Gerald I. Shulman, Kitt Falk Petersen
Metabolism encompasses all chemical reactions in the body's cells, necessary to sustain life. These chemical processes allow the body to grow, reproduce, maintain structure, and respond to changes in its environment. These processes can be anabolic, in which energy is used in the formation of substances such as proteins or nucleic acids, or catabolic, in which organic substances are broken down during cellular respiration to harvest energy to support cellular processes such as biosynthesis, transport of molecules or ions across cell membranes, or locomotion. A healthy, sedentary young man weighing 70 kg requires 2100 kcal (~30 kcal/kg body weight) N58-1 to sustain resting metabolism for 1 day, an amount known as the resting metabolic rate (RMR). The number of calories increases with increased activity, illness, or other stress. For example, the metabolic rate can rise 2- to 3-fold with exposure to a cold environment, or up to 10-fold during heavy exercise. The basal metabolic rate (BMR) is a clinical term for metabolism that is measured under standardized conditions in which the subject (1) has had a full night of restful sleep, (2) has been fasting for 12 hours, (3) is in a neutral thermal environment (see p. 1196), (4) has been resting physically for 1 hour, and (5) is free of psychic and physical stimuli. In adults the BMR (units: kilocalories per hour and per square meter of body surface area) is ~5% higher for males than for females and falls with age. The BMR is less than the RMR.
Contributed by Edward Masoro, Emile Boulpaep, Walter Boron
One gram of tissue from a small mammal has a higher resting metabolic rate (RMR; see p. 1170) than the same mass of tissue from a larger mammal (e.g., a human). By plotting heat production versus body weight for animals over a range of five orders of magnitude, Max Kleiber showed that metabolic rate (in kcal/day) is not proportional to body weight (in kg) but to body weight raised to the power .
Allometry is the study of how body size relates to various biological parameters (e.g., metabolic rate, anatomic parameters).
Kleiber M. Body size and metabolic rate. Physiol Rev. 1947;27(4):511–541.
Regulation of energy metabolism in humans involves a complex interplay among ingested nutrients, hormones, and interorgan exchanges of substrates to maintain a constant and adequate supply of fuel for all organs of the body. Because energy acquisition by the body is intermittent, whereas energy expenditure is continuous, the body needs to store and then parcel out energy in a carefully coordinated fashion. Insulin (see pp. 1035–1050) is the key hormone that orchestrates this exchange and distribution of substrates between tissues under fed and fasting conditions. Glucagon (see pp. 1050–1053), catecholamines (see pp. 1030–1031), cortisol (see pp. 1018–1026), and growth hormone (see pp. 990–995) play major roles in energy regulation at times of acute energy needs, which occur during exercise, under conditions of stress, or in response to hypoglycemia. The major organs involved in fuel homeostasis are (1) the liver, which is normally the major producer of glucose; N58-2 (2) the brain, which in the fasted state is the major utilizer of glucose; and (3) the muscle and adipose tissue, which respond to insulin and store energy in the form of glycogen and fat, respectively.
Role of the Kidneys in Fuel Homeostasis
Contributed by Kitt Petersen, Gerald Shulman
Although the kidneys can also produce glucose by gluconeogenesis, their net contribution to whole-body glucose production is typically <10% except under conditions of prolonged fasting, when they can contribute up to 30% to 40%.
The purpose of this chapter is to review how humans utilize energy and the means by which the body manages its energy stores during times of feeding, fasting, and exercise.