Food Intake and Digestion

Will BrownLecture 3

Questions?

• Does everyone have a lab manual?• Reminder: Lab worksheets due Thursday at

7PM• Any questions for me? – Subject material– Logistics– Testing/Grading

Objectives and Content

• Objective: Outline the process of digestion and the major organs and tissues involved

• Content– “Eat this: not that!”– Digestive system• Anatomy and physiology of digestive tract• Common digestive issues

Which should you eat?

Nutrition Facts

Serving Size: 2 cups popped

70 cal

4 g fat (2 g saturated)

50 mg Sodium

Nutrition Facts

Serving Size: 2 cups popped

80 cal

3.5 g fat (0.5 g saturated, 1.5 trans)

40 mg Sodium

McDonald’s: Which one?McDopnald’s Big N’ Tasty

Nutrition Facts

Serving Size: One burger

460 cal

24 g fat (8 g saturated; 1.5g trans)

720 mg Sodium

Nutrition Facts

Serving Size: One sandwich

530 cal

20 g fat (3.5 g saturated)

1150 mg Sodium

Starbucks: Which one?

Venti Caramel Cappuccino

Venti Caramel Latte

Nutrition Facts

180 cal

4 g fat (2.5 g saturated)

18 g Sugar

Nutrition Facts

1320 cal

8 g fat (5 g saturated)

43 g Sugar

Digestive System: Overview

• In order to be used, food must be extensively altered for the human body to use

• This process has 2 parts– Digestion – the breakdown of food; both

mechanical and chemical– Absorption – The uptake of nutrients from the

intestine• Takes place in the Gastrointestinal (GI) tract– A long tube that stretches from mouth to anus

Digestive system: Overview

• Mechanical Digestion – Physical breakdown of food– Chewing– Peristalsis – muscle contractions along the GI tract

that move and mix food (process called motility)• Chemical Digestion – The breakdown of food

by chemicals secreted by the GI tract– Stomach acid– Pepsin

Digestive Tract: Overview

• Involved in waste elimination• Houses commensal bacteria– Vitamin K– Biotin

• Most processes under autonomic control– Nervous system– Endocrine system

Mouth

• Responsible for primary mechanical digestion– Teeth– Tongue – to a lesser degree

• Taste – allows for sensation of flavors– 5 main flavor components: salty, sweet, bitter,

sour and umami; taste buds on tongue– Nose is highly involved in taste sensation• Volatile compounds reach the nasal passages

– Signals rest of GI to prepare for food

Mouth

• Produces Saliva via the salivary glands– Provides lubrication– Acts as a solvent

• Produces mucus– Lubricates food and surfaces of GI tract

• Produces Salivary Amylase– Enzyme that begins the breakdown of starch

Enzymes: A slight digression

• Follow “Lock and Key” model– An enzyme only acts on one substrate– Lactase only digests lactose (milk sugar)

• Enzymes are diffusion rate limited• Sensitive to pH– Stomach is acidic, small intestine is alkaline– Enzymes designed to work in acidic pH do not

work so well in basic conditions

Esophagus

• Connects the pharynx (the back of the mouth) to the stomach

• The epiglottis covers the trachea when food is swallowed

• Peristalsis begins at the top of esophagus

Esophagus

• Esophageal sphincter – muscle at the end of esophagus that prevents stomach contents from entering the esophagus– There are numerous sphincters along the GI; all

prevent flow of contents from going backwards• The reason for “heartburn” is stomach

contents in the esophagus

Stomach

• Large sac at the end of the esophagus• Holds ~1 quart of food– This can be different for each person; depends on genetics

and diet• Once in the stomach, food mixes with gastric juice– Maintains acidic environment of stomach– Destroys biological activity of foreign enzymes– Activates stomach enzymes– Makes minerals soluble

• Release of chyme into small intestine is about 1 tsp at a time

Anatomy of Stomach

Physiology of Stomach

• Mucous producing cells produce mucous that coats the entire stomach and prevents self-digestion

• Parietal cells produce hydrochloric acid• Chief cells produce digestive enzymes• Releases gastrin (a hormone) in order to control

release of other digestive enzymes and acid• Releases intrinsic factor – required for vitamin B-12

absorption• Can absorb alcohol

Small Intestine

• Pyloric sphincter joins stomach and small intestine• 3 parts of the SI are the duodenum, jejunum and

ileum• ~ 10 ft long• Called small because of narrow diameter; 1 in• More peristalsis to mix food with digestive juices– Enzymes to breakdown carbohydrates, protein, and fat– Prepares vitamins and minerals for absorption

Gastric = stomach; digestive = small intestine

Small intestine

• Lining folded numerous times – creates villi• Villi are fingerlike projects– Trap food as it moves to enhance absorption– Covered with absorptive cells that have a highly

folded cap called microvilli– Creates a surface area 600 times greater than if it

were a normal tube

Absorptive cells

• Responsible for transporting nutrients and digestive food across the intestinal barrier and into the body

