DIGESTION: WHAT
HAPPENS TO THE FOOD
WE EAT
INGRID ADAMS, PHD, RD, LD
MATTHEW SAULNIER,RD,LD
HOW DO YOU MAKE FOOD DECISIONS?
• What did you eat yesterday?
• Why?
• What will you eat for the rest of your life?
• How will you decide?
• Is it important to make wise food decisions?
Why or why not?
• What role do you have to play in this
situation?
The Digestive Organs
DIGESTION: THE ENZYMES
• Digestive enzymes are produced in the salivary
glands, mouth, stomach, small intestine, and
pancreas.
• The pancreas produces digestive enzymes and
secretes them into the duodenum via the
pancreatic duct.
• Most digestive enzymes are inactive until they are
secreted into the GI tract – protects our cells from
self-digestion.
DIGESTION
• Digestion occurs along the
gatrointestinal (GI) tract.
• Digestion requires enzymes and other
chemical and physical factors.
MOUTH
Carbs Fats Protein
Food is chewed
Salivary glands:
Amylase
Lipase
CARBOHYDRATE DIGESTION
• Mouth
• Starch digestion begins in the mouth. Salivary glands
secrete salivary amylase.
• Salivary amylase is an enzyme that breaks down starch into
smaller polysaccharides and disaccharides.
CARBOHYDRATES – WHAT IS
DIGESTED?
• Starch is the most common carbohydrate in the
American diet. It is a large molecule composed of
linked glucose units.
• Digestion breaks starch down into single glucose
molecules.
STOMACH
HCl
Pepsin
Lipase
CARBOHYDRATE DIGESTION
• Stomach
• Starch digestion stops in the stomach where it is to acidic for
salivary amylase to work.
• Small Intestine
• The majority of carbohydrate digestion occurs in the small
intestine.
• The pancreas produces pancreatic amylase, which is secreted
into the small intestine once food enters from the stomach and
breaks down food into very small chains of glucose
• The cells that line the small intestine break down carbohydrates
into simple sugars, which are absorbed directly into the blood
stream.
SMALL INTESTINE
Duodenum
Liver – produce bile which is stored
in the gall bladder
Pancreas
Amylase
Lipase
Trypsin
Chymotrypsin
Dietary fibers, other large
molecules (undigested proteins,
triglycerides, starches, etc…)
LARGE INTESTINE
CARBOHYDRATE DIGESTION
• Absorption
• Glucose units are absorbed into the intestinal wall and pass
into the bloodstream.
• Dietary fibers: NOT DIGESTED OR ABSORBED in the small
intestine
PROTEIN DIGESTION
• Mouth
• No digestion of protein occurs here.
• Stomach
• Stomach acid, hydrochloric acid, beings to break down
protein molecules.
• Hydrochloric acid activates an enzyme called pepsin,
which breaks down protein into small polypeptides.
PROTEIN DIGESTION
• Small Intestine
• Once these polypeptides enter the small intestine, enzymes
form the pancreas and intestine break down these chains
into amino acids.
• Intestinal cells absorbed amino acids just like
carbohydrates.
• Absorption
• Amino acids enter the intestinal cells and pass into the
bloodstream.
FAT DIGESTION
• Digestion of fat is slightly different from
carbohydrates and protein.
• Mouth
• Lingual lipase mixes with food. Lingual lipase plays a more
prominent role in children.
• Stomach
• Lingual lipase acts in the stomach where it prefers a more
acidic environment.
FAT DIGESTION
• Small Intestine
• Triglycerides mix with the bile from the gallbladder and form
micelles. The water soluble lipases from the pancreas and
intestine act on the triglycerides in the suspended micelles.
• The main products of triglyceride digestion are fatty acids
and monoglycerides.
• Absorption
• Fatty acids, glycerol, and monoglycerides pass into the
intestinal cells and are reassembled into triglycerides. They
are then packaged into chylomicrons and enter the
lacteals of the lymphatic system.
WHAT IS NOT ABSORBED IN THE SMALL
INTESTINE?
• Dietary fibers, other large molecules (undigested
proteins, triglycerides, starches, etc…)
• Many food additives are digested to be
unobservable, for safety and to reduce caloric
content of the diet.
AFTER ABSORPTION, THEN WHAT?
• Fats, cholesterol, and fat soluble vitamins are
repackaged in the intestinal cell, sent through the
lacteal (lymphatic system), enter the general
circulation through the thoracic duct.
• Most other nutrients are sent through capillaries
from the small intestine to the portal vein and go
directly to the liver.
• Liver processes sugars to glucose; amino acids to
whatever amino acids are needed.
PROBLEMS WITH DIGESTION
HIATAL HERNIA
• Portion of stomach
pressed into chest
cavity through
esophageal hiatus
(passageway for
esophagus) of
diaphragm
• Symptoms: GERD,
dysphagia
(swallowing
difficulty), heartburn,
and epigastric pain.
