Total metabolic rate is the amount of energy used or expended by the body in a given amount of time. It is often expressed in kilocalories per hour or per day. The main direct determinants of total metabolic rate are as follows:
Factor 1—the basal metabolic rate, which usually constitutes about 55% to 60% of the total metabolic rate.
Factor 2—the energy used to do skeletal muscle work.
Factor 3—the thermic effect of foods. The metabolic rate increases for several hours after a meal, apparently because of the energy needed for metabolizing foods.
Energy Balance and Body Weight
Our bodies maintain a state of energy balance, in which the body’s energy input equals its energy output. Energy input per day equals the total calories (kilocalories) in the food ingested per day. Energy output equals the total metabolic rate expressed in kilocalories. If calorie intake and energy output are not equal, changes in body weight may occur:
▪Body weight remains constant (except for possible variations in water content) when the total calories in the food ingested equal the total metabolic rate.
▪Body weight increases when energy input exceeds energy output.
▪Body weight decreases when energy input is less than energy output—when the total number of calories in the food eaten is less than the total metabolic rate.
Foods are stored primarily as glycogen and fats. Many cells (except for skeletal muscle) catabolize carbohydrates first, then fats. If there is no food intake, almost all of the glycogen is estimated to be used up in a matter of 1 or 2 days. Then, with no more carbohydrate to act as a fat sparer, fat is catabolized. The amount of fat available determines the length of time that an individual can catabolize fat as a reserve source of energy. Finally, with no more fat available, tissue proteins are catabolized. Because significant amounts of protein are not “stored” for use in catabolism, important structural and functional proteins are quickly depleted. For this reason, severe starvation will eventually lead to death.