Download Digestion

Judith E. Brown
www.cengage.com/nutrition/brown
How the Body Uses Food:
Digestion and Absorption
Unit 7
Prof. Albia Dugger • Miami-Dade College
My Body, My Food
• Each day, about 5% of our body weight is
replaced by new tissue
• Cell maintenance is one of many body
functions that requires nutrients
How Do Nutrients in Food Become
Available for the Body’s Use?
• Digestion
• Processes that convert ingested food into
substances that can be absorbed by the intestinal
tract and used by the body
• Absorption
• Processes that transfer nutrients from the
digestive system into body fluids
The Digestive System
Overview
• In the digestive system, organs such as the
liver and pancreas secrete digestive juices
that break foods down into very small
particles that can be absorbed and used by
the body
Absorbable Forms of Nutrients
• Carbohydrates are broken down into
monosaccharides
• Proteins are broken down into amino acids
• Fats are broken into fatty acids and glycerol
• Vitamins and minerals are absorbed without
being broken down
Chemical Processes
• Salivary glands, stomach, and pancreas
produce over a hundred enzymes involved in
digestion of carbohydrates, proteins, and fat
• Carbohydrates, proteins, and fat each have
their own set of digestive enzymes
Key Terms
• Enzymes
• Proteins that speed up chemical reactions but are
not used up in the reactions
• Present in large amounts in the digestive system
• Bile
• Fluid produced by the liver, stored in the
gallbladder, and released into the small intestine
• Makes fats more accessible to lipase by breaking
it down into small droplets
Carbohydrates
• Most carbohydrates in our diet consist of
starch, monosaccharides, and disaccharides
• Starch
• Complex carbohydrates made up of complex
chains of glucose molecules
• Primary form of carbohydrate storage in plants
Carbohydrates
• Monosaccharides
• Simple sugars consisting of one sugar molecule
• Glucose, fructose, and galactose
• Disaccharides
•
•
•
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Simple sugars consisting of two sugar molecules
Sucrose (glucose + fructose)
Lactose (glucose + galactose)
Maltose (glucose + glucose)
Functions of Digestive Enzymes
A Closer Look
• In the mouth:
• Saliva contains salivary amylase and lipase that
begin to break down carbohydrates and lipids
• In the stomach:
• Food is liquefied, mixed with digestive juices, and
gradually ejected into the small intestine
A Closer Look
• In the small intestine:
• Liquefied food is mixed with digestive enzymes
• Carbohydrates, proteins, and fats are reduced to
substances that can be absorbed
• Vitamins and minerals are released
• Nutrients are absorbed through the large surface
area (villi) of the small intestine
A Closer Look
• In the large intestine:
• Bacteria break down undigested fiber and other
complex carbohydrates
• Breakdown products, water, and sodium are
absorbed
• Remains are collected and excreted
Surfaces of Small and Large Intestines
Summary of Digestion
On the Side
• Just thinking about food can start your
salivary glands producing digestive juices
Absorption
• Absorption is the process by which the
products of digestion are taken up by the
lymphatic system and circulatory system for
distribution to body cells
• Nutrients enter lymph vessels and blood
vessels in villi lining the intestines
Key Terms
• Lymphatic system
• Network of vessels that absorb some products of
fat digestion and transport them to the
bloodstream
• Circulatory system
• Heart, arteries, capillaries, and veins that circulate
blood and nutrients through the body
• Blood vessels absorb breakdown products of
carbohydrates and proteins
Absorption in Villi
Beyond Absorption
• Cells use nutrients directly
• For energy, body structures, regulation of body
processes
• Cells convert nutrients into other substances
• Glycogen, hormones, other substances
Digestion and Absorption are Efficient
• Most of the nutrients we consume in food are
digested and absorbed
• 99% of carbohydrates
• 92% of proteins
• 95% of fat
• Most dietary fiber is not digested
Digestive Disorders
• Digestive disorders are
one of the leading
reasons for medical
visits and
hospitalization
Key Terms
• Heartburn
• Condition that results when acidic stomach
contents are released into the esophagus, usually
causing a burning sensation
• Hemorrhoids
• Swelling of veins in the anus or rectum
Key Terms
• Irritable bowel syndrome (IBS)
• Disorder of bowel function characterized by gas,
abdominal pain, diarrhea and/or constipation
• Duodenal and stomach ulcers
• Open sores in the lining of the upper part of small
intestine (duodenum) or stomach
Constipation
• Characterized by difficulty passing hard, dry
stools; may cause bleeding hemorrhoids
• Caused by intestinal tract disorders,
immobility, medication, habitual laxative use,
inadequate dietary fiber consumption
Good Sources of Fiber
• 25-30 g fiber/day helps prevent constipation
Myths About Constipation
Ulcers and Heartburn
• Ulcers develop when the protective lining of
the stomach or duodenum is damaged, and
the lining is eroded by stomach acid and
digestive enzymes
• Duodenal ulcers are associated with bacterial
(H. pylori) infection
Key Terms
• Probiotics
• Nonharmful bacteria and some yeasts that help
colonize the intestinal tract with beneficial
microorganisms, and that sometimes replace
colonies of harmful microorganisms
• Most common strains: Lactobacilli and
Bifidobacteria
Heartburn
• Stomach acid enters the esophagus when the
valve at the top of the stomach opens too
often or does not close tightly
• Occasional heartburn is not dangerous
• Chronic heartburn can develop into GERD and
cause serious damage
Factors Related to Heartburn
• Many factors can
trigger heartburn
• Medications are
available to reduce
symptoms
Irritable Bowel Syndrome (IBS)
• Causes of IBS are uncertain
• Associated with anxiety, depression
• Affects quality of life
• No single therapy is entirely effective
• Treatments address symptoms of abdominal
pain, bloating, diarrhea and constipation
Diarrhea
• Diarrhea
• The presence of three or more liquid stools in a
24-hour period
• May be due to bacterial- or viral-contaminated
food or water, infectious diseases, or nutrient
deficiencies
Diarrhea
• Depletes body fluids and nutrients, and can
produce malnutrition
• Can lead to dehydration, kidney malfunction, and
death, especially in children
• Treated with rehydration fluids
• Avoid sugary drinks
• Eat solid foods to provide nutrients
Normal Functions
• Flatulence
• Gas produced by bacteria or from swallowing air
• Associated with certain foods, heartburn,
digestive disorders, antibiotics
• Stomach growling
• Occurs when gas and fluids are mixing in your
stomach – especially when stomach is empty
Problems with Lactose
• Lactose maldigestion
• Reduced digestion of lactose due to low
availability of the enzyme lactase
• Genetically determined
• Lactose intolerance
• GI symptoms resulting from consumption of more
lactose than can be digested by lactase
• Flatulence, abdominal pain, diarrhea
Problems with Lactose
• Most adults have
problems digesting
lactose
Lactose Maldigestion
Managing Lactose Maldigestion
• If symptoms occur after consuming lactose,
see your health care provider
• Consume dairy products that are fermented
or pretreated with lactase drops
• Consume dairy products in small amounts
regularly to encourage digestive bacteria
Reduced Lactose Products
• Cultured milk products
in which most of the
lactose is digested are
generally well-tolerated