and vice versa .

After each phase, subjects completed 5m and10m sprint tests, a repeated-effort agility test,and tennis skill tests.

As expected, glycerol hyper-hydrationincreased fluid retention (by around 900ml) bycomparison with placebo. However, theexercise-induced dehydration resulted insimilar losses of weight (from fluid loss) in bothgroups. Despite the fact that this loss wasmodest (less than 3%), the measured sprinttimes were significantly slower for both groupsafter phase 2 than after phases 1 and 3, whilethere were no significant differences betweengroups for the agility and tennis skill tests.

The researchers concluded that, while the

glycerol regimen provided better hydrationstatus than placebo, this was not reflected inperformance benefits.

Another study conducted last year set out to

compare the effectiveness of glycerol and waterhyper-hydration in cyclists working under hot,humid conditions (12). Seven moderately-to-welltrained subjects ingested either a glycerolsolution or the same volume of placebo 2.5hours before a race-simulation exercise, in

which they cycled as far as possible over a 60-minute period. While the glycerol groupsweated more during the trial, there were nosignificant between-groups differences in coretemperature, power output and total distancecycled.

Although there appears to be conflictingevidence about the performance benefits of glycerol hyper-hydration, you might think it fairto assume that it definitely enhances waterretention.

However, a recent Canadian study reportedon a trained triathlete who retained more water

with water alone than with glycerol (13) . Theresearchers postulated that this might havehappened because the plain water wasintegrated into the body fluid pools more slowlythan the glycerol solution. With just one subject,it is difficult to draw firm conclusions, but thisstudy suggests there are some people whorespond unusually to glycerol administration,and this may help to explain why some glycerolstudies have drawn a blank.

It is fair to say that, for most athletes,imbibing a glycerol solution does produce an

increase in total body water. What is less clear is whether this actually enhances performance.This is partly because we dont fully understandhow glycerol works in the body. We do knowthat the kidneys dont excrete glycerol rapidlyso that it stays in the body and holds water withit. But more research is needed to find out

whether glycerol works by increasing theamount of fluid inside cells or in the circulation.

No consensus on glycerol usageOverall, the current weight of evidence is tiltedin favour of glycerol, but only in events where

substantial dehydration is likely to be a problem ie long, strenuous events in hot and humidconditions. However, there is no consensus onthe best way to take glycerol solution, or on

whether certain kinds of plain water hyper-hydration protocols might offer similarbenefits.

So should you take glycerol? Unless yourevent is long and due to take place in hot/humidconditions, resulting in unavoidabledehydration, there is probably little point. Theevidence also suggests that taking glycerol

before less vigorous events is not particularly worthwhile. And where the benefits are likely to be

marginal, you should also be aware that glycerol

References1. Journal of AppliedPhysiology 1995,79, 2069-20772. Exercise Sport

Science Review 1993, 21, 297-3303. Medicine and

Science in Sportsand Exercise 1990,22, 477-4834. Journal of AppliedPhysiology 1997,83, 860-8665. Journal of AppliedPhysiology 1996,79, 2069-20776. International

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PAGE 15

HYDARATION THERAPY

Contributors: Andrew HamiltonBSc, MRSC, trainedas a chemist and isnow a consultant to

the fitness industry and an experienced

science writerRon Maughanis professor of sportand exercise sciencesat LoughboroughUniversity

ingestion is associated with such side effects asstomach upsets, headaches and blurred visionat higher doses. If you are tempted to tryglycerol, make sure youve tried properhydration strategies using good old water orfluid replacement drinks first! Glycerol shouldbe considered as a last resort, not the first.

If you do decide to try glycerol, you mightlike to use the protocol that producedsignificant hyper-hydration in one of the studiesmentioned above (5). Bear in mind, though, thatit involves drinking nearly two litres of fluid, which will lead to a weight increase of 3% for a70kg athlete and 4% for one weighing 50kg! If youre a runner, you may find that this extra

mass outweighs, quite literally, any potentialperformance benefits!

Montners glycerol ingestion protocol,beginning 150 minutes (2.5 hours) beforeexercise:l 0 minutes drink 5ml per kg of your body

weight of a 20% glycerol solution (1 partglycerol to 4 parts water);l 30 minutes drink 5ml/kg of water;l 45 minutes drink 5ml/kg of water;l 60 minutes drink 1ml/kg of a 20% glycerolsolution and 5 ml/kg of water;l 90 minutes drink 5ml/kg of water;l 150 minutes begin exercise.

Andrew Hamilton

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