Fluids, Electrolytes and Hydration

Stephen M. Simons, MD, FACSMCredit: Diana Heiman, MD

Objectives

• Discuss optimum hydration and effects of dehydration on exercise

• Discuss the best methods for fluid replacement

• Review the electrolyte composition of sports drinks

Optimum Hydration• Performance decreases with water loss of 2% or

more of body weight

• Prehydration prior to exercise helps avoid excessive losses during exercise (goal to be euvolemic)

• Measuring body weight prior to and after exercise can estimate sweat rates

• Electrolytes also need to be replaced- more or less depending on sweat rate and salt content

Sweat• Major means of cooling the exercising body

• Losses vary greatly person to person and with different types of activities

• Contains:

• Water

• Electrolytes

• Sodium*

• Potassium

• Calcium

• Magnesium

• Chloride*

• Electrolytes must be replaced

• Excessive salty sweating can lead to hyponatremia

• *increased losses with increased sweat rate and dehydration

Sweat• Water loss (sweat) during exercise is dependent on:

• Exercise Intensity

• Ambient temperature and humidity

• Clothing

• Body surface area/ body weight

• Acclimatization

• Metabolic efficiency in the exercise

Hourly Sweat Rates

Sweat

• 1 L of sweat loss

• = 1kg body weight loss

• = ~ 580 kcal loss

Sweat Rates in Different Sports

ACSM Position Stand on Exercise and Fluid Replacement 2007

Measures of Hydration Status

ACSM Position Stand on Exercise and Fluid Replacement 2007

Measures of Hydration Status

• Can’t rely on thirst, dizziness, headache, tachycardia, dry mouth, skin turgor to predict dehydration

Effects of Dehydration on Performance• With dehydration: increased strain and perceived exertion for

the same task

• Exaggerated in warm-hot conditions and with greater degree of dehydration

• Less effect of dehydration seen in cold conditions

• Dehydration (>2%BW) decreases aerobic performance, especially in warm-hot conditions

• Dehydration(2%BW) may decrease mental/ cognitive performance

• Dehydration (3-5%BW) doesn’t decrease anaerobic performance or muscle strength.

Dangers of Impaired Fluid Balance• Dehydration:

• Increases risk for exertional heat illness

• Muscle cramps (salty sweaters)

• Increases risk for renal damage and rhabdomyolysis

• Hyperhydration

• Hyponatremia - free water intake greater than sweat loss or large sweat sodium losses

Exercise-Associated Hyponatremia• First recognized in Comrades Marathon (South

Africa, reported in 1971)

• Associated with over drinking of hypotonic fluids and/or excessive salt loss.

• Athletes at higher risk: women and older adults, smaller athletes who run slowly, sweat less and drink hypotonic fluids before, during and after the race; CF gene carriers, football and tennis players who over hydrate to prevent cramps or get hypotonic IV hydration.

Exercise-Associated Hyponatremia

• Beware of diagnosing the endurance athlete with dehydration and encouraging hypotonic fluid replacement.

• Symptoms occur with rapid decline in levels below 130 mmol/L

• Symptoms increase with rapidity of decline, the longer it remains low and the lower it goes.

Exercise-Associated Hyponatremia• Symptoms at levels of 125 mmol/L or less:

• Headache, vomiting, swollen hands and feet, restlessness, fatigue, confusion and disorientation (cerebral edema), wheezing (pulmonary edema)

• Progression of symptoms with levels below 120 mmol/L:

• cerebral edema with seizure, coma, brainstem herniation, respiratory arrest and death.

Exercise-Associated Hyponatremia

• Treatment:

• Get a sodium level immediately!

• Hypertonic fluid replacement immediately and transport to the ED with CLEAR DIAGNOSIS

Fluid Replacement- Pre-Exercise• Goal is to start euhydrated

• If not euhydrated from prior exercise (<12 hour interval between activities), drink slowly at least 4 hours prior to exercise.

• If urine is dark or no urine output, drink more 2 hours prior to exercise

• Add salt to beverages or eat salty food to assist with fluid retention.

Fluid Replacement- During Exercise• Goal is to prevent excessive dehydration (>2%BW)

• Fluid replacement should mirror losses (0.4-1.8 L/hr) and not be excessive, especially in exercise >3 hours.

• Periodically drink, don’t drink at every water station

• Marathoners safe to drink 04-0.8 L/hr

• Slower, thinner athletes most at risk of over-hydration in activity > 3 hours.

• Salty sweaters need to replace electrolytes during exercise to avoid hyponatremia.

Marathon Hydration Levels and Body Weight Loss

ACSM Position Stand on Exercise and Fluid Replacement 2007

= Overhydration and Weight gain

= Insufficient Hydration and Weight loss

Fluid Replacement- After Exercise• Goal is to fully replace fluid and electrolyte deficits

• If mild deficits, normal eating and drinking will replace losses with time (~12 hours)

• If more moderate deficits, more aggressive replacement is warranted

• 1.5 L Fluid/kg weight loss

• Replacing fluid without sodium leads to excessive urine production and delayed euhydration

• IVF should be used only when losses exceed >7% BW with inability to ingest oral fluids

• No benefit vs. oral rehydration

Sports Drinks• Institute of Medicine - recommended composition of sports drinks for

prolonged physical activity in hot weather:

• 20-30 mEq/L sodium

• 2-5 mEq/L potassium

• 5-10% carb

• Concentrations >8% delay gastric emptying

• Ranges are variable to account for different exercise conditions, durations and intensity

• Carb intake is important only for extended periods of exercise (>1 hour)

Sports Drinks

• Gatorlytes add 700 mg Na and 200 mg K: also 70mg Ca and 40 mg Mg.

• Only really want to use in salty sweaters or bad crampers.

• 8 oz = 1/4 L goal is 115-173 mg NA and 20-80 mg K per 8 oz.

• 1 mEq Na=23 mg, 1 mEq K=39 mg

Caffeine and Alcohol• Caffeine, in small amounts (<180 mg/d), does

not lead to dehydration.

• Alcohol does increase urine output, especially in higher doses, use in the rehydration period can delay full rehydration.

Caffeine Content

• Red Bull = 80 mg/ 8.3 oz

• Fix Extreme Ulta Shot = 400 mg/0.17 oz

Summary• Proper hydration is important to good performance

• Dehydration negatively impacts performance

• Rehydration should be performed orally when possible

• Salty sweaters are at risk of hyponatremia if not replacing electrolyte losses

• Sports drinks are a good way of replacing losses, but often don’t contain the recommended electrolyte concentrations.

Reference

• 2007 ACSM Position Stand

• Exercise and Fluid Replacement