The science of nutrition 4th edition thompson manore vaughan chapter 03

Chapter 3 Lecture

Chapter 3:
The Human Body:
Are We Really
What We Eat?

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Why Do We Want to Eat?
• Hunger: physiological drive for food
• Nonspecific
• Can be satisfied by a variety of different foods
• Appetite: psychological desire to consume
specific foods
• Aroused by environmental cues
• Anorexia: physiological need for food, yet no
appetite
• Can occur with illnesses or side effect of
medication
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Why Do We Want to Eat? (cont.)
• Role of nerve cells
• Hypothalamus triggers feelings of hunger or
satiation (fullness)
• Integrates signals from nerve cells in other body
regions and from chemical messengers
• Special cells lining the stomach and small intestine

send signals to the brain to indicate if they are full
or empty

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Why Do We Want to Eat? (cont.)
• Role of hormones
• Hormones: chemical messengers secreted
into the bloodstream by endocrine glands to
help regulate body functions
• Pancreatic hormones, insulin and glucagon,
maintain blood glucose levels
• Ghrelin and cholesytokinin control hunger and
satiety
• Leptin acts on the hypothalamus to suppress
hunger

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Why Do We Want to Eat? (cont.)
• Foods have differing effects on our feelings of
hunger
• Proteins have the highest satiety value
• High-fat diets have a higher satiety value than
high-carbohydrate diets

• Bulky meals (high in fiber and water) distend
the stomach and promote a sense of satiety
• Solid foods are more filling than semisolid
foods or liquids

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Why Do We Want to Eat? (cont.)
• Foods stimulate our senses:
• Sight
• Smell
• Taste
• Texture (mouth feel)
• Hearing
• Social and cultural cues
• Learned experiences

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What Happens to the Food We Eat?
• Food undergoes three processes:
• Digestion: large food molecules are broken
down to smaller molecules, mechanically and
chemically
• Absorption: process of taking these products

through the intestinal wall
• Elimination: undigested portions of food and
waste products are removed from the body

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What Happens to the Food We Eat? (cont.)
• Gastrointestinal (GI) tract:
• A series of organs arranged in a long tube that
work together to process foods
• The GI tract includes:
• Organs such as the stomach, intestines
• Sphincters: muscles that control the passage
of food material from one organ to the next

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Digestion Begins in the Mouth
• Cephalic phase of digestion:
• Hunger and appetite work together to prepare
the GI tract for digestion
• First thought of food (nervous system)
stimulates the release of digestive juices
• Chewing moistens the food and mechanically
breaks it into smaller pieces

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Digestion Begins in the Mouth (cont.)
• Saliva contains digestive juices secreted by the
salivary gland in the mouth
• Taste receptors detect distinct tastes:
• Bitter, sweet, salty, sour, and umami
• Olfactory receptors detect aromas of foods

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Digestion Begins in the Mouth (cont.)
• Enzymes are complex chemicals that induce
chemical changes in other substances to speed
up bodily processes
• Salivary amylase begins starch digestion
• Bicarbonates neutralize acids
• Mucus moistens the food and oral cavity
• Antibodies and lysozymes fight oral bacteria

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Digestion Begins in the Mouth (cont.)

• The mass of food chewed and moistened in the
mouth is called bolus
• The epiglottis covers the opening to the trachea
during swallowing
• The esophagus propels food from the mouth to
the stomach
• Peristalsis is the muscular contractions that
move food through the GI tract

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© 2017 Pearson Education, Inc.


The pH Scale
• pH = potential of hydrogen
• Measures the potential of a substance to release
or take up hydrogen ions in solution
• Put another way, pH is a measure of a
compound's acidity or alkalinity
• Pure water is neutral/human blood is close to
neutral
• Tissues lining the stomach are generally
protected from the effects of acidity

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Digestion in the Stomach
• Gastrin: a hormone secreted by stomach lining
cells that stimulates gastric juice
• Gastric juice contains:
• Hydrochloric acid (HCl) denatures proteins
and converts inactive enzyme pepsinogen to
pepsin
• Pepsin: enzyme to digest protein
• Gastric lipase: enzyme to digest fat
• Intrinsic factor: protein to absorb vitamin B12

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Digestion in the Stomach (cont.)
• Chyme: liquid product of mechanical and
chemical digestion in the stomach
• Mucus layer protects the stomach lining from
the acid in gastric juices
• Bicarbonate neutralizes the acid near the
surface of the stomach's lining

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Digestion in the Small Intestine
• Small intestine is composed of three sections
• Duodenum, jejunum, and ileum
• Ileocecal valve (sphincter) connects the small
intestine to the large intestine
• Most digestion and absorption take place in the
small intestine

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© 2017 Pearson Education, Inc.