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Mass Exodus continued...

"Metabolism is based upon three different factors," Peeke tells WebMD. "The first factor is genetics. We're good, but we can't fudge with that yet -- give us time, however.

"Number two is thyroid function, and interestingly enough, here's where we get gender specificity. Women have much greater thyroid issues than men, by a at least 10 to 1, and it's quite gradual, so women may find that they're losing some of that metabolic edge during their 40s also because thyroid issues begin to spring up."

The third factor affecting metabolism, Peeke says, is muscle mass. In the 40s and beyond, "lifestyle changes rather dramatically and it's sort of a keen grasp of the obvious that everyone's sitting on their ass. So what's happening is if you don't use it, you lose it, and in your 40s you don't just lose it, it melts."

Recent research suggests that women on average will lose muscle mass twice as fast as men the same age, and that can make a huge difference in their ability to lose or at least maintain weight, Peeke says. Muscle is far more "metabolically active" than fat, meaning that lean, more muscular people have an easier time burning calories at rest than to people with higher proportions of body fat.

"Let's say I've worked out at the gym and I have a new pound on board, or, for that matter, I take an old muscle mass on me that's untrained and now I train it and preserve that pound. That muscle mass may now burn between 35 to 50 calories extra a day, versus the same pound of fat, which would burn anywhere from 5-10 calories a day.

"So it's extremely important to know that muscle is very metabolically active and that you don't want to lose it. That being said, a typical can man can lose over the course of the age of 30 through the age of 50 anywhere between 5 and 10 pounds of muscle mass. A woman could definitely lose that -- that's a given because she, through repeated dieting and decreased physical activity, will lose that," Peeke says.

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