Women who gain weight progressively in the decades approaching middle-age may be at much higher risk for developing breast cancer after menopause, reports a study presented this month at the American Association for Cancer Research (AACR) annual meeting. It’s long been known that being overweight or obese increases the likelihood of developing breast cancer, but this study is the first to look at how weight gain over time affects cancer risk.

According to the AACR, an increase in BMI of 5 kg/m2 is about equivalent to a 30-pound weight gain for a woman who is 5’4”. Though this is a considerable amount of weight, it’s also important to note that it occurred over several decades.

The researchers, led by Laura Sue at the National Cancer Institute, followed over 72,000 women who were between the ages of 55 and 74 years old when the study began. Of these women, 3,677 developed breast cancer after the onset of menopause. The researchers did not include in their analysis women who had been on hormone replacement therapy, since this alone can increase the odds of developing breast cancer.

Sue summarizes the results: “[c]ompared with women who maintained approximately the same BMI, those who had an increase of 5 kg/m2 or more between age 20 and study entry had a nearly twofold increased risk of breast cancer.” According to the AACR, an increase in BMI of 5 kg/m2 is about equivalent to a 30-pound weight gain for a woman who is 5’4”. Though this is a considerable amount of weight, it’s also important to note that it occurred over several decades, since the women entered the study in middle-age. In this example, it is simply gaining a pound a year from age 20 to 50 if you are 5/4" (if you are taller, a 30-pound weight gain would affect your BMI less