Losing lean muscle after age 25?

Your personal trainer would be wrong. Muscle is a totally different tissue type than a fat cell. Muscle cells don’t become fat cells, nor the other way around. A person’s number of fat cells is a constant, and you simply gain or lose muscle mass, typically by breaking down the cells & then they rebuild after a work out, following the restoring of the glycogen that makes up the bulk of the weight of the muscle tissue, which serves as the food for those cells.

What probably happened in your case is that you gained size in your fat cells, which gives you more fat, and you lose glycogen stores in your muscle tissue, because as your body no longer needed that energy store, it was consumed. Between the consuming of the glycogen & the lack of exercise, your body transformed a higher allocation than previous of your daily diet into fat stores.

Hope that helps explain things, muscle doesn’t turn into fat.