Can you build muscle and lose body fat at the same time? For a natural athlete, the answer is probably not. To gain tissue, whether it be muscle or fat, you need to have caloric surplus. Then the question becomes whether the protein is integrated into regenerating muscle fibers that have been torn during workouts. That means that enough has to be eaten to get enough calories to get above basal energy expenditure as well as workout calories utilized. To burn fat, your body must utilize carbohydrate stores in the form of muscle and liver glycogen first and then fat can be metabolized. To lose fat, you must be in relative caloric deficit to utilize this pathway. In general, you can keep body fat stores about the same and gain muscle if you are very careful with diet and nutrition. In this way, you are not inhibiting your ability to generate muscle but there is so little caloric excess left over that body fat stores are not increased. Similarly, slow dieting cycles will spare more muscle tissue than faster dieting cycles.
There are many factors that influence this balance including natural hormone production, tissue receptor sensitivity, protein intake, micronutrient intake, sleep, and strength training regimens employed. There are also genetic and environmental components that come into play as well as relative age and overall health. It is important to distinguish the natural bodybuilder or strength athlete from the enhanced athlete using performance enhancing drugs like anabolic steroids and human growth hormone. Many of the physiques gained in this fashion are short-lived without continued exogenous hormone administration, which has numerous deleterious effects on overall health. In addition, some patients may have low testosterone or growth hormone levels and require hormone replacement therapy.
Dr. Kenneth Hughes, Los Angeles Plastic Surgeon