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Aerobics and myostatin: an odd couple?

The thought of doing aerobic exercise is anathema to many bodybuilders. According to bodybuilding dogma, doing aerobics will melt muscle faster than butter thrown into a hot frying pan. The usual reason provided for this alleged negative effect of aerobics is that doing aerobics stimulates the release of cortisol. Cortisol is an adrenal stress hormone that is the primary catabolic hormone in the body.Any type of overtraining is associated with an increase in blood cortisol levels. Cortisol also has an inverse relationship with anabolic hormones,including testosterone, growth hormone, and insulin. When cortisol is up, these other hormones are usually down, and vice-versa. The problem with not doing aerobics is that aerobics is the best type of exercise to promote fat oxidation, or “fat burning.” Oxidation of body fat requires larger amounts of oxygen, and the term “aerobics” literally means “with oxygen”. Aerobics is also said to blunt the release of anabolic hormones, particularly testosterone. Yet, some studies show that doing aerobics can increase testosterone levels, unless you overdo it. A new study found an unexpected association between doing aerobics and one of the primary blocks to muscle growth in the body.

Myostatin is a protein discovered by scientists from Johns Hopkins University in 1997. Animals lacking this protein display huge muscles, and when lab animals, such as rats and mice, are specially bred to lack the myostatin protein, they to become the rodent equivalent of Mr.Olympia. Even humans who don’t fully express myostatin show extraordinary levels of muscular development, along with a significant lack of excess body fat. Myostatin may be a key factor in explaining those individuals who are often called “easy gainers.” Several pro bodybuilders have also been rumored to partially lack full expression of myostatin, although the effect would be difficult to separate from their use of various anabolic drugs, including growth hormone and anabolic steroids.  The discovery that blocking myostatin can result in larger muscles has led to the development of several food supplements touted to block myostatin. Thus far, none of these “myostatin blockers” has proven effective. While there are a few genuine myostatin blockers, all are experimental, and none are sold commercially. And that’s probably a good thing, since information about precisely how myostatin works in the body is ongoing, and the long-term effects of using any myostatin blocker aren’t yet established.

One easy way to lower myostatin levels in the body is through exercise. Several studies have shown that lifting weights lowers myostatin levels by an average of about 40%. This is one way that lifting weights promotes muscle growth. In the new study, the focus was any connection between doing aerobics and myostatin. The study began with measuring myostatin levels in 10 insulin-resistant, middle-aged men, average age, 53. The myostatin measurements were obtained before and after the men did moderate level (1,200 calories a week at 40-50% of peak oxygen level intensity) for six months. In another part of the study, mice were injected with myostatin. The results showed that aerobics lowered myostatin levels in the men by an average of 37%. There was also a strong correlation between myostatin levels and insulin sensitivity. This was apparent in the mice directly injected with myostatin, who all showed increased insulin resistance. A 44% increase in myostatin was associated with a significant reduction in insulin activity in cells of both muscle and liver. Based on this, the study authors suggest that an increase in myostatin that occurs with inactivity may play a role in the insulin insensitivity that ensues from a lack of exercise. From a practical angle, the fact that moderate aerobics lowers myostatin may mean that it may be more compatible with muscle growth than most bodybuilders and others think. On the other hand, typical extended aerobic sessions of one hour or more are known to interfere with muscle protein synthesis, which isn’t good for muscle growth promotion. The solution may be to do shorter sessions of interval aerobics, marked by alternate periods of high and low intensity done for shorter times. This type of aerobics may offer the benefits minus the possible liabilities related to aerobic exercise.

Hittel D,et al. Myostatin decreases with aerobic exercise and associates with insulin resistance.Med Sci Sports Exer 2010: in press.

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