Long periods of inactivity will cause your muscles to shrink! Build and maintain your muscles, use it or lose it or however you want to say it. Your muscles will shrink (atrophy) if you don't continue to do strength training (muscle does not ever change into fat). And, the body fat will come back with a vengeance! Fat cells will shrink but they will fill up again when you are inactive for a long period of time.
Scientists have learned that abdominal fat cells secrete inflammatory proteins that cause damage to blood vessels and other cells. These inflammatory chemicals are also implicated in the development of coronary artery disease and Metabolic Syndrome!
If you are not exercising, you could be hit with a double dose of likely bad news. First, you will probably gain weight (interpreted fat). Also, your metabolism begins to slow down in your early thirties. Your metabolism will slow down even more because you are also losing muscle mass. Muscle mass speeds up your metabolism because your body has to work harder to maintain it.
In your early thirties, you can begin to lose as much as one-half pound of muscle each year. Inactivity will make this problem even worse. So, you begin to lose muscle mass and gain body fat.
Rapid weight loss can trick you into thinking that your fat cells are shrinking. When you have rapid weight loss through severe calorie restriction, much of that weight loss will be water weight. So, you find yourself regaining this weight quickly when you begin to eat the carbohydrates, proteins and fats that your body needs. Why? Because you will also replenish water stores in your body's cells.
Fortunately, muscle mass can be maintained/increased (and body fat decreased) throughout your life with regular weight training (includes bodyweight exercises). Weight training also helps to keep your bones strong (preventing osteoporosis).
Also, control your calorie intake by eating smart and often. Eat protein with every meal. Protein helps you feel fuller for a longer period of time, helps keep blood sugar levels from spiking and repairs your muscles after a tough workout.