Compared to normal-weight sedentary people and the obese, athletes actually have lower levels of most hormones at rest. However, this does not represent a global hormone deficiency, rather it demonstrates that athletes are more sensitive to the hormones and respond more vigorously when exposed to lower levels of the hormones. This phenomenon was relatively unappreciated until the discovery of leptin. When leptin was being developed as a drug, it was believed to represent the great hope in combating obesity. Unfortunately, clinical trials failed, as it was discovered that most obese people generate plenty of leptin- since it's generated by fat cells- but they are resistant to its effects.21,22 Administering additional leptin had minimal effect. Athletes and those exposed to exercise training develop greater leptin sensitivity.23

     Catecholamines are more immediately potent and respond rapidly to changes in activity. In sedentary people, this promotes fat oxidation, but it does not have a direct impact on subcutaneous fat (the stores of fat directly under the skin). This is due to what is known as tissue-specific effects of catecholamines. Different receptors respond to catecholamines; some promote fat loss, others constrict blood vessels closing off blood flow. In fat cells and muscle, ß-receptors dominate, which turn on the fat-burning machinery in a cell. In blood vessels, especially those in the subcutaneous fat tissue, α2-receptors dominate, which constrict the blood vessels, preventing hormones from getting into the subcutaneous area to stimulate fat release and preventing the released fat from reaching the circulation.24-26

     One might wonder why the body prevents sedentary people from tapping into the subcutaneous fat, but allows athletes to access these energy stores. Likely, the matter is incidental to another function of the skin: thermoregulation. As athletes generate a lot of body heat during exercise, they have to release the heat through the skin.27 By conditioning the body to open the subcutaneous and dermal circulation during exercise, athletes now have the ability to tap into their subcutaneous fat stores. This is due to lower α2-receptor sensitivity.

     In contrast, it appears the fat oxidizing effects of catecholamines are increased through higher ß-receptor sensitivity.25,26 Thus, relative to catecholamines, athletes have more fat oxidation.

    The Role of Insulin
Insulin shows a similar pattern. Obese people are prone to type II diabetes and metabolic syndrome. These conditions include a pronounced resistance to insulin. As insulin strongly inhibits lipolysis, even at normal physiologic levels, chronically elevated insulin levels can rapidly lead to increased fat storage and a predisposition toward obesity.28 Athletes, as a group, have a much better degree of insulin sensitivity, so their insulin levels remain low.3 Thus, under similar conditions, athletes have less resistance to releasing and burning stored fat during exercise.

     The other hormones mentioned- testosterone, estradiol, thyroid hormone, growth hormone and cortisol- demonstrate similar features. Immediately during exercise, these hormones peak, but at rest, athletes have similar or lower levels than their sedentary counterparts.3 Primarily, this is due to improved sensitivity and the body's ability to respond to the hormone stimulus.

     Another interesting feature noted in some exercise studies is a change in the cellular structure of athletes. The mitochondria, the part of the cell where fat oxidation occurs, actually increases in size and number.29 This means not only does the active muscle of athletes receive more fatty acids to burn as energy, the muscle also has more "furnaces," increasing the body's ability to rely upon fat stores for energy instead of the more fragile carbohydrate stores.

     The ideal for fitness fanatics would be to have a physiology that burned fat almost exclusively for most of the body's resting and exercise needs. This would preserve carbohydrate stores for immediate, high-energy work and protect protein stores from the catabolic demands of gluconeogenesis. Unfortunately, the human body is relatively inefficient at burning fat.30

     Fortunately, the body adapts to a training lifestyle, making changes that improve the amount of fat made available for energy and offering a greater potential for burning fat for energy. This includes changes in the levels of several key metabolic hormones and a general increase in the sensitivity to the hormonal stimuli. These adaptations allow for a greater mobilization of stored fat, preservation of carbohydrate stores (especially during the recovery/replenishment period after exercise) and protection against muscle breakdown.

     For dedicated athletes, the fat loss benefits of exercise go well beyond the mathematical equation of calories in minus calories burned. The bodybuilding lifestyle creates a physiology that improves health and works on physique enhancement even while you sleep.

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