Chapter Summary

Nutritional Requirements

1. Food provides molecules used in cell respiration for energy.
1. The metabolic rate is influenced by physical activity, temperature and eating; the basal metabolic rate is measured as the rate of oxygen consumption when such influences are standardized and minimal.
2. The energy provided by food and the energy consumed by the body are measured in units of kilocalories.
3. When the caloric intake is greater than the energy expenditure over a period of time, the excess calories are stored primarily as fat.
2. Vitamins and elements serve primarily as cofactors and coenzymes.
1. Vitamins are divided into those that are fat-soluble (A, D, E, and K) and those that are water-soluble.
2. Many water-soluble vitamins are needed for the activity of the enzymes involved in cell respiration.
3. The fat-soluble vitamins A and D have specific functions but share similar mechanisms of action, activating nuclear receptors and regulating genetic expression.

Regulation of Energy Metabolism

1. The body tissues can use circulating energy substrates, including glucose, fatty acids, ketone bodies, lactic acid, amino acid, and others, for cell respiration.
1. Different organs have different preferred energy sources.
2. Circulating energy substrates can be obtained from food or from the energy reserves of glycogen, fat, and protein in the body.
2. Eating behavior is regulated, at least in part, by the hypothalamus.
1. Lesions of the ventromedial area of the hypothalamus produce hyperphagia, whereas lesions of the lateral hypothalamus produce hypophagia.
2. A variety of neurotransmitters have been implicated in the control of eating behavior. These include the endorphins, norepinephrine, serotonin, and cholecystokinin.
3. Adipose cells, or adipocytes, are both the targets of hormonal regulation and themselves endocrine in nature.
1. In children, circulating saturated fatty acids promote cell division and differentiation of new adipocytes. This activity involves the bonding of prostaglandin, 15d-PGJ₂, with a nuclear receptor known as PPAR_g.
2. Adipocytes secrete leptin, which regulates food intake and metabolism, and TNF_a, which may help to regulate the sensitivity of skeletal muscles to insulin.
4. The control of energy balance in the body is regulated by the anabolic and catabolic effects of a variety of hormones.

Energy Regulation by the Islets of Langerhans

1. A rise in plasma glucose concentration stimulates insulin and inhibits glucagon secretion.
1. Amino acids stimulate the secretion of both insulin and glucagon.
2. Insulin secretion is also stimulated by parasympathetic innervation of the islets and by the action of intestinal hormones such as gastric inhibitory peptide (GIP).
2. During the intestinal absorption of a meal, insulin promotes the uptake of blood glucose into skeletal muscle and other tissues.
1. This lowers the blood glucose concentration and increases the energy reserves of glycogen, fat, and protein.
2. Skeletal muscles are the major organs that remove blood glucose in response to insulin stimulation.
3. During periods of fasting, insulin secretion decreases and glucagon secretion increases.
1. Glucagon stimulates glycogenolysis in the liver, gluconeogenesis, lipolysis, and ketogenesis.
2. These effects help maintain adequate levels of blood glucose for the brain and provide alternate energy sources for other organs.

Diabetes Mellitus and Hypoglycemia

1. Diabetes mellitus and reactive hypoglycemia represent disorders of the islets of Langerhans.
1. Insulin-dependant diabetes mellitus occurs when the beta cells are destroyed; the resulting lack of insulin and excessive glucagon secretion produce the symptoms of this disease.
2. Non-insulin-dependant diabetes mellitus occurs as a result of a relative tissue insensitivity to insulin and inadequate insulin secretion; this condition is aggravated by obesity and improved by exercise.
3. Reactive hypoglycemia occurs when the islets secrete excessive amounts of insulin in response to a rise in blood glucose concentration.

