On average, American women gain about a pound a year between their mid-40s and their mid-50s—the time of life that matches up with the years just before and after menopause. It's enough to make a levelheaded woman suspicious: Is rising weight due to dropping hormones?

Probably not, says Barbara V. Howard, lead author of one of the most recent research papers to come out of the Women's Health Initiative (WHI), a multi-year, government-sponsored study of women's health in the years beyond menopause. "Gaining weight seems to be a trend in this country, in men and women," she says. "I don't think menopause adds anything special."

That news may be hard to swallow for women who find their bodies changing just as menopause arrives. But those extra pounds at midlife—which bring with them an increased risk of heart disease, diabetes and some cancers—are more likely due to aging than hormones.

As both men and women get older, the number of calories their bodies use declines. Scientists think this is due in part to a reduction in the body's lean muscle mass. Muscle, as anyone who's heard of resistance training knows, burns more calories than fat, even while people are at rest. Replace muscle mass with fat, and you burn fewer calories. For women, at least one recent study suggests that decreasing levels of hormones may help lower the number of calories burned at rest, too, but evidence for this is not yet conclusive.

"I think menopause is a convenient thing to blame," says Howard, whose most recent research, published in the Jan. 4 issue of the Journal of the American Medical Association, found that women who ate a low-fat, high-carbohydrate diet after menopause were less likely to gain weight than women who ate more fat. But diet probably is not the sole explanation for midlife weight gain. What really seems to be the culprit is people's tendency to turn down their level of physical activity as they get older. As Howard puts it, "If you look at that stage of life—the kids leave, you may be less busy at home—it's likely you become more sedentary."

Changing levels of hormones at menopause may have some effect on women's proportion of lean mass to fat, says Barbara Sternfeld, a senior research scientist at Kaiser Permanente of Northern California who has studied exercise and weight gain in women around the time of menopause. Hormones may also affect where that fat settles on the body—around the waist rather than a more even distribution. Those changes, she says, may give women the impression that menopause is the major cause of their newly padded bodies when, again, aging is a more likely explanation.

Everyone gets older, but that doesn't mean weight gain is inevitable. "What the studies so far suggest," Sternfeld says, "is that women who maintain or increase physical activity are going to see less of these body changes."

Sternfeld's research has found that women who are physically active have less body fat, maintain smaller waists and gain less weight than those who are not physically active. The WHI study co-authored by Howard found that the low-fat, high-carbohydrate diet recommended for years by government agencies, while out of favor during the low-carb diet craze, appears to help women maintain their weight, if not shed pounds. And the Healthy Women's Lifestyle Project, a five-year study completed in 1999 that followed women through menopause, concluded that women who both ate a low-calorie diet and got regular exercise lost weight, while women who were not on the diet and exercise plan gained an average of just over five pounds after one year.

Not that it's particularly easy to keep midlife weight off. The women in the diet and exercise group of the Healthy Women's Lifestyle Project ate about 1,300 calories a day and dedicated themselves to regular moderate-intensity exercise, such as walking briskly for 10 to 15 miles a week. In contrast, a 2004 analysis by the U.S. Centers for Disease Control found American women were eating just under 1,900 calories a day. And for years, the number of Americans who report they are physically active in their leisure time has held steady at just three in 10.

"The message is people need to move," says Sternfeld. "They need moderate intensity activity on most if not all days of the week for a minimum of 30 minutes." By moderate intensity, Sternfeld means an activity comparable to walking at a 20-minutes-per-mile pace.

"In certain respects, it's easier to keep at it as you get older," she says. "Your family responsibilities and household responsibilities may not be so pressing as when you have little kids."

It also may help to realize that your exercise time "is time for yourself that you should guard jealously," Sternfeld adds. "It will help you feel well and good about yourself, and to be strong, fit and have a lot of energy."

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