Saladin Outline Ch.26 Page 27
Saladin 5e Extended Outline
Chapter 26
Nutrition and Metabolism
I. Nutrition (pp. 1014–1025)
A. Body weight is determined by the body’s energy balance. (p. 1014)
1. If intake exceeds output, weight is gained; if output exceeds intake, weight is lost.
2. Weight usually remains stable over many years’ time and seems to have a homeostatic set point.
3. In humans, the set point varies greatly, and body weight results from a combination of hereditary and environmental influences.
4. 30% to 50% of the variation in human weight appears due to heredity, and the rest to environmental factors such as eating and exercise.
B. Appetite is controlled in the body by peptide hormones and regulator pathways that affect short-term and long-term appetite and body weight. (pp. 1014–1017)
1. Some hormones are termed gut–brain peptides because they act as chemical signals from the GI tract to the brain.
2. Three peptides work as short-term regulators of appetite over periods of minutes to hours.
a. Ghrelin is secreted by parietal cells in the fundus of the stomach, especially when it is empty.
i. It produces the sensation of hunger and stimulates the hypothalamus to secrete growth hormone–releasing hormone.
ii. Within an hour after eating, ghrelin secretion ceases.
b. Peptide YY (PYY) is secreted by enteroendocrine cells in the ileum and colon, which sense that food has arrived as it enters the stomach.
i. PPY is secreted long before the chyme reaches the ileum, and in quantities proportion to calories consumed.
ii. It is a member of a family of hormones related to neuropeptide Y (NPY).
iii. The primary effect of PPY is to signal satiety and terminate eating.
iv. It remains elevated well after a meal and acts as a ileal brake to prevent the stomach from emptying too quickly.
c. Cholecystokinin (CCK) is secreted by enteroendocrine cells in the duodenum and jejunum.
i. It stimulates bile and pancreatic enzymes.
ii. It also stimulates the brain and sensory fibers of the vagus nerves, producing an appetite-suppressing effect.
3. Other peptides are involved in long-term regulation of appetite, metabolic rate, and body weight over week to years; two examples, leptin and insulin, work as “adiposity signals.”
a. Leptin is secreted by adipocytes throughout the body, and its level is proportional to one’s fat stores.
i. This is the brain’s primary way of knowing how much body fat we have.
ii. Animals with a leptin deficiency or defect in leptin receptors exhibit hyperphagia and extreme obesity.
iii. With few exceptions, obese humans are not leptin deficient and are not aided by leptin injections.
iv. Obesity is linked to unresponsiveness to leptin—a receptor defect.
v. Adipose tissue is increasingly seen as an important source of multiple hormones influencing energy balance.
b. Insulin is secreted by the pancreatic beta cells and stimulates glucose and amino acid uptake and glycogen synthesis—but it also has receptors in the brain.
i. On these receptors it functions like leptin as an index of the body’s fat stores.
ii. It has a weaker effect than leptin.
4. The arcuate nucleus of the hypothalamus is important in appetite regulation.
a. All five peptides described above had receptors in the arcuate nucleus in addition to other target cells in the body.
b. The arcuate nucleus has two neural networks involved in hunger.
i. One network secretes neuropeptide Y (NPY), a powerful appetite stimulant.
ii. The other secretes melanocortin, which inhibits eating.
iii. Ghrelin stimulates NPY secretion, whereas insulin, PPY, and leptin inhibit it.
iv. Leptin also stimulates melanocortin secretion. (Fig. 26.1)
v. Leptin inhibits secretion of appetite stimulants called endocannabinoids, which resemble tetrahydrocannabinol (THC) of marijuana.
Insight 26.1 Obesity
5. In addition to the action of gut–brain peptides, hunger is also stimulated partly by gastric peristalsis.
a. Mild hunger contractions begin soon after the stomach is emptied and increase over a few hours.
b. Food intake is terminated not only by PPY and CCK, but also in ways similar to the way that water intake slakes thirst.
i. Merely chewing and swallowing food briefly satisfies the appetite even if the food does not reach the stomach.
ii. Inflating a balloon in the stomach also inhibits hunger in the same way a meal would.
iii. Satiation is short lived in these situations, and lasting satiation depends on the absorption of nutrients.
6. Appetite involves not only how much but also what kind of food is consumed.
a. In humans, different neurotransmitters seem to govern the appetite for different classes of nutrients.
b. Norepinephrine stimulates appetite for carbohydrates, galanin for fatty food, and endorphins for protein.
