Another old wives tale. As far as I am still aware, it is physically impossible to turn muscle tissue into fat tissue. Kind of like trying to turn glass into gold, It just can\'t be done.
So why do some people still think that this is possible? Maybe its down to a lack of education and probably a lot to do with "word of mouth"? And a little of what the eye sees!
People use former athletes, bodybuilders and even regular exercisers that have lost their athletic look as an example, when they ask the question, "why do people get fat when they stop exercising"? However, they have not thought about the actual changes of lifestyle that have taken place that lead to this perception of muscle turning into fat.
Anyone who exercises at any level will also have a nutritional requirement to aid the body to deal with the extra stress that it is put under during the period of exercise. Also, the body requires the right balance of nutrients to help in the repair process after periods of exercise.
When someone appears to be less toned, to have put on weight or turned fat, then this is generally down to a couple of factors.
Firstly, If the calories you consume whilst on an exercise program is maintained when you stop doing that exercise program then you will put weight on.
Secondly, If you stop training, over time, the muscle tissue will diminish (atrophy - http://en.wikipedia.org/wiki/Muscle_atrophy). Assuming you do not change your eating habits to account for this change in lifestyle, the extra calories consumed will be stored as fat! Creating a false impression that the muscle tissue has turned into fat.
The rules are still, in general, as simple as 1,2,3:
1. If you consume more calories than you expend then you will get fatter.
2. If you manage to consume the same as you expend then you will roughly remain the same weight.
3. If you consume less than you expend then you will lose weight.
As always there are certain factors that might change the equation but on simple terms it kinda works!
So the answer is simple, lead a balanced lifestyle with a healthy diet and frequent periods of exercise and you will look fine, feel great and chances are, live a better quality of life for a lot longer!