• Short lived – rapidly turned over and renewed• Dead cells are sloughed off, digested and

reabsorped• Have a highly folded cap, called microvilli,

further increases surface area of SI

Absorption

• Passive diffusion – Higher concentration in lumen than in body so nutrients move with a gradient– Fats, water, some minerals– No energy required

• Facilitated diffusion – Compounds require carrier to assist in diffusion– Fructose– No energy required

Absorption

• Active absorption – Nutrients in low concentration in lumen are taken up into the body, usually with carrier– Glucose, and amino acids– Requires energy input

• Phagocytosis/pinocytosis – Cell membrane creates a fold that closes off to create a vessicle which can move things from lumen to dorsal side of cell– Phago=eat or devour; pino=sip or drink– In infants, Immune substances from mothers milk– Requires energy; highest energy requirements

Absorption• Water soluble compounds enter blood stream via capillaries

– Glucose– Amino acid– Most vitamins– Some minerals

• Fat and fat soluble compounds enter the lymph system and then enter the blood stream later

• Once absorbed into the blood, goes directly to the liver– Hepatic portal– Detoxifies blood– If pregnant, then heads directly to uterus

Large Intestine

• Transition from SI to LI is called the ileocecal sphincter – prevents LI contents from re-entering the SI

• Very little nutrient absorption occurs in LI• Anatomically different from SI– No villi– No digestive enzymes– High number of mucous producing cells

• Mucous protects LI from bacteria and helps hold feces together

Large Intestine• Nutrients absorption does occur

– Water - ~10-30% of total– Some fatty acids– Sodium and potassium– Some vitamins

• Houses numerous commensal bacteria – Most are beneficial – probiotic– Recent studies have demonstrated that there are over 2000 types

of bacteria that can live in the LI• Most people have between 200-500 different species

– After birth the colon is rapidly colonized; usually from the mother– Another term to know – prebiotic; substances that help probiotics

grow

Rectum and Anus

• Last portion of LI• Feces held in rectum until pushed into the

anus for elimination• The anus contains 2 sphincters– Internal anal sphincter – autonomic control– External anal sphincter – voluntarily controlled

Accessory organs

• Not part of GI tract but play important role in aiding digestion

• Gall Bladder– Houses bile (which is made in the liver)– Bile release under hormone control

• Released due to presence of fat in stomach– Bile acts as “soap” to emulsify fat for absorption– Components of bile can be recirculated and reused – process

known as enterohepatic circulation– Bile can also house substances for excretion that are to large

to exit via the kidneys

Accessory organs

• Pancreas– Makes hormones and pancreatic juices– Hormones: insulin and glucagon; both act on blood sugar

levels– Pancreatic juices: bicarbonate, water, and digestive

enzymes• Bicarbonate is a basic substance that neutralizes acidity of

chyme as it moves from stomach to SI• Important because not as much mucous in SI compared to

stomach; low pH would cause ulceration of SI

• Liver: makes bile and cleans blood after absorption

Digestion Review• Mouth• Esophagus - ~10 in• Stomach – Holds ~1 quart

– Food held 2-4 hours• Small Intestine – 3-10 hours

– Duodenum – 10” long– Jejunum – 4’ long– Ileum – 5’ long

• Large intestine – up to 72 hours– 3.5 ft long– Cecum, ascending, transverse,

descending, and sigmoid colon

Organ FunctionMouth & Salivary Gland

Chew food; Perceive taste; Moisten food with Saliva; Lubricate food w/ Mucous; Release starch via salivary amylase; Initiate swallowing reflex

Esophagus Lubricate w/ mucous; Move food to stomach via peristalsisStomach Store, mix, digest; kill microorganisms w/ acid; Release protein

digesting enzymes(pepsin); Lubricate & protect stomach w/ mucous; Regulate emptying into SI; Produce intrinsic factor for B-12 absorption

Liver `Produce bile to aid in fat digestion and absorptionGallbladder Store, concentrate and release bile into SIPancreas Secrete Na bicarbonate and enzymes for digesting carbs, fat &

proteinSmall Intestine Mix and propel contents; Lubricate w/ mucous; Digest and

absorbLarge Intestine Mix and propel contents; Absorb Na, K, and water; House

commensal bacteria; lubricate w/ mucous; Synthesize some vitamins and short-chain fatty acids; Form feces