C. A calorie is defined as the amount of heat that will raise the temperature of 1 g of water 1°C; one thousand calories is a Calorie in dietetics and a kilocalorie (kcal) in biochemistry. (p. 1017)
1. In physiology, calories are a measure of the capacity to do biological work.
2. Nearly all dietary calories come from carbohydrates, proteins, and fats.
a. Carbohydrates and proteins yield about 4 kcal/g when completely oxidized; fats yield about 9 kcal/g.
b. Alcohol (7.1 kcal/g) and sugary foods can promote malnutrition by providing “empty calories”—energy without nutritional diversity.
c. In sound nutrition, energy needs are met by complex foods that provide proteins, lipids, vitamins, and other nutrients.
d. “Fuel” in nutrition discussions refers to a chemical oxidized solely or primarily to extract energy, usually to make ATP for other physiological processes.
D. A nutrient is any ingested chemical that is used for growth, repair, or maintenance of the body. (p. 1017)
1. Nutrients fall into six major classes: water, carbohydrates, lipids, proteins, minerals, and vitamins. (Table 26.1)
a. Macronutrients include water, carbohydrates, lipids, and proteins because relatively large quantities are needed.
b. Micronutrients include minerals and vitamins because only small quantities are required.
2. Recommended daily allowances (RDAs) of nutrients were first developed in 1943 by the National Research Council and National Academy of Sciences and have been revised several times since.
a. An RDA is a liberal but safe estimate of the daily intake that would meet the nutritional needs of most healthy people.
b. Consuming less than the RDA does not necessarily mean malnutrition, but the probability of malnutrition increases according to the amount of deficit and length of time.
3. Essential nutrients are those the body cannot synthesize and include minerals, most vitamins, eight of the amino acids, and one to three of the fatty acids.
E. About 440 g of carbohydrates are present in the body of a well-nourished adult, most of which (325 g) is in the muscle as glycogen. (pp. 1018–1019)
1. Sugars function as a structural component of other molecules including nucleic acids, glycoproteins, glycolipids, ATP, and related nucleotides.
2. They can also be converted to amino acids and fats.
3. Most of the carbohydrate serves as fuel—an easily oxidized source of chemical energy.
a. Some cells, such as neurons and erythrocytes, depend almost exclusively on carbohydrates as an energy source; hypoglycemia (low blood glucose) can cause nervous system disturbances felt as weakness or dizziness.
b. Blood glucose concentration is carefully regulated mainly through the interplay of insulin and glucagon.
i. These hormones regulate the balance between glycogen and free blood glucose.
ii. It is important to consume enough carbohydrate to ensure adequate stores of glycogen for periods of exercise and fasting (including sleep).
4. Carbohydrate intake influences the metabolism of other nutrients.
a. Fat is oxidized as fuel when glucose and glycogen levels are too low to meet energy needs; conversely, excess carbohydrate is converted to fat.
b. Excessive reduction of carbohydrate intake in an effort to lose fat may lead to incomplete fat oxidation, formation of ketone bodies, and metabolic acidosis.
5. Carbohydrates are required in greater amounts than any other nutrient, with an RDA of 125 to 175 g.
a. The brain alone consumes about 120 g of glucose per day.
b. Most Americans get 40% to 50% of their calories from carbohydrates, but highly active people should get up to 60%.
c. Carbohydrate consumption in the United State has become excessive over the last century because of fondness for sweets, increased use of sugar in processed foods, and reduced physical activity.
i. In the last century, average consumption of sugar has increased almost 27-fold, from 4 lb per year to 106 lb per year.
d. The three principle forms of dietary carbohydrates are monosaccharides, disaccharides, and polysaccharides (complex carbohydrates).
i. The only nutritionally significant polysaccharide is starch.
ii. Only trivial amounts of glycogen are present in cooked meats.
iii. Cellulose is not considered a nutrient because it is indigestible.
e. The three major dietary disaccharides are sucrose, lactose, and maltose.
f. The monosaccharides—glucose, galactose, and fructose—arise mainly from digestion of starch and disaccharides.
i. Ultimately all carbohydrate digestion generates glucose.
ii. Outside the hepatic portal system, glucose is the only monosaccharide present in the blood in significant quantity, and so it is known as blood sugar.
iii. Normal blood sugar is normally maintained at 70–110 mg/dL in venous blood.
g. Ideally, most carbohydrate intake should be in the form of complex carbohydrates, but a typical American now obtains only 50% of his or her carbohydrates from starch and the other 50% from sucrose and corn syrup.
