I have been weight-training for several years and have made gains on muscle
loss. A few months ago I started running as a means to shed some body fat
and I found out that I love it. I keep increasing my mileage, but I have
heard that exercising aerobically for extended periods of time may result in
muscle loss. The explanation for this was that after a certain amount of
time your body might start using muscle tissue as a source of energy, and at
the same time decrease production of a product call leucine (sp?), which
promotes muscle growth. Is there any truth in this? If so, is there a time
limit that I should place on my runs to avoid muscle loss? Thanks for your
help.
Cesar, it is unlikely that you would lose muscle secondary to sub-maximal
aerobic exercise. By definition, during the Krebbs cycle, aerobic exercise
results in the conversion of fat for the production of ATP's. You have enough
fat to be able to run for ages and ages at submaximal levels. Should your total
caloric intake be less than 12 calories per lean pound of mass, then you might
have a lack of calories, and in that case, to meet your total caloric need each
day, you might get into a state where you break down muscle tissue to feed your
baseline metabolic needs, more so if you also exercise for extended periods of
time. You have to sum calories in vs calories out to get a sense if you are
really at risk for the breakdown of muscle tissue.
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