The total increase in fatty acid uptake during exercise was 50 percent lower in obese subjects and 35 percent lower in overweight men than in the lean subjects. Normally, moderate-intensity exercise leads to a twofold to threefold increase in the burning of stored fat. Exercise also promotes greater use of fat stored in muscle, known as intramuscular fat. That’s due to increased secretion of catecholamines, such as epinephrine and norepinephrine, from the adrenal glands during exercise.
With increased bodyfat levels the body secretes lower levels of catecholamines during exercise, blunting release of fatty acids from fat cells into the blood. On the other hand, the study also showed that higher bodyfat levels also led to increased use of intramuscular fat, so the level of fat oxidation is similar. Still, fatter men release less of the type of fat that most people focus on—fat stored systemically in fat cells.
Decreased use of systemic fat by the obese involves not only decreased catecholamine release during exercise but also a heightened stimulation of alpha-adrenergic receptors. Unlike beta-adrenergic cell receptors, which favor the use of fat, alpha-adrenergic fat-cell receptors inhibit fat use. A preponderance of alpha-adrenergic receptors in women’s lower bodies explains why it’s so difficult for most women to lose fat in that area. Interestingly, obese men burn fat much as women do in their lower bodies—with great difficulty.
Obese men also usually have higher resting insulin levels. That’s caused by insulin resistance resulting from larger fat-cell volume. Insulin blocks the release of fat during exercise, an effect usually opposed by increased catecholamine secretion. Catecholamine secretion is blunted in the obese, however, leading to a vicious metabolic cycle.
The question is how those with higher bodyfat levels can overcome their considerable fat-oxidation problems. Ephedrine and mahuang supplements simulate the effects of catecholamines in the body, including their effects on bodyfat release. Such supplements are no longer available, due to inaccurate reports about their so-called health dangers. Caffeine may also help release catecholamines to a limited degree, but the effect is usually transient.
As for the problem of alpha-adrenergic receptors, a supplement based on yohimbe may block the effects of those fat-blunting receptors. But yohimbe must be taken on an empty stomach at a dose of about 0.2 milligrams per kilogram of bodyweight. Food obliterates yohimbe’s fat-oxidation properties.
The easiest way to overcome the metabolic fat oxidation deficit, though, is simply to have patience and lose the excess fat. When that happens, the obese are likely to burn fat as easily as their leaner peers.
1 Mittendorfer, B., et al. (2004). Excess bodyfat in men decreases plasma fatty acid availability and oxidation during endurance exercise. Am J Physiol Endocrinol Metab. 286:E354-62.
©,2012, Jerry Brainum.Any reprinting in any type of media, including electronic and foreign is expressly prohibited.
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