Very common question that I get asked is the relationship between muscles and fat. Are they the same? Can they convert into each other? If I stop training will my muscle turn into fat? If I train will my fat turn into muscle? All good questions. There is a simple and scientifly based answer.
Muscles cannot turn to fat. When you exercise, your muscles become larger and stronger because exercise causes extra protein building blocks (amino acids), to deposit in muscles. All day long, amino acids pass from your muscles into your bloodstream and then back into muscles. Exercise forces amino acids back into muscles.
If you stop exercising, a reduction of amino acids go into your muscles. The results of this is your muscles get smaller. Your body is unable to store extra protein, so amino acids that are not moved to your muscles move to your liver, which uses them for energy or converts them into fat for storage. So if you stop exercising, you have to eat less or you will gain weight, likely fat.