Rectum Hold feces and expel via anus

From Figure 3-10 pg 97

Important Secretions and Products of GISecretion Site of Production Purpose

Saliva Mouth Partial starch digestion,lubrication

Mucous Mouth, stomach, SI, LI Protects GI tract; lubricates food

Enzymes Mouth, stomach, SI and Pancreas

Promotes digestion so nutrients are small enough for digestion

Acid Stomach Promotes digestion; other functions

Bile Liver (stored in gall bladder) Aids in fat digestion in SI by solubilizing fat in water using bile acids, cholesterol, and lecithin

Bicarbonate Pancreas and SI Neutralizes stomach acid in SI

Hormones Stomach, SI, and pancreas Stimulate production/release of acid, enzymes, bile, and bicarbonate. Helps regulate peristalsis and overall GI tract flow

Table 3-3 pg.99

Common Digestive Issues

• Lactose intolerance – inability to digest lactose (milk sugar); More on this in carbohydrate lecture

• Diverticulosis – Inflammation of intestinal tract; more on this in carbohydrate lecture

Common Digestive Issues: Ulcers• Erosion of stomach lining, esophagus, or duodenum• Loss of protective mucous layer• In younger people, primarily in SI; older is stomach• Millions will develop ulcers sometime in their life resulting in lots of

health care $ spent• Symptoms include pain ~2 hours after eating• Causes; originally thought to be caused by high stomach acid

– H. pylori infection found in 80% of ulcers– Stress– Smoking is a complicating factor

• Treatments– Antibiotics for H. pylori– Proton pump inhibitors (Prilosec, Nexium, and Prevacid) block acid formation– H2 blockers (Tagamet, Zantac, and Axid) block histamine related acid secretion– Reduce smoking– Limit use of NSAIDs

Common Digestive Issues: Heartburn

• ~50% of Americans experience heart burn occasionally• AKA: acid reflux• Recurrent form called gastroesophageal reflux disease (GERD)• Esophagus has limited mucous protection so easily eroded causing pain• Symptoms-For GERD must occur >2 times/week

– Pain– Coughing– Nausea

• Long term untreated GERD can lead to Barrett’s Esophagus• Treatments

– OTC antacids – short term quick releif– Proton pump inhibitors (Prilosec, Nexium, and Prevacid) block acid formation– H2 blockers (Tagamet, Zantac, and Axid) block histamine related acid secretion

Common Digestive Issues: Constipation

• Difficult or infrequent evacuation of bowels• Common causes– Ignoring “urges” for prolonged periods– LI muscle spasms– Ca and Fe supplements – Medications and antacids

• Treatments– High fiber foods and plenty of water; good for mild constipation– Develop more regular bowel habits– Laxatives

Common Digestive Issues: Irritable Bowel Syndrome

• Not to be confused with IBD (Inflammatory Bowel Disease)• ~25 million adults report IBS symptoms

– Cramps and pain– Gassiness and abdominal distention– Bloating– Irregular bowel function – diarrhea, constipation or both, mucous in feces

• Worth noting that 50% of IBS patients report some type of abuse – verbal or sexual are the most common

• Treatments – very individualized– High fiber foods– Elimination diets which focus on dairy, caffeine and gas-forming foods– Herbal supplements– Probiotics– Low fat diet and more frequent small meals

• Aside from discomfort and upset there are no other health concerns

Common Digestive Issues: Diarrhea• Usually lasts a few days• Symptoms

– Very loose or liquid stool– Increased frequency of bowel movements

• Causes– Usually infection in intestine – mostly due to bacteria or virus– Less common is consumption of foods not readily absorbed –

sorbitol or very high fiber foods such as bran• Treatment

– Lots of fluids during affected stage – first 24 to 48 hours• Important for those at high risk of dehydration – infants & elderly

– If lasting longer than 7 days – seek medical help

Common Digestive Issues: Gallstones

• Affects 10-20% of adults in US• Results from formation of solid crystal particles of bile that form

in gallbladder• Causes

– Excess weight – especially in women– Genetic background – Native Americans have increased risk– Advanced age - >60 years– Reduced gallbladder activity – reduced contraction– Altered bile composition– Diabetes– Diet – low fiber food– Rapid weight loss or prolonged fasting

Common Digestive Issues: Gallstones

• Symptoms– Pain in upper right abdomen– Gas– Bloating– Nausea and/or vomitting

• Treatment – surgical removal of gallbladder is most common• Prevention

– Avoid becoming overweight – especially for women– Avoid rapid weight loss - >3lbs/week– ↓ animal protein and ↑ plant protein; ie: nuts– High fiber diet

Less Common Digestive Disorders

• Cystic Fibrosis– Inherited disease caused by overproduction of mucous– Usually affects upper respiratory tract– Pancreas produces thick mucous that blocks ducts

which causes cell death – result is ↓digestive enzymes• Celiac disease– Allergic reaction to gluten

• Gluten is common in certain cereals like wheat and rye– Damages Absorptive cells reducing surface area in SI– Removing gluten from diet usually results in

amelioration of disease