a. It appears that 30% of the variation in weight among people is due to variation in environmental influences, and 70% is due to hereditary variation.

a. The satiety center has neurons called glucostats that rapidly absorb blood glucose after a meal. One hypothesis on hunger is that glucose uptake causes the satiety center to send inhibitory signals to the hunger center and thus suppresses the appetite.

a. Some cells, such as neurons and erythrocytes, must have energy in the form of glucose, thus blood glucose levels are carefully maintained by insulin and glucagon.

a. Because carbohydrates are rapidly oxidized, they are required in greater amounts than any other nutrient. The RDA is 125 to 175 g. The brain alone consumes about 120 g of glucose per day.

b. Carbohydrate consumption in the United States has become quite excessive over the past century due to a combination of fondness for sweets, increased use of sugar in processed foods, and reduced levels of physical activity.

c. Dietary carbohydrates come in three principal forms: monosaccharides, disaccharides, and polysaccharides. The only nutritionally significant polysaccharide is starch.

d. The three disaccharides are sucrose, lactose, and maltose. The monosaccharides - glucose, galactose, and fructose - arise mainly from the digestion of starch and disaccharides.

e. Outside of the hepatic portal system, glucose is the only monosaccharides present in the blood in any quantity, and is thus known as blood sugar.

a. Nearly all dietary carbohydrates come from plants - grains, legumes, fruits, and root vegetables.

a. Dietary fiber refers to all fibrous materials of plant and animal origin that resist digestion. Most fiber is from cellulose, pectin, gums, and lignin, all derived from plant matter. Fiber is an essential component of the diet.

b. Fiber in the intestines absorbs water and increases its bulk by 40% to 100%. It binds and carries away toxins and other undesirable chemicals, thus reducing the risk of colon cancer.

c. Fiber is categorized as water- soluble or insoluble.

d. Water-soluble fiber found in oats, beans, peas, carrots, brown rice, and fruits reduces blood cholesterol and low-density lipoprotein levels.

e. Cellulose, hemicellulose, and lignin are water-insoluble fibers and aid digestion.

a. Fat has glucose-sparing and protein-sparing effects as long as there is enough fat available to meet the energy needs of tissues.

b. Vitamins A, D, E, and K are fat-soluble vitamins, which depend on dietary fat for their absorption by the intestine.

c. Phospholipids and cholesterol are major structural components of plasma membranes and myelin. Cholesterol is also important as a precursor of steroid hormones, bile salts, and vitamin D.

a. The average adult needs 80-100 g of dietary fat per day. Fat should account for no more than 30% of the daily caloric intake, and no more than 10% of daily calories should come from saturated fat.

b. A typical American consumes 40-50% of calories from fat, and ingests twice as much cholesterol as the recommended limit.

c. Most fatty acids can be synthesized by the body, except for linoleic and linolenic acids, which are essential fatty acids.

a. Saturated fats are predominantly of animal origin, with the exceptions of coconut and palm oil.

b. The richest source of cholesterol is egg yolk, but it is also prevalent in milk products, shellfish, organ meats, and meat in general. Cholesterol does not occur in foods from plant origins.

c. Saturated fat intake must be controlled because it stimulates cholesterol synthesis.

a. Lipids would stick to the insides of blood vessels if not transported as lipoproteins, structures with a core of cholesterol and triglycerides and a coating of proteins and phospholipids. The coating allows the lipids to be transported in the bloodstream and also serves as a recognition marker for cells that absorb the lipoproteins.

b. Lipoproteins are classified as chylomicrons, high-density lipoproteins (HDLs), low-density lipoproteins (LDLs), and very low-density lipoproteins (VLDLs). Lipoproteins vary not only in size and density but more importantly in composition and function.

c. Chylomicrons form in the absorptive cells of the small intestine and are transported through lacteals and lymphatic vessels before entering the bloodstream. Endothelial cells in capillaries have lipoprotein lipase that hydrolyzes triglycerides into glycerol and fatty acids. These products pass through the capillary walls into adipocytes where they are reconstructed into triglycerides and stored. The remainder of the chylomicron is removed and degraded by the liver.

d. VLDLs, produced by the liver, transport lipids to adipose tissue for long term storage. Lipoprotein lipase converts VLDLs to LDLs, which contain mostly cholesterol. Cells needing the cholesterol have membrane receptors for LDLs and absorb them by receptor-mediated endocytosis.

e. HDL production begins in the liver, which releases a protein shell that travels in the blood and picks up cholesterol and phospholipids, and thus removes excess cholesterol from the body.

f. It is necessary to maintain serum cholesterol within limits, and most authorities agree that it should not exceed 200 mg/dL. High HDL levels are seen as protective because cholesterol is removed from circulation and transported to the liver for disposal.

