Human energy expenditure can be divided into three components: basal or resting metabolic rate (RMR), the thermic effect of food (TEF), and the thermic effect of activity (TEA). The stoichiometric relationships between oxygen consumption and the heat release that occurs with biologic substrate oxidations are similar to those seen in chemical combustion. As a result, the rate of energy expenditure and substrate oxidation can be determined by measuring heat losses (direct calorimetry) or by measuring oxygen consumption and carbon dioxide production.
RMR has been operationally defined as the calories expended per unit time by a relaxed person who is in a thermoneutral environment and who has been fasting for 12 to 18 hours. The RMR defines that energy which is necessary for the basic maintenance of the body. This includes energy utilized for the movement of the heart and respiratory muscles, for maintenance of ionic gradients between cells and the body fluids, for synthesis of new protein, and for the maintenance of body temperature. The TEF is defined as the elevation of metabolic rate occurring after food ingestion. It includes the cost of the absorption, metabolism, and storage of the food within the body. The TEA is the energy expended with activity and exercise.
The increase of caloric intake over expenditure leads to the accretion of fat and an increase in weight. The increase in fat is accompanied by a proportional increase in lean body mass. Thus, for every pound of excess weight added to the body, about two-thirds is fat and one-third is lean. Although the predominant increase in fat-free mass (FFM) is in muscle, other organs are also involved. It has been shown that the RMR is related to the FFM. As a result, as individuals gain weight, they increase their RMR.
On the other hand, what happens to the TEF is more complex. Some studies in obese persons have shown a decrease in TEF whereas others have not. This is most likely related to the insulin sensitivity of the subjects. The more insulin resistant an obese individual is, the more trouble he/she will have in glucose oxidation and disposal, so that postprandial thermogenesis will be decreased. Obese individuals with normal glucose tolerance will have normal TEF, whereas those with impaired glucose tolerance will tend to have decreased TEF.
The TEA is increased in obese individuals per unit of activity. That is, for a given activity, obese persons expend more energy because they are carrying around a greater weight. However, obese persons tend to be very sedentary, so that they actually are likely to spend fewer minutes per day in any type of activity.
Overall, in either room calorimeters or using doubly labeled water in the free-living state, obese persons expend more total 24-hour energy than age- and sex-matched nonathletic normal weight persons.
The best known study of metabolism during starvation was conducted by Ancel Keys and his coworkers at the University of Minnesota in the late 1940's. They studied 32 young male volunteers, who were placed on a diet that provided about two-thirds of their usual calories for 24 weeks. The young men lost more than 70 percent of their fat and about 24 percent of their FFM. The RMR of these volunteers decreased by 40 percent after the 24 weeks of starvation. This decreased RMR can be ascribed primarily to the decrease in lean body mass (LBM). However, the RMR also decreased if expressed per unit of remaining lean tissue, suggesting that other hormonal changes had an important impact. The TEF also decreased, partly because smaller meals were being eaten by the subjects, although the influence of hormonal changes could also have played a role. In addition, TEA decreased, both because the men moved about much less and were moving a much lighter total body, requiring less work and caloric output.