But the key word here is "overboard." Cortisol levels don't begin to rise during aerobics until after about an hour of steady aerobics. If you do less than that in one session, the rise in cortisol is negligible. There is an exception to this, however. Those on either restricted low calorie diets, or diets very low in carbohydrate, would have a higher release of cortisol during aerobics because of the restricted energy intake features of both diets. In addition, just lifting weights also boosts cortisol, particularly near the end of the workout. This workout-induced rise in cortisol, however, tends to occur more in less experienced trainees. With training experience, the body tends to adapt to regular exercise, and as a result less cortisol is produced during and after the workout session.
Another reason why there's a greater chance of losing muscle with aerobics is because muscle protein synthesis ceases during aerobics (and weight-training as well). The reason for this is that as far as the body is concerned, energy production is more vital than muscle protein synthesis. So the primary focus during exercise is maintaining energy to power the muscle activity. One way the body does this, besides cortisol release, is through the release of a protein called AMPK. AMPK functions as an energy sensor in muscle, and when levels of ATP decline in muscle, AMPK is secreted. Among other functions, AMPK promotes the use of fat as a fuel, which is a primary goal of aerobic exercise. But the other side of the coin is that AMPK also abruptly halts all protein synthesis. Since the body mantains a balance between anabolic and catabolic processes, if protein synthesis ceases during exercise, catabolic or breakdown processes begin to dominate.
But there may be a easy nutritional antidote to this dilemma. A new study had 10 subjects who ingested a protein drink containing 10 grams of protein, with either 1.87 or 3.5 grams of added leucine during a cycling test. Leucine is one of three branched-chain amino acids, and is the amino acid most associated with increased muscle protein synthesis. It does this by promoting the activity of another protein called mTOR. Another group of subjects drank an essential amino acid drink with no added leucine. Those in the leucine group showed a 33% increase in muscle protein synthesis. The leucine group also showed less muscle protein breakdown during exercise. It's important to note that this effect is mainly associated with aerobic exercise of extended duration, not weight-training. But it also means that those who fear losing muscle from doing aerobics while in restricted calorie or carb diets can supplement with leucine prior to the aerobic session, and not only not lose any muscle, but actually boost muscle protein synthesis by a third.
Pasiakos, SM ,et al. Leucine-enriched essential amino acid supplementation during moderate steady state exercise enhances leucine-enriched muscle protein synthesis, Am J Clin Nutr 2011;94:809-818.
©,2013 Jerry Brainum. Any reprinting in any type of media, including electronic and foreign is expressly prohibited.