a. For persons of average weight, the RDA of protein is 44-60 g, depending on age and sex. A higher intake is recommended under conditions of stress, infection, injury, and pregnancy. Infants and children also require more protein relative to body weight.

b. Essential amino acids are those, which the body cannot synthesize. The nutritional value of a protein depends on whether it is a complete protein (with all the necessary essential amino acids), or an incomplete protein (lacking one or more essential amino acids). Meats and dairy products are complete proteins; plant sources are incomplete proteins but can be used in a complimentary fashion to provide complete proteins.

c. Protein quality is also determined by net protein utilization, which is the percent of amino acids in a protein that the human body uses.

a. The animal proteins of meat, eggs, and dairy products closely match human proteins in amino acid composition, thus animal proteins are complete proteins.

b. An exclusive vegetarian diet cannot meet all of your nutritional needs unless supplemented with vitamin B₁₂ and essential amino acids.

c. Proteins are our chief dietary source of nitrogen. Nitrogen balance is the state in which nitrogen ingestion equals rate of excretion (as waste material mostly).

d. Someone in positive nitrogen balance is in a period of growth, like a child or pregnant woman. Negative nitrogen balance means that body proteins are being broken down and used as fuel. This condition may occur is carbohydrate and fat intake are insufficient to meet the need for energy.

e. Nitrogen balance is affected by growth hormone and sex hormones that promote protein synthesis. Glucocorticoids promote protein catabolism during times of stress.

a. Minerals constitute about 4% of the body mass, with three-quarters of this being the calcium and phosphorus in bones and teeth.

b. Table 26.3, p. 939, summarizes adult mineral requirements and dietary sources. Broadly speaking, the best sources of minerals are vegetables, legumes, milk, eggs, fish, shellfish, and some meats.

a. Vitamins were originally named with letters in the order of their discovery, but also have chemically descriptive names. Most of the vitamins must be obtained from the diet but the body synthesizes some of them from precursors called provitamins. Vitamin K, pantothenic acid, and folic acid are also produced by bacterial flora in the large intestine.

b. Vitamins, their requirements, and some dietary sources are listed in Table 26.4, p. 940.

c. Vitamins are classified as water-soluble or fat-soluble. The water-soluble vitamins include the B vitamins and vitamin C. In general, these vitamins are not stored by the body to any great extent, and excesses are excreted in the urine. They seldom accumulate to excess.

d. The fat-soluble vitamins are vitamins A, D, E, and K. They can be stored in the body and are more likely to reach toxic levels if ingested in excess. However, deficiency in vitamin A is the world's most common vitamin deficiency.

a. Glycolysis involves splitting a glucose molecule into two molecules of pyruvic acid.

b. Anaerobic fermentation, which occurs in the absence of oxygen, reduces pyruvate to lactic acid.

c. Aerobic respiration occurs in the mitochondria in the presence of oxygen and oxidizes pyruvic acid to carbon dioxide and water.

a. The two coenzymes of special importance to glucose catabolism are NAD⁺ (nicotimamide adenine dinucleotide) and FAD (flavin adenine dinucleotide). Both are derived from B vitamins: NAD⁺ from niacin and FAD from riboflavin.

a. The matrix reactions are shown in Fig. 26.4, p. 944 (Transp. 460), where the reaction steps are numbered to correspond with text descriptions on p. 944-945.

b. After the matrix reactions, there is nothing left of the organic matter of the glucose; its carbon atoms have all been carried away as CO₂ and exhaled.

c. The citric acid cycle not only oxidizes glucose metabolites but is also a pathway and a source of intermediates for the synthesis of fats and nonessential amino acids.

a. The membrane reactions have two purposes: (1) to further oxidize NADH and FADH₂ and (2) to regenerate NAD⁺ and FADH and make them available again to earlier reaction steps.

b. The membrane reactions are carried out by a series of compounds called the electron-transport chain. Most members of the chain are bound to the inner mitochondrial membrane. The members of the chain are flavin mononucleotide, iron-sulfur centers, coenzyme Q, copper ions, and several cytochromes.

c. Fig. 26.5, p. 945 (Transp. 461), shows the order in which electrons are passed along the chain.

d. Hydrogen atoms are split apart as they are transferred from coenzymes to the chain. The protons are released to the mitochondrial matrix. Each electron carrier in the chain becomes reduced when it receives an electron pair and oxidized again when it passes the electrons along to the next carrier. Energy is liberated at each transfer.

e. The final electron acceptor in the chain is oxygen. Each oxygen atom accepts two electrons from cytochrome a₃ and two protons from the mitochondrial matrix. The result is a molecule of water, the body's primary source of metabolic water.

