A healthy human body contains a large amount of water and smaller amounts of fat and lean tissues. A small amount of body fat (about 4% in men and 10% in women) is essential for good health. However, too much body fat contributes to illnesses such as heart disease, diabetes, and high blood pressure. Regular exercise helps to keep body fat at healthy levels.

Lean tissue cells, especially muscle cells like the ones shown here, have a higher metabolic rate--they use more energy than fat cells do. In other words, lean tissue cells burn more calories than fat cells. That’s why you can increase your metabolic rate (and lose body fat) by exercising and increasing your muscle mass.

As we age, muscle cells wear out and die. These cells are not replaced, so people tend to lose muscle and gain body fat as they age. Although you may hear people say it, muscle doesn’t “turn into” fat—that’s impossible. To preserve as much muscle mass as possible, it’s important to exercise regularly throughout our lives.

“Spot reducing,” trying to lose fat in a particular body area or part by exercising, does NOT work. For example, climbing many flights of stairs each day will not eliminate body fat stored in the thighs. Exercising a specific muscle group, however, will improve your fitness and increase strength. So climbing all those stairs will make your legs stronger and may improve their appearance by increasing muscle mass.