Gaining a Pound a Year After Age 20 Nearly Doubles Women's Breast Cancer Risk

7 ways to keep your weight steady as you age.

Video: What Is Breast Cancer?

Gaining a pound or two a year after age 20 is the norm for most Americans, which explains why two-thirds of us are overweight by the time we hit our 50s. Not only does that put us at increased risk of heart disease and diabetes, but it can also increase a woman's chances of developing postmenopausal breast cancer. A new National Cancer Institute study of 72,000 women found that those who had a normal body mass index at age 20 and gained through the decades to become overweight—an increase of at least 5 BMI units, which is equivalent to a 30- pound gain for a 5- foot, 4- inch woman­—had nearly double the risk of developing breast cancer after menopause compared to women who kept their weight steady as they aged. (The average 60- year-old woman's risk of developing breast cancer by age 65 is about 2 percent; her lifetime risk is 13 percent.)

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"Weight gain is a major risk factor for breast cancer," and could play as much of a role as other known risk factors, like family history of cancer, or the age at first menstruation or childbirth, says study coauthor Regina Ziegler, an epidemiologist at NCI. That's probably because the accumulation of excess body fat over time increases the level of estrogen in the body, which is thought to fuel the growth of most postmenopausal breast cancer tumors. Interestingly, the study also found that women who started off overweight or obese at age 20 didn't have any increase in breast cancer risk, which contradicts other research showing that obesity increases the risk of postmenopausal breast cancer as well as several other cancers including colon, endometrial, kidney, and esophageal, according to the NCI's website.

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Unfortunately, few women are able to maintain their post-college dress size, a testament to how tough it is to prevent that mid-life bulge. (Nearly 57 percent of the study participants failed to do so.) "As you progress through mid-life, you'll find your metabolism naturally slows down," says Jana Klauer, a New York City physician and nutritionist specializing in obesity treatment and author of How the Rich Get Thin. In other words, if you maintain the same Big Mac habits you had in college, you'll pack on pounds. So what does it take to keep the scale steady? "A lot of effort," says Klauer. Here are her 7 steps for beating the odds:

1. Cut back on calories. "Your metabolic rate peaks in your 20s," says Klauer, because your body is still adding bone mass and churning out a lot of hormones to keep you fertile. After age 30, your metabolism slows by about 5 to 7 percent per decade. That means if you were eating about 2,500 calories per day to maintain your weight in your 20s, you'd have to eat about 125 to 175 fewer calories each day to keep the scale from inching upward. By the time you hit your 50s, you'd need to cut back by 300 to 500 calories a day to keep the same waistline you had in your 20s.

2. Sweat, sweat, sweat. If you don't want to cut your food intake too much, increase your calorie-burning activities. Those who aren't counting calories probably need to exercise for about an hour each day, every day of the week to truly keep the pounds off, according to a recent study published in the Journal of the American Medical Association. The researchers looked at 34,000 nondieting women (average age 54) and found that those who worked out for an hour a day—the equivalent of a brisk walk—kept themselves slim through the years. While those who worked out less gained weight, they still benefitted from lower risks of heart disease and diabetes, compared to their sedentary counterparts.

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3. Lift weights. Muscle burns more calories than fat, so building muscle helps keep your metabolism revved up as you age. Unfortunately, your body begins to shed muscle in your 40s if you don't do anything to maintain it. Doing resistance training with free weights or weight machines at the gym three or four days a week can go a long way towards helping you retain muscle and boost your metabolism. Klauer recommends hiring a personal trainer for three or four sessions to learn the proper form and prevent injuries.