Previously Exercising Muscle

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Previously Resting Muscle

Previously Resting Muscle

FIGURE 11 Postulated interaction between previously exercising muscle and previously resting muscle via the Cori cycle during recovery from prolonged arm exercise. (From Ahlborg G, Wahren J, Felig P. Splanchnic and peripheral glucose and lactate metabolism during and after prolonged arm exercise. J Clin Invest 1986;77:690-699).

Increasing or decreasing the total amount of fat stored is achieved primarily by changes in the volume of the fat droplets within each adipocyte by the mechanisms of fat deposition and mobilization already discussed. In addition, the number of adipocytes is not fixed and may increase throughout life by multiplication and differentiation of precursor cells. Conversely, fat cells may become depleted of their triglyceride stores and undergo dedifferentiation and apoptosis. The process of adipo-genesis depends on locally produced growth factors and cytokines as well as hormones. Insulin and cortisol promote differentiation of human preadipocytes and tend to favor accumulation of body fat, whereas GH inhibits both differentiation and storage. These chronic actions of insulin and GH are consistent with short-term actions already discussed, but the effects of cortisol are unexpected in light of its short-term actions to promote lipolysis and decrease fatty acid reesterification. However, removal of the adrenal glands in experimental animals prevents or reverses all forms of genetic or experimentally induced obesity. Additionally, chronic excess production of glucocorticoids in humans is associated with increased body fat, especially in the torso (truncal obesity), the face (moon face), and between the scapulae (buffalo hump).

For most people, body fat reserves are maintained at a nearly constant level throughout adult life despite enormous variations in daily food consumption and energy expenditure. Figure 12 summarizes the findings of five independent studies of changes in body weight and fat mass with aging in about 12,000 individuals. Although total body fat increased with increasing age, the increase averaged less than a gram per day when averaged over a period of 50 years and corresponded to a daily positive energy balance of about 6 Calories. Assuming that daily energy consumption averages about 2000 Calories, the intake of fuel in a mixed diet matched the rate of energy utilization with an error of 0.3%. However, the affluence, ready access to high-calorie foods, and technology that fosters sedentary activities in contemporary society have so distorted the balance between caloric intake and energy expenditure that 30% of the American population is now classified as obese.

An understanding of the mechanisms that govern long-term fuel storage requires an understanding of the regulation of energy expenditure as well as food intake. Physical activity accounts for only about 30% of daily energy expenditure. Sixty percent of energy consumption is expended at rest for maintenance of ion gradients, renewal of cellular constituents, neuronal activity, and to support cardiopulmonary work. The remaining 10% is dissipated as the thermogenic effect of feeding and the consequent processes of assimilation.

FIGURE 12 Cross-sectional data showing changes in body weight and fat content with aging obtained in five independent studies. (From Forbes GB, Reina JC. Adult lean body mass declines with age: Some longitudinal observations. Metabolism 1970;19:653-663).

Neither resting nor thermogenic energy expenditure are fixed, but are adjustable in a manner that tends to keep body fat reserves constant. In the experiment illustrated in Fig. 13, normal human subjects were overfed or underfed in order to increase or decrease body weight by 10%. They were then given just enough food each day to maintain their new weight at a constant level. Energy utilization increased disproportionally in the overfed subjects and decreased disproportionally in the underfed subjects. Such compensatory changes in energy expenditure opposed maintenance of the change from initial body fat content. How such changes in energy expenditure are brought about is not understood. One possibility is that metabolic efficiency may be regulated by varying expression of genes that encode proteins that uncouple ATP generation from oxygen consumption (see Chapter 39).

Hypothalamic Control of Appetite and Food Intake

Studies such as those illustrated in Figs. 12 and 13 and many older observations gave rise to the idea that the mass of the fat storage depot is monitored and maintained at a nearly constant set point through feedback mechanisms that regulate food consumption and energy expenditure (Fig. 14). Clinical observations and studies in experimental animals established that the hypothalamus coordinates the drive for food intake with such energy-consuming processes as temperature regulation, growth, and reproduction. Various injuries to the hypothalamus can produce either insatiable eating behavior accompanied by severe obesity, or food avoidance and lethal starvation. A complex neural network interconnects "satiety centers'' in the medial hypothalamus and "hunger centers'' in the lateral hypothalamus with each other, with autonomic integrating centers in the hypothalamus and brain stem, and en o 8

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