Metabolism is the physical and biochemical modifications of compounds in living organisms. For human beings, metabolism primarily deals with processing food, along with the atmospheric gas oxygen, into energy needed to maintain a living state. Metabolic rate refers to the speed or rate at which this process happens. Metabolic rate is also known as energy expenditure.

Metabolic rate is made up of four processes: (1) basal or "resting" metabolism, which includes energy needed to maintain electrochemical balance in cells, energy for growth and repair of structural components of the body, and energy expended during the mechanical work of the cardiovascular and respiratory systems at rest; (2) energy needed for exercise or physical work; (3) energy needed to maintain a "core" body temperature; and (4) energy used during the eating and digestion of food (sometimes called the thermic effect of food).

At rest, internal organs such as the liver, brain, heart, and kidneys together can account for about 60 percent of metabolic rate, and muscle accounts for about 15 percent. During exercise or physical work, the energy requirements of muscle can increase manifold. The thermic effect of food can account for ~10 percent of metabolic rate. The rest is made up by metabolism of other organs. Because modern persons typically live in a thermoneutral environment, the energy needed to maintain body temperature is very little and thus contributes very little to daily metabolic rate.

There are several ways to directly measure metabolic rate. Heat production (a by-product of metabolism) can be measured by placing a person in a suit or chamber that measures heat production. This method is called direct calorimetry, and requires expensive and sophisticated equipment. A simpler method is to measure oxygen consumed over a time period. This method is called indirect calorimetry. These are direct ways to measure metabolic rate.

There are several "field methods" used to indirectly measure metabolic rate. Behavioral observation and time and motion analysis can be used for supervised subjects. Questionnaires and interviews of subjects can be used. Movement assessment devices such as pedometers and accelerometers can be worn by subjects. These methods and devices sacrifice accuracy for ease and/or decreased cost. Metabolic rate can be affected by a number of factors, including age, gender, amount of fat and muscle mass, time of day, heredity, certain drugs and disease states, and exercise or physical work.

To affect the metabolic rate, changes must occur in one of the four processes listed above. For the U.S. population, the energy needed to maintain core temperature and for eating and digestion of food cannot be altered much. Thus, the two ways to alter metabolic rate are to change basal metabolism, and/or change the amount of exercise and muscular work.

The largest contributor to basal metabolism that can be reasonably and voluntarily altered is the amount of muscle mass a person has. By incorporating resistance training, muscle mass can be increased which should in theory raise basal metabolic rate. Conversely, when people consume fewer calories, or diet, typically both muscle and fat mass are lost. Thus, dieting tends to lower basal metabolic rate, contributing to an overall reduced daily metabolic rate. The simplest thing to do to increase metabolic rate is regular exercise. The benefits of modern life have contributed greatly to reductions in muscular work, and thus metabolic rate. Regular exercise can counteract this reduction in metabolic rate.

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