So slow weight loses are perhaps the best. We lose much less muscle that way. Of course, as described, the decreased energy state of the cell due to either a negative energy balance created by caloric restriction or a negative energy balance created by exercise is the most obvious cause. Also the rise in glucocorticoids, reduced dietary protein, and the decreased infusion of leucine cause higher rates of protein degradation. Once again, it is obvious that calories are most important.
Exercise decreases protein synthesis and it also increases protein degradation.
However, if there is not enough energy to the cell, as in enough calories, protein synthesis is slowed or stopped and degradation rates are increased. So exercise might well help to protect or slow degradation, by causing increases in synthesis afterwards. However, it doesn't stop degradation in periods of negative energy, because rates of protein synthesis are very dependent on caloric intake and, once again, energy to the cell. Newsholme, E.A., et al. Low carbohydrate availability impairs high rates of aerobic energy production because the shift to fats as the predominant fuel leads to either a reduction in the rate of acetyl-CoA units or an inability to maintain an adequate level of Krebs cycle intermediates.