Metabolic Regulation by Adrenal Hormones, Thyroxine, and Growth Hormone

1. The adrenal hormones involved in energy regulation include epinephrine from the adrenal medulla and glucocorticoids (mainly hydrocortisone) from the adrenal cortex.
1. The effects of epinephrine are similar to those of glucagon. Epinephrine stimulates glycogenolysis and lipolysis, and activates increased metabolism of brown fat.
2. Glucocorticoids promote the breakdown of muscle protein and the conversion of amino acids to glucose in the liver.
2. Thyroxine stimulates the rate of cell respiration in almost all cells in the body.
1. Thyroxine sets the basal metabolic rate (BMR), which is the rate at which energy (and oxygen) is consumed by the body under resting conditions.
2. Thyroxine also promotes protein synthesis and is needed for proper body growth and development, particularly of the central nervous system.
3. The secretion of growth hormone is regulated by releasing and inhibiting hormones from the hypothalamus.
1. The secretion of growth hormone is stimulated by a protein meal and by a fall in glucose, as occurs during fasting.
2. Growth hormone stimulates catabolism of lipids and inhibits glucose utilization.
3. Growth hormone also stimulates protein synthesis, and thus promotes body growth.
4. The anabolic effects of growth hormone, including the stimulation of bone growth in childhood, are believed to be produced indirectly via polypeptides called insulin-like growth factors, or somatomedins.

Regulation of Calcium and Phosphate Balance

1. Bone contains calcium and phosphate in the form of hydroxyapatite crystals. This serves as a reserve supply of calcium and phosphate for the blood.
1. The formation and resorption of bone are produced by the action of osteoblasts and osteoclasts, respectively.
2. The plasma concentrations of calcium and phosphate are also affected by absorption from the intestine and by the urinary excretion of these ions.
2. Parathyroid hormone stimulates bone resorption and calcium reabsorption in the kidneys. This hormone thus acts to raise the blood calcium concentration.
1. The secretion of parathyroid hormone is stimulated by a fall in blood calcium levels.
2. Parathyroid hormone also inhibits reabsorption of phosphate in the kidneys, so that more phosphate is excreted in the urine.
3. 1,25-dihydroxyvitamin D₃ is derived from vitamin D by hydroxylation reactions in the liver and kidneys.
1. The last hydroxylation step is stimulated by parathyroid hormone.
2. 25-dihydroxyvitamin D₃ stimulates the intestinal absorption of calcium and phosphate, resorption of bone, and renal reabsorption of phosphate.
4. A rise in parathyroid hormone, accompanied by the increased production of 1,25-dihydroxyvitamin D₃ helps to maintain proper blood levels of calcium and phosphate in response to a fall in calcium levels.
5. Calcitonin is secreted by the parafollicular cells of the thyroid gland.
1. Calcitonin secretion is stimulated by a rise in blood calcium levels.
2. Calcitonin, at least at pharmacological levels, acts to lower blood calcium by inhibiting bone resorption and stimulating the urinary excretion of calcium and phosphate.

After studying this chapter, students should be able to . . .

1. state some of the factors that influence metabolic rate and explain the significance of the basal metabolic rate and state how the metabolic rate.
2. distinguish between the caloric and anabolic requirements for food, and define the terms essential amino acids and essential fatty acids.
3. distinguish between fat-soluble and water-soluble vitamins and describe some of the functions of different vitamins.
4. define the terms energy reserves and circulating energy substrates and explain how these sources of energy interact during anabolism and catabolism.
5. describe the regulation of eating and discuss the endocrine control of metabolism in general terms.
6. describe the regulation of adipocyte development and the roles of adipocytes in the regulation of hunger and tissue responsiveness to insulin.
7. describe the actions of insulin and glucagon and explain how the secretion of these hormones is regulated.
8. explain how insulin and glucagon regulate metabolism during feeding and fasting.
9. describe the symptoms of insulin-dependant and non-insulin-dependant diabetes mellitus and explain how these conditions are produced.
10. describe the metabolic effects of epinephrine and glucocorticoids.
11. describe the effects of thyroxine on cell respiration and explain the relationship between thyroxine levels and basal metabolic rate.
12. describe the symptoms of hypothyroidism and hyperthyroidism, and explain how these conditions are produced.
13. describe the metabolic effects of growth hormone and explain why growth hormone and thyroxine are needed for proper body growth.
14. describe the actions of parathyroid hormone, 1,25-dihydroxyvitamin D₃, and calcitonin, and explain how the secretion of these hormones is regulated.
15. describe how 1,25-dihydroxyvitamin D₃ is produced and explain why this compound is needed to prevent osteomalacia and ricket.