• Rx: weight control
and methods used to
control GERD
DISEASES OF THE STOMACH AND SMALL
INTESTINE—PEPTIC ULCER DISEASE (PUD)
PUD: Ulceration of the gastric or duodenal
mucosa penetrating the submucosa with
potential for perforation
Antrum
Duodenum
LACTOSE INTOLERANCE
• Insufficient lactase production in brush border
• Poor digestion of lactose
PRIMARY LACTASE NON-PERSISTENCE
PREVALENCE IN US
Northern European American
Mexican American
African American
Asian American
Native American
Vietnamese American
5%
62%
80%
90%
95%
100%
LACTOSE INTOLERANCE—SYMPTOMS
• Symptoms usually begin 30min to 2hrs after consuming
lactose
• Crampy pain
• Bloating, flatulence
• Fermentation of lactose by colonic bacteria (CO2, H2, methane)
• diarrhea
• Lactose draws water into gut by osmosis   peristalsis  fluid,
electrolyte, nutrient losses
• Nausea
LACTOSE INTOLERANCE—SYMPTOMS
• Severity of symptoms depend on
• Degree of lactase insufficiency
• Lactose load
• Many lactase insufficient individuals can
tolerate some lactose
• Some enzyme activity often present to varying
degrees
• Fermentation of undigested lactose by colonic
microorganisms
• Up to 1 cup of milk (12g of lactose) WITH MEAL
can be well tolerated without significant
symptoms by those who are lactose
maldigesters
SOURCES OF LACTOSE
• Lactose naturally present only in milk and
products made from milk
• 7% human milk
• 5% cows milk
• 4.5 % goat milk
• Usually less lactose in milk products than in milk
• Yogurts—less lactose than milk; may contain active
bacterial cultures capable of metabolizing lactose
• Ice cream—fair amount compared with milk
• Cottage cheese—contains little lactose; can usually
be eaten with no adverse effects
• Hard cheeses—contain less than cottage cheese
LACTOSE INTOLERANCE
• Remove lactose to the level of tolerance
• Consume lactose containing foods with meals
• dilutes lactose and slows its movement
• Elimination of lactose worsens tolerance  less
favorable mix of colonic microflora
• Fermented milk products (yogurt with active
cultures)
• Non-fermented milk products (acidophilus milk
or yogurt milk) may or may not be tolerated
• Chocolate and whole milk better tolerated than
skim
• Aged cheese well tolerated
• Cottage cheese and ice cream—lower lactose
content than milk
LACTOSE INTOLERANCE—MNT
• Lactose free milk
• Enzyme replacement—lactase tablets
• Dairyease®, Lac-Dos®, Lactaid®, Lactrase®,
RiteAid® Dairy Relief™, Surelac®, Walgreens® Dairy
Digestive™
• Check medication labels
• powder filler used in medication may contain
lactose
• Teach patients to read foods labels for hidden
sources
• Ca, vitamin D, vitamin B12, and riboflavin
supplementation depending on milk intake
CELIAC DISEASE
Also known as:
Gluten sensitive enteropathy
Celiac sprue
Nontropical sprue
Prevalence: 1 in 133
Higher prevalence in females
Underdiagnosed—symptoms resemble those of
other diseases
• Caused by an allergic reaction to protamine in
certain grains
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CELIAC DISEASE (CD)



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Also known as:
 Gluten sensitive enteropathy
 Celiac sprue
 Nontropical sprue
Prevalence: 1 in 133
Higher prevalence in females
Underdiagnosed—symptoms resemble those of other
diseases
Caused by an allergic reaction to protamine in certain
grains
OFFENDING GRAINS
Grain
Prolamin
Wheat
gliadin
Rye
secalin
Barley
hordein
Oats?
avenin
Certain peptides from these proteins induce
the celiac response in sensitive individuals.
OFFENDING GRAINS
• Triticale – hybrid of rye and wheat
• Spelt – wheat
• Kamut – wheat
• Einkorn – wheat
• Emmer – wheat
• Wheat (bulgur, wheat bran or germ, wheat
germ oil, wheat starch or food starch,
couscous, pastas made from wheat, matza,
seitan, semolina)
• Barley (malt)
“GLUTEN”
• Protein found only in wheat
• The main protein in wheat
• Composed of glutenin and gliadin
• The term “gluten” has come to mean any
cereal peptides inducing a celiac response
• What is the celiac response?
• Both cellular and humoral
• Lymphocyte infiltration of the intestinal epithelium
• Flattening of the villi
VILLUS ATROPHY
The Internet Pathology Laboratory, University of Utah
CLASSIC SYMPTOMS
• Weight loss, wasting
• Weakness, malaise
• Failure to thrive (FTF), short stature
• Dental enamel defects
• Iron deficiency anemia (Fe malabsorption;
occult blood loss)
• Steatorrhea
• Diarrhea due to malabsorption
• Constipation in some cases
• Irritability in children
• Abdominal discomfort, bloating, flatus,
belching, GERD
• Physical signs of nutrient deficiency
More on symptoms
• Many celiacs do not exhibit the classic
symptoms
• Some have no clinical GI symptoms
• Because of many and varied symptoms:
– Average time to diagnosis CD: 11 years!
Dermatitis herpetiformis (DH)
medline plus
Graphics: Microsoft Online, USDA Food and Nutrition Services
Ingrid Adams, Ph.D., R.D. L.D.
Associate Professor , Nutrition and Food Science, University of
Kentucky
Extension Specialist for Physical Activity and Weight
Management
Date: February 27, 2015
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