f. As electrons are passed along the electron transport chain, some of the energy is used to drive proton pumps that remove H⁺ from the mitochondrial matrix and pump it into the space between the inner and outer mitochondrial membranes. This creates a steep electrochemical gradient across the inner mitochondrial membrane.

g. The inner membrane is permeable to H⁺ only through specific channel proteins called ATP synthase. As H⁺ flows through these channels, it creates an electrical current. ATP synthase harnesses the energy of this current to drive ATP synthesis. This process is called the chemiosmotic mechanism.

a. When glucose is in excess, the liver stores glucose as glycogen through the process of glycogenesis.

b. When glucose in the blood declines, glycogen can be converted back to glucose through glycogenolysis.

c. Gluconeogenesis is the synthesis of glucose from noncarbohydrates such as fats and amino acids.

a. Lipogenesis refers to the synthesizing of triglycerides from other molecules. These pathways are summarized in Fig. 26.9, p. 950; Transp. 465.

a. Breaking down fat to be used as fuel is lipolysis. These pathways are also summarized in Fig. 26.9, p. 950.

b. Inadequate carbohydrate intake interferes with the complete oxidation of fat. Under such circumstances, fat oxidation leads to ketogenesis and potentially to ketoacidosis.

a. Some amino acids in the pool can be converted to others. Free amino acids also can be converted to glucose and fat or directly used as fuel. Such conversions involve either deamination, amination, or transamination.

a. The first step in using amino acids as fuel is to deaminate them. After removal of the -NH₂ group, the remainder of the molecule is called a keto acid. Depending on which amino acid is involved, the resulting keto acid may be converted to pyruvic acid, acetyl CoA, or one the acids of the citric acid cycle.

a. When deamination reactions occur, the -NH₂ becomes ammonia, NH₃, which is extremely toxic to cells and cannot be allowed to accumulate. The liver quickly converts the ammonia to its less toxic form, urea, by a pathway known as the ornithine cycle. Urea is then excreted in the urine.

a. The liver can make many amino acids from other amino acids or from citric acid cycle intermediates by transamination reactions. The essential amino acids, however, must be obtained from the diet.

a. Absorbed sugars are transported by the hepatic portal system to the liver. Most glucose passes through the liver and is available to body cells. Excessive glucose is stored as glycogen or body fat.

b. Fats enter lymph as chylomicrons and initially bypass the liver. Lipoprotein lipase removes fats from the chylomicrons for uptake by the tissues, especially adipose and muscular tissue.

c. Amino acids, like sugars, circulate first to the liver. Most pass through and become available for protein synthesis. Some are removed by the liver for protein synthesis and fatty acid synthesis after deamination.

a. The absorptive state is regulated largely by insulin, which stimulates nearly all cells to absorb glucose. Neurons are an exception and absorb glucose at their own independent rate.

a. Glucose is drawn from the body's glycogen reserves or synthesized from other compounds (gluconeogenesis).

b. Adipocytes and hepatocytes hydrolyze fats and convert the glycerol to glucose. After 4 to 5 days of fasting, the brain begins to use ketone bodies as supplemental fuel.

c. If glycogen and fat reserves are depleted, the body begins to use proteins as fuel. The first to go are skeletal muscle proteins, and starvation results in wasting away of muscle mass.

a. Postabsorptive metabolism is regulated mainly by the sympathetic nervous system and several hormones.

b. As blood glucose drops, the pancreas releases glucagon that promotes glycogenolysis and gluconeogenesis, raising blood glucose level. It also promotes lipolysis and a rise in FFA levels, thus making both glucose and lipids available for fuel.

c. The sympathoadrenal system richly innervates adipose tissue and can mobilize stored energy reserves as needed.

d. Growth hormone has the opposite effects of insulin and raises blood glucose concentrations.

a. When blood temperature is too high, the heat-losing center signals the dilation of dermal arterioles, conducting more heat to the body surface. If this fails, sweating is triggered.

b. When blood temperature drops too low, the heat-promoting center causes dermal vasoconstriction and the body may resort to shivering thermogenesis.

c. During colder seasons, the body raises its metabolic rate by 20-30% through nonshivering thermogenesis.

a. Heat cramps are painful muscle cramps that result from excessive electrolyte loss in the sweat.

b. Heat exhaustion results from more severe electrolyte loss, and is characterized by hypotension, dizziness, vomiting, and sometimes fainting.

c. During heat stroke, the body temperature can rise to as high as 43^oC (110^oF). Brain cells misfunction, hypothalamic mechanisms break down, and convulsions, coma, and death may suddenly occur.

a. If the core temperature falls below 33^oC (91^oF), the metabolic rate drops so low that heat production cannot keep pace with heat loss, and the temperature falls further.

b. A body temperature below 24^oC (75^oF) is usually fatal.

CHAPTER ESSAY: Alcohol and Alcoholism (p. 957)