This is a common misconception many people have. No matter how firmly you may believe it however, muscle will never ever turn to fat. Conversely, fat will also never turn to muscle.

I know what you’re going to ask.

Why do bodybuilders (or anyone with a lot of muscle) get so fat when they stop working out?

The answer is quite simple. To gain muscle, a person must consume more calories than they expend (or burn). People who have a lot of muscle, consume a lot of food. They must do this because their metabolism is higher (from the amount of muscle they have and exercise they perform), and therefore their caloric intake must also be higher.

When a person with a diet this large decides to stop working out, they often won’t modify their diet accordingly.

As day’s go by, and the metabolism begins to slow (due to lack of exercise), but the calories consumed stay constant at their high numbers, a person will begin to gain fat.

This isn’t to say the person will lose all their muscle (because they won’t), but it’ll be hard to see under the fat which is covering it.

The moral of the story: if you’re going to stop exercising, modify your diet accordingly, so you’re burning equal or less calories than you consume.