Between the late 30s and late 40s, it's not uncommon for both men and women to gain 10 pounds. The gain may come on relatively suddenly, or more gradually, at a general rate of one pound a year, even though you haven't made any major changes to your diet or exercise routine.
In men, body weight tends to increase until the mid-50s, then begins to decrease, according to the American Geriatric Society (AGS). Women's body weight tends to increase until the late 60s, then decreases, although at a slower rate than for men.
People in less high-tech cultures don't show this pattern of weight change, the AGS says. This could mean that it's not the aging process in itself that causes the weight gain, but changes in eating or activity levels.
In women, hormonal shifts in perimenopause start the process. For women, as menopause approaches the ovaries begin to produce less estrogen, a powerful chemical that influences a variety of physiological factors, including mood, blood cholesterol and bone mineral density. Meanwhile, fat cells begin to take over the ovaries' job.
"Women don't stop making estrogen; they simply start making it in fat cells, particularly those in the abdominal area, which are the largest sites for estrogen production," says Pamela Smith, R.D., author of When Your Hormones Go Haywire.
In women who have gone through menopause, the biggest factor in weight gain appears to be less physical activity, according to the journal Postgraduate Medicine .
With age, both men and women gradually lose muscle mass at an average of a half-pound a year and gain that much in body fat, according to Tufts University. The loss of muscle mass at midlife is even more pronounced in men because of the loss of testosterone, a male hormone that regulates muscle mass and strength. In both men and women, however, the loss of muscle mass and strength usually leads to becoming less active.
The key to keeping excess pounds at bay at any age is a healthy diet and plenty of exercise, including strength training.
Dieting by slashing your calorie intake or manipulating your diet's ratio of fat, protein and/or carbohydrates to shed pounds is perhaps the worst thing you can do to counter midlife weight gain.
"If you try to fight it by dieting, your fat cells are going to fight back even stronger by trying to conserve the calories you do take in even more," says Debra Waterhouse, R.D., author of Outsmarting the Midlife Fat Cell.
Moreover, cutting calories drastically isn't effective because even if you lose weight, you're apt to gain back twice as much, Ms. Waterhouse says.
Altering your intake of fat, protein or carbohydrates also can further upset the balance your body needs at this time, inviting more hormonal hurricanes.
"Cutting fat from your diet, for example, can threaten your fat cells, causing them to grow larger by 20 percent," says Ms. Waterhouse. On the other hand, consuming too much fat (more than 30 to 35 percent of your total daily calories) also can lead to weight gain since fat is a concentrated source of calories.
The solution is to make sure your diet contains small amounts of unsaturated, heart-healthy fat, which is found in olive and canola oil, fish, avocados and nuts.
Your diet also should contain a mix of fruits, vegetables, whole grains, lean protein and low-fat foods rich in calcium.
Don't wait to weight train
If you haven't started a strength-training routine yet, now is definitely the time to start a two- or three-day-a week program.
"Strength training can be the very thing that stops the weight-gain process from getting out of control," says Ms. Smith.
Weight training helps boost metabolism by reversing the natural loss of muscle mass that's otherwise part of the aging process. More muscle mass on board means your body burns calories at a faster rate, even when you're at rest.
Try mini meals
To boost metabolism (the rate you burn calories) and control hormonally related cravings, it's also important to eat small, balanced meals or snacks about every three hours.
"Eating that frequently helps prevent overeating, and helps maintain optimum blood sugar levels to keep your energy level up and your mood more stable," says Ms. Waterhouse. "Keeping your diet and exercise in balance is really the key to maintaining a healthy weight."