EXSS Nutrition Ch 5: Gastric Emptying, Digestion, and Absorption

Gastrointestinal (GI) tract

The digestive system that processes food and provides nutrients to the body.

Primary function of the GI tract

To provide the body with nutrients.

Mechanical digestion

Physically breaking food into smaller pieces.

Chemical digestion

Breaking large food molecules into smaller molecules using digestive chemicals and enzymes.

Why must food be digested?

Large molecules such as starch, fat, and protein must be broken into smaller molecules that can be absorbed.

Products of carbohydrate digestion

Sugars.

Products of fat digestion

Fatty acids.

Products of protein digestion

Amino acids.

GI tract defense

Protects the body from pathogens such as E. coli and Salmonella.

Length of the GI tract

About 6–8 meters long.

Main parts of the GI tract

Mouth, esophagus, stomach, small intestine, colon, rectum, and anus.

Accessory digestive organs

Pancreas, liver, and gallbladder.

Where does most absorption occur?

Mostly in the small intestine, with additional absorption in the large intestine.

Mouth

Mainly responsible for mechanical digestion.

Salivary glands

Secrete fluid and digestive enzymes.

Stomach

Secretes hydrochloric acid and protein-digesting enzymes.

Pancreas

Secretes sodium bicarbonate and digestive enzymes.

Liver

Produces/secretes bile acids.

Gallbladder

Stores and concentrates bile.

Small intestine

Digests food and absorbs water, nutrients, and electrolytes.

Large intestine

Primarily absorbs electrolytes and water.

Chewing

Mechanically reduces food size and increases surface area for digestion.

Why does chewing increase digestion?

It increases the surface area available for digestive enzymes.

Why is chewing important for plant foods?

It helps destroy indigestible cellulose cell walls and release nutrients.

Three pairs of salivary glands

Parotid, sublingual, and submandibular glands.

Esophagus

Transports food from the mouth to the stomach.

Peristalsis

Rhythmic squeezing contractions that move food through the GI tract.

Esophageal sphincter

A valve that opens to allow food into the stomach and closes to help prevent reflux.

Heartburn

Occurs when stomach acid leaks back into the esophagus.

Stomach length

About 20–25 cm.

Three parts of the stomach

Corpus/body, antrum, and fundus.

Pyloric sphincter

The valve that controls emptying of food from the stomach into the small intestine.

Three main functions of the stomach

Stores food, mixes food with gastric secretions, and regulates emptying into the small intestine.

Chyme

The liquid or paste-like mixture formed when food is mixed with stomach secretions.

Stomach volume

About 1.5 L when empty and up to about 6 L when full.

Gastric glands

Glands in the stomach wall that secrete digestive juices and hydrochloric acid.

Stomach absorption

Very little absorption occurs in the stomach except for some water.

Small intestine

An organ where most digestion and absorption occur.

Length of the small intestine

About 2–3 meters.

Three parts of the small intestine

Duodenum, jejunum, and ileum.

Duodenum

The first section of the small intestine; about 30 cm long.

Jejunum

The middle section of the small intestine.

Ileum

The final section of the small intestine; about the last 1 meter.

Where does about 95% of absorption occur?

In the duodenum and jejunum.

Folds of Kerkring

Folds in the small intestine that increase its surface area for absorption.

Villi

Small fingerlike structures lining the small intestine that greatly increase surface area.

Microvilli

Tiny structures on intestinal cells that form the brush border and further increase surface area.

Brush border

The layer of microvilli covering intestinal cells.

Why are folds, villi, and microvilli important?

They dramatically increase the surface area available for nutrient absorption.

Total increase in absorption surface area

About 600-fold compared with a simple tube with a flat surface.

Approximate surface area of the small intestine

Up to about 250 m², larger than a tennis court.

Transit time

The amount of time food spends in a particular section of the GI tract.

Total GI transit time

Food can spend about 1–3 days in the GI tract before elimination.

Small intestine transit time

About 3–10 hours.

Small intestine motility

Peristalsis and mixing contractions move chyme toward the large intestine.

Gallbladder function

Stores and concentrates bile.

Bile

Helps digest and absorb fat.

When does bile secretion increase?

After a meal, especially after a meal high in fat.

Bile salt recycling

Bile salts are reabsorbed in the distal ileum and returned to the liver to be resecreted.

Pancreas

A large gland located beneath the stomach that releases bicarbonate and digestive enzymes.

Sodium bicarbonate from the pancreas

Buffers the hydrochloric acid coming from the stomach.

Pancreatic digestive enzymes

Break down carbohydrates, proteins, and fats.

What stimulates pancreatic juice secretion?

Chyme entering the upper small intestine.

Large intestine

About 1.5 m long and includes the colon, rectum, and anal canal.

Four sections of the colon

Ascending, transverse, descending, and sigmoid colon.

Main functions of the colon

Absorbs water and electrolytes and stores feces.

Feces

Chyme after it enters the large intestine.

Parasympathetic stimulation of the GI tract

Generally increases GI motility.

Intrinsic GI regulation

Regulation occurring within the GI tract that can override external autonomic regulation.

Hormonal regulation of the GI tract

Hormones released by GI organs regulate digestion, secretion, and motility.

Gastrin

A stomach hormone that increases hydrochloric acid and pepsinogen secretion.

Secretin

A small-intestine hormone that stimulates water and bicarbonate secretion from the pancreas.

Cholecystokinin (CCK)

A small-intestine hormone that stimulates pancreatic enzyme secretion and gallbladder contractions while inhibiting gastric motility and secretion.

GIP

Gastric inhibitory peptide; inhibits gastric motility and secretion.

GLP-I

Glucagonlike peptide I; inhibits gastric motility and secretion.

Guanylin

A hormone from the ileum and colon that removes sodium chloride and water from feces.

Digestion

Digestion begins when food is ingested and may take about 4–6 hours to complete.

Digestive enzymes

Specific enzymes break down different types of macronutrients.

Undigested substances

Substances that cannot be digested are fermented by bacteria or expelled.

Daily digestive secretions

About 6–7 L of digestive secretions enter the GI tract each day.

What happens to most digestive secretions?

Almost all are reabsorbed as chyme moves through the intestines.

Gut microbiota

The microorganisms living in the gastrointestinal tract.

Gut microbiota and energy

Can influence how much energy is harvested from ingested food, especially protein and soluble fiber.

Gut microbiota and vitamins

Can synthesize vitamin K and several B vitamins.

Gut microbiota and fiber

Can digest soluble fiber and ferment it into short-chain fatty acids.

Gut microbiota and immunity

Helps stimulate the immune system and control pathogens.

Gut microbiota and mood

Can send signals from the gut to the brain and influence mood.

Gastric emptying (GE)

The movement of stomach contents into the small intestine.

High meal/drink volume

Generally promotes faster gastric emptying.

High energy density

Slows gastric emptying.

Very hot or cold food/drinks

Can slow gastric emptying.

High exercise intensity

Slows gastric emptying, especially above about 80% VO2max.

Dehydration

Can slow gastric emptying.

Extreme body temperature

Can slow gastric emptying.

Psychological stress and anxiety

Can slow gastric emptying.

Sex and gastric emptying

The slides state that gastric emptying is slower in women than in men.

Thought, smell, and sight of food

Can affect gastric emptying.

GI problems during exercise

GI problems are common, especially among endurance athletes.

Percentage of distance runners with exercise-related intestinal problems

About 30–50%.

Common exercise-related GI complaints

Nausea, heartburn/reflux, abdominal pain, loose stool, diarrhea, and vomiting.