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, Maximal oxygen uptake, endurance training, and age. Endurance-trained subjects possess greater maximal oxygen uptake than sedentary subjects, regardless of age.

regulation of insulin receptors and reduced insulin release after chronic exercise is ideal therapy in type 2 diabetes (non-insulin-dependent), a disease characterized by high insulin secretion and low receptor sensitivity. In persons with type 2 diabetes, a single episode of exercise results in substantial glucose transporter translocation to the plasma membrane in skeletal muscle.

AGING, IMMUNE, AND PSYCHIATRIC RESPONSES

Maximal dynamic and isometric exercise capacities are lower at age 70 than at age 20. There is overwhelming evidence, however, that declines in strength and endurance with advancing age can be substantially mitigated by training. Changes in functional capacity, as well as protection against heart disease and diabetes, do increase longevity in active persons. However, it remains controversial if chronic exercise enhances lifespan, or if exercise boosts the immune system, prevents insomnia, or enhances mood.

As People Age, the Effects of Exercise on Functional Capacity Are More Profound Than Their Effect on Longevity

The influence of exercise on strength and endurance at any age is dramatic. Although the ceiling for oxygen uptake during work gradually falls with age, the ability to train toward an age-appropriate ceiling is as intact at age 70 as it is at age 20 (Fig. 30.8). In fact, a highly active 70-year-old, otherwise healthy, will typically display an absolute exercise capacity greater than a sedentary 20-year-old. Aging affects all the links in the chain of oxygen transport and use, so aging-induced declines in lung elasticity, lung diffusing capacity, cardiac output, and muscle metabolic potential take place in concert. Consequently, the physiological mechanisms underlying fatigue are similar at all ages.

Regular dynamic exercise, compared with inactivity, increases longevity in rats and humans. In descriptive terms, the effects of exercise are modest,- all-cause mortality is reduced, but only in amounts sufficient to increase longevity by 1 to 2 years. These facts leave open the possibility that exercise might alter biological aging. While physical activity increases cellular oxidative stress, it simultaneously increases antioxidant capacity. Food-restricted rats experience increased life span, and exhibit elevated spontaneous activity levels, but the role exercise may play in the apparent delay of aging in these animals remains unclear.

Acute Exercise Transiently Alters Many Circulating Immune System Markers, but the Long-Term Effects of Training on Immune Function Are Unclear

In protein-calorie malnutrition, the catabolism of protein for energy lowers immunoglobulin levels and compromises the body's resistance to infection. Clearly, in this circumstance, exercise merely speeds the starvation process by increasing daily caloric expenditure and would be expected to diminish the immune response further. Nazi labor camps of the early 1940s became death camps, partly, by severe food restrictions and incessant demands for physical work—a combination guaranteed to cause starvation.

If nutrition is adequate, it is less clear whether adopting an active versus a sedentary lifestyle alters immune respon-sivity. In healthy people, an acute episode of exercise briefly increases blood leukocyte concentration and transiently enhances neutrophil production of microbicidal reactive oxygen species and natural killer cell activity. However, it remains unproven that regular exercise over time can lower the frequency or reduce the intensity of, for example, upper respiratory tract infections. In HIV-positive men and in men with AIDS and advanced muscle wasting, strength and endurance training yield normal gains. There is also incomplete evidence that training may slow progression to AIDS in HIV-positive men, with a corresponding increase in CD4 lymphocytes.

Exercise May Help Relieve Depression, but Its Efficacy and Neurochemical Effects Are Uncertain

In healthy people, prolonged exercise increases subsequent deep sleep, defined as stages 3 and 4 of slow-wave sleep (see Chapter 7). This effect is apparently mediated entirely through the thermal effects of exercise, since equivalent passive heating produces the same result. Whether or not exercise can improve sleep in patients with insomnia is not known.

Clinical depression is characterized by sleep and appetite dysfunction and profound changes in mood. Whether acute or chronic exercise can help relieve depression remains unproven. The two most prominent biological theories of depression—the dysregulation of central monoamine activity and dysfunction of the hypothalamic-pituitary-adrenal axis—have received almost no study with regard to the impact of exercise.

Panic disorder patients, often characterized by agoraphobia, have reduced exercise capacity. Although sodium lactate infusion does provoke panic in these patients, the anxiety mediator appears to be hypernatremia, not lactate, even strenuous exercise with substantial lactic acidosis will not trigger panic attacks in these individuals.

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