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Digestion and Absorption of
Food Nutrients
•Proteins break down to amino
•Carbohydrates break down to
simple sugars
•Fats break down to fatty acids and
• Digestion and absorption do not take place
one step at a time as isolated functions
– 92-97% of food is digested
– Most substances must be broken down to
simple form
• When contents pass too slowly (and too
much water is removed) = constipation
Absorption in Small Intestines
Villi-expand absorptive
Most nutrients absorbed
Nutrients move from the
villi to the blood to be
transported all over the
Absorption from Large Intestines
• Site of water, salt, vitamin
absorption (leaving a mass in a
semi-solid state)
• Almost everything of nutritional
value has been utilized by time it
reaches the large intestines -leaving only waste
• Feces contains some water, dead
intestinal cells, bacteria, nonabsorbed remains of digestion
Absorption from Large Intestines
• Helpful bacteria in the large
intestine feed on the “leftovers”
and make vitamins like vitamin K
and B
Nutrition and GI Disorders
• Acid Reflux
– Reduced by smaller meals, less fat
• Ulcers
– Caused by bacteria
• Constipation
– When too much water is removed in the large
Nutrition and GI Disorders
• Constipation
– Hard, dry, infrequent stools
– Reduced by high fiber, fluid
intake, exercise
• Diarrhea
– Loose, watery, frequent
– Symptom of
– Can cause dehydration
• Diverticulosis
– Pouches along colon
– High fiber diet reduces
Gastric emptying research tells us:
Maximal rates
of 1-1.2 L /hr
with gastric
volumes of
100 - 200 mL.
Effects of Solution Characteristics on
the Rate of Gastric Emptying
Solution characteristic
Rate of emptying
Volume of the solution
Increases with larger volumes
Caloric content
Decreases as the caloric
Decreases with hyperosmolar
Faster for cooler fluids than
warm solutions
Decreases with more acidic
Summary: Digestion (general)
• Digestion breaks down complex molecules into
simpler substances for absorption. Selfregulating processes control the liquidity, mixing
and transit time of the digestive mixture
• Physically altering food in the mouth makes it
easier to swallow, and increases access to
enzymes and other digestive substances.
Swallowing transfers the food to the esophagus
where peristalsis propels it into the stomach
Summary (cont.)
• In the stomach, food contents mix as hydrochloric acid
and enzymes continue the breakdown process. Little
absorption takes place in the stomach except for some
water and alcohol.
• About 90% of digestion occurs along the first half of
the 3-m long small intestines. Highly specific intestinal
enzymes act on more complex CHO, lipid and protein
structures, splitting them into simpler subunits. Bile,
produced in the liver and secreted by the gallbladder
emulsifies lipid droplets, thus facilitating digestive
enzyme action.
Summary (cont.)
• The villi of the small intestine
provide a tremendous surface area
to absorb digested nutrients.
• The large intestine serves and the
final path for absorption of water
and electrolytes, and storage of
undigested food residue (feces).