Flab at 40 (cont.)

Mass Exodus

When it comes to pinning blame on changes in metabolism there are handful of prime suspects, says Pamela Peeke, MD, assistant clinical professor of medicine at the University of Maryland School of Medicine in Baltimore, who specializes in nutrition and stress, particularly among adults on the far side of 40.

"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.

Old Wives' Tales?

Of course, if you wait long enough, say about 25 years, the weight gains that started to accelerate may begin to reverse themselves, says a researcher who studies metabolism in people in their 70s, 80s, and beyond.

"People tend to gain weight steadily, on average -- not everybody -- and get more fat and tend to lose lean mass up to about age 65, and then what happens is that there's a downward trend: Now people start to kind of slowly lose weight -- again, not everybody, but the trend is that as you get older -- the general population I see is in the 70s and 80s -- they tend to lose weight," says Michi Yukawa, MD, MPH, acting instructor in the department of medicine and the division of gerontology and geriatric medicine at the University of Washington in Seattle.