A new study confirms what menopausal women have known all along: The change of life can make you fat.

For years, scientists have acknowledged the fact that women have a tendency to pack on extra pounds at menopause. Until recently, however, their research failed to show that dwindling levels of the female hormone estrogen were primarily responsible for the weight gain.

The results of a new animal study conducted by researchers at the University of Texas Health Science Center at San Antonio suggest that, as suspected, estrogen deficiency can lead to the accumulation of excess weight.

After surgically removing the ovaries of aged female rats, the researchers found that these "menopausal" rodents gained twice as much weight as aged female rats with functioning ovaries. When estrogen replacement was given to rats that had their ovaries surgically removed, there was no such increase in weight.

Blood tests revealed that the menopausal rats had high levels of leptin, a hormone produced by fat cells that is known to play a major role in the regulation of body weight and appetite. High levels of leptin correlate with increased body fat in both humans and rodents.

Among the menopausal rats receiving estrogen replacement, leptin levels were roughly 70 percent lower, similar to the levels measured in the female rats with functioning ovaries.

In a series of related animal experiments, University of Cincinnati researchers uncovered additional clues supporting the role of estrogen in the prevention of obesity. The scientists demonstrated that estrogen receptors located in the brain collectively function as a master switch to control food intake, energy expenditure and body fat distribution.

Applied to women, these findings suggest that supplemental estrogen could help reduce appetite and prevent some of the weight gain associated with menopause. Although many menopausal women would welcome the weight-related benefits of hormone therapy, many are leery of the associated side effects.

The results of the Women's Health Initiative, the largest study to date to test the effectiveness of hormone therapy, suggest that treatment with estrogen and progestin increase the risk of heart attack in postmenopausal women. After the findings of the study were released in 2002, many physicians began advising their female patients to avoid taking supplemental hormones.

Follow-up studies found that the risk of heart attack is greatest in women who start taking hormones 10 years or more after menopause. In light of these findings, hormone therapy is now conservatively recommended for the short-term treatment of menopausal symptoms.

In addition to hot flashes, night sweats and poor-quality sleep, many menopausal women experience significant increases in appetite and corresponding weight gain. On average, women accumulate an extra pound each year after the age of 40, with the greatest increases in weight occurring during menopause.

As weight creeps up in menopausal women, body composition and fat distribution patterns typically begin to change. With each passing year, the percentage of body fat tends to increase, as the percentage of muscle mass tends to decline.

While pre-menopausal women often struggle with fat deposits in the "saddlebag" portion of their anatomy, much of the newly acquired fat in menopausal women is deposited around the waist. As it turns out, excess fat deposits in the abdomen are far more worrisome than a little extra padding around the hips and thighs.

Abdominal obesity is known to increase the risk for a number of dangerous diseases. In women, a waist circumference of 35 inches or more significantly increases the chances of developing type 2 diabetes, heart disease and certain cancers.

Accumulating additional inches and pounds during menopause is rather common, but it isn't necessarily inevitable. Most women can avoid significant weight gain and reduce their risk of developing serious health problems by exercising more and eating less.

At the age of 50, many women require roughly 200 fewer calories per day than they did at the age of 20. Although exercise can help prevent weight gain, a number of studies show that postmenopausal women tend to be less physically active than pre-menopausal women of the same age.

Just 30 minutes a day of moderate-intensity activity is all it takes to reap a number of important health benefits.

While hormone therapy may help some menopausal women avoid packing on extra pounds in the short-run, the treatment isn't recommended for long-term use. For menopausal women who are determined to stay as fit and as healthy as possible, positive changes in eating and exercising habits are likely to yield the greatest success.

Rallie McAllister, M.D., M.P.H., is a family physician in Kingsport, Tenn., and author of "Healthy Lunchbox: The Working Mom's Guide to Keeping You and Your Kids Trim." Her Web site is http://www.rallieonhealth.com. To find out more about Rallie McAllister, M.D., and read features by other Creators Syndicate writers and cartoonists, visit the Creators Syndicate Web page at www.creators.com.

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