The changes in a woman’s metabolism that can cause weight gain after menopause actually start years earlier, a recent study suggests.

Researchers found that the greatest increases in the overall percentage of body fat and decreases in lean muscle mass occurred during perimenopause, a transitional phase that typically starts when women are in their 40s.

“Perimenopause, which is during middle age, is the most valuable time to make some lifestyle changes,” said Abbie Smith-Ryan, an associate professor of exercise physiology and the co-director of the Human Performance Center at the University of North Carolina, Chapel Hill, who is the co-author of the study published March 2 in the journal Menopause. “It’s also a time where I find women tend to neglect themselves the most because they are taking care of everyone else.”

Perimenopause can be accompanied by irregular periods, fluctuations in estrogen and changes in mood and sleep. It’s also when many women notice that fat starts collecting in their bellies, even though they’ve neither cut back on exercise nor increased calorie intake.

By the time women get to menopause, which begins about age 51 for many, it’s harder for the body to use fat for fuel, Smith-Ryan said.

“We think the largest changes are in the muscles,” she added. There is an increased resistance to muscle building as women approach menopause, she explained. “Some of this has to do with the muscle not being as sensitive to the protein we consume and the weights we lift as the transition occurs,” she said.

And that means women in perimenopause need to consume more protein and lift heavier weights for calories to turn into muscle rather than flab.

To look at the impact of perimenopause and menopause on body composition and on energy expenditure during moderate exercise, Smith-Ryan and her colleagues recruited 72 women between ages 35 to 60. Twenty-four were premenopausal; 24 were perimenopausal; and 24 were postmenopausal.

At the outset, the researchers asked their study volunteers to fill out questionnaires on diet, activity and sleep.

Next, the women’s body composition was evaluated. When the researchers compared the three groups, they found that women in the perimenopausal group had the most unfavorable composition overall — more weight accumulated as fat rather than muscle.

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The perimenopausal group also had greater fat deposition in the abdominal region.

During moderate exercise, the postmenopausal group was the least able to use fat to power muscles. The researchers found that negative changes in metabolism and body composition peaked during the perimenopausal period.

But, perimenopause was also the time when the women’s metabolisms were most flexible, most likely to be affected by changes in diet and exercise, they found. In other words, women could see the most benefit from improving their diets — raising the ratio of protein to carbs — and bumping up both aerobic exercise and weightlifting.

Although the new study is small, there are messages women can take from it, said Nancy Reame, the emeritus Mary Dickey Lindsay professor of health promotion and risk reduction in the Columbia University School of Nursing.

“Perimenopause is the time when you should start paying attention, if you haven’t before, to your body and to your metabolism,” she said. “It’s a time of slowing down. And a time to think about weight gain and diet and nutrition in ways that can help you eat well and have more energy.”

Reame suggests women start tracking their periods when they are in their mid-30s so they can anticipate perimenopause. “It’s a way of engaging with your body,” she said. “You can look at it as one of the biomarkers of the beginning of changes.”