Insight 26.2 Evolution of the Sweet Tooth
6. Nearly all dietary carbohydrates come from plants.
a. Sucrose is refined from sugarcane and sugar beets.
b. Fructose is present in fruits and corn syrup.
c. Maltose is present in some foods such as germinating cereal grains.
d. Lactose is the most abundant solute in cow’s milk (about 4.6% lactose by weight).
7. Dietary fiber refers to all fibrous materials of plant and animal origin that resist digestion.
a. Fiber is an essential component of the diet, with an RDA of about 30 g.
b. Water-soluble fiber includes pectin and certain other carbohydrates found in oats, beans, peas, carrots, brown rice, and fruits.
i. It reduces blood cholesterol and low-density lipoprotein (LDL).
c. Water-insoluble fiber includes cellulose, hemicellulose, and lignin.
i. It apparently has no effect on cholesterol or LDL levels, but softens the stool and increases its bulk by 40% to 100%, reducing the risk of constipation and diverticulitis.
d. Dietary fiber has no clear effect on the incidence of colorectal cancer, and excessive fiber can interfere with the absorption of iron, calcium, magnesium, phosphorus, and some trace elements.
F. Lipids make up 15% and 25% of the reference male and female body, respectively, by weight. (pp. 1019–1021)
1. Fat accounts for most of the body’s stored energy, and small amounts of phospholipid, cholesterol, and other lipids also play vital structural and physiological roles.
a. A well-nourished adult meets 80% to 90% of his or her resting energy needs from fat.
b. Fat is superior to carbohydrate for energy storage for two reasons.
i. Carbohydrates are hydrophilic, absorb water, and expand to occupy space, while fats are hydrophobic and is more compact.
ii. Fat is less oxidized than carbohydrate to begin with and contains over twice as much energy (9 kcal/g compared with 4 kcal/g).
c. Fat has glucose-sparing and protein-sparing effects—as long as enough fat is available, protein is not catabolized for fuel and glucose is spared for neurons.
d. Vitamins A, D, E, and K are fat soluble, so they depend on dietary fat for their absorption by the intestine; vitamin deficiency may result if less than 20 g of fat per day is eaten.
e. Phospholipids and cholesterol are major structural components of membranes, and cholesterol is an important precursor of hormones, bile acids, and vitamin D.
f. Adipose tissue has important protective and insulating functions.
2. Fat should account for no more than 30% of the caloric intake, and no more than 10% should be saturated fat.
a. Cholesterol intake should not exceed 300 mg/day.
b. One egg yolk has 240 mg/day.
c. A typical American consumes 40% to 50% of calories from fat and twice as much cholesterol as recommended.
d. Essential fatty acids are those that cannot be synthesized by the body and include linoleic acid and possibly linolenic and arachidonic acids.
3. In terms of sources, saturated fats are predominantly of animal origin but are also present in some plant products such a coconut and palm oils.
a. Processed foods such as hydrogenated oils and vegetable shortening are also high in saturated fats.
b. Unsaturated fats are present in nuts, seeds, and most vegetable oils.
c. Excessive consumption of saturated and unsaturated fats is a risk factor for diabetes mellitus, cardiovascular disease, and breast and colon cancer.
d. The richest source of cholesterol is egg yolks, but it is also prevalent in milk products, shellfish, organ meats, and other mammalian meat.
4. Serum lipoproteins transport hydrophobic cholesterol and triglycerides through the aqueous blood plasma.
a. A lipoprotein is a tiny droplet complex consisting of a core of cholesterol and triglycerides with a protein and phospholipid coating.
i. The coating enables the lipids to remain suspended in the blood and also serves as a recognition marker for cells that absorb them.
ii. The complexes are often referred to as serum lipoproteins because their concentrations are expressed in terms of a volume of blood serum, not whole blood.
b. Lipoproteins are classified into four major categories by their density: chylomicrons, very-low-density lipoproteins (VLDLs), low-density lipoproteins (LDLs), and high-density lipoproteins (HDLs); they also vary with size—smaller lipoproteins are generally more dense because of the increased proportion of protein to lipid. (Fig. 26.2a)
c. Chylomicrons (75–1,200 nm in diameter) form in the absorptive cells of the small intestine and pass into the lymphatic system.
i. The endothelial cells of the blood capillaries have a surface enzyme, lipoprotein lipase, that hydrolyzes chylomicron triglycerides so they can pass through the capillary walls into adipocytes.
ii. Some free fatty acids (FFAs) remain in the blood plasma bound to albumin.
iii. The remainder of a chylomicron after the triglycerides are removed, called a chylomicron remnant, is degraded by the liver.