Liz Neporent is a diet and fitness expert and author of 12 fitness bestsellers. She regularly appears on national TV programs and is the president of Wellness 360, a New-York based wellness provider. You can also follow her on Twitter @lizzyfit.

If I stop exercising, my muscles turn to fat, right? Is this fit or fiction – Margie Semilof, Mass.

Photo: jupiterimages

Fat and muscle are two distinctly different substances. When you look at them under the microscope, fat looks like chicken coop fencing and muscle looks like frayed electrical wiring. If you stop exercising, your muscles will simply "atrophy," a fancy word for shrink. I think the muscle-turns-to-fat myth got started because so many former athletes gain weight after they hang up their athletic shoes. Basically, once they retire they eat too much and don't get enough exercise; they become poster children for the fit-to-fat crowd. If, for some reason, you do stop exercising, your muscles will simply deflate -- and your waistline will likely inflate.

But if your hard-as-steel muscles aren't magically melting down into soft, squishy fat, what does happen to your body when you gain weight? By way of explanation, a short biology lesson:

Your fat cells, known as adipocytes, multiply in number when you're a baby and during puberty. Once you become an adult, you produce new fat cells only during pregnancy (what we moms do for our kids!) or if you weigh at least 200 percent more than a healthy body weight. Otherwise, your existing fat cells simply expand in size, filling up with lipids (the form in which your body stores fat) until they reach capacity.

When you gain weight, your fat cells act like ity-bity storage balloons that swell as they fill up with more and more lipids. You're stuck with the same number unless you suddenly pork up to epic proportions, but those suckers really can s..t..r..e..t..c..h. Most people have between 25 and 35 billion fat cells, and women have more than men. Some obese people have as many as 75 billion. Scientists think that the more adipocytes you have, the more difficult it is to lose weight and the more weight you are capable of gaining.

When you lose weight, the fat cells do shrink down in size but the only way to decrease your fat cell count is to have liposuction. Once you've had fat cells sucked out of you, science says they probably don't, but possibly might, grow back.

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