Muscle and fat tissue

Some people[who?] incorrectly believe that muscle tissue will turn into fat tissue once a person stops exercising. This is not literally true — fat tissue and muscle tissue are fundamentally different — but the common expression that "muscle will turn to fat" is truthful in the sense that catabolism of muscle fibers for energy can result in excess glucose being stored as fat. Moreover, the composition of a body part can change toward less muscle and more fat, so that a cross-section of the upper-arm for example, will have a greater area corresponding to fat and a smaller area corresponding to muscle. This is not muscle "turning into fat" however — it is simply a combination of muscle atrophy and increased fat storage in different tissues of the same body part. Another element of increased fatty deposits is that of diet, as most trainees will not significantly reduce their diet in order to compensate for the lack of exercise/activity.