My interest is fat reduction. The owner of the gym told me that the first few minutes of my run burnt off residual ( temporary energy ) and then the next 20 minutes, my body would take energy from stored fat. I was in agreement with him up to this point, until he said that after that time, my body would switch from using fat for energy to using muscle tissue for energy. And so I should not do an aerobic exercise for more than 40 minutes due to my body type. ANYBODY HEARD OF THIS ONE???
Larry DeLuca, EdM, CSCS
In order to maintain glucose homeostasis during exercise, amino acids can be catabolized as part of the process of gluconeogenesis. Estimates of the contribution of amino acids to total energy expenditure range from 5% to 10% of total energy. This becomes more of a factor during sustained endurance exercise, especially if one is trying to maintain a more competitive intensity (e.g. trying for a personal best in the marathon rather than a easy 10-mile jog). But, like everything else in exercise, this is an *acute* response that is temporary and which does not affect long-term body composition independent of the other factors that occur in one's daily life (overall activity, diet, etc).
I guess what I am trying to do is point out that most exercise *myths* are not complete fairy tales. They often have some basis in fact, but are *micro* processes that are *misgeneralized* into *macro* changes.
The body does not *switch* anything on or off, as a rule. Everything exists in a type of dynamic balance. Exercise temporarily disrupts that balance, but after the exercise session, the body reestablishes equilibrium. It is possible to kind of push that dynamic balance into a different direction so that the body reestablishes the equilibrium at a different level, but that takes sustained changes in either an exercise routine, overall lifestyle, or both.
Just like your body replenishes glycogen stores following exercise, it will replenish amino acids as well. While there is evidence that suggests that sustained endurance training can negatively affect increases in muscle mass, the average fitness exerciser is not going to notice any difference--especially if one follows a balanced diet of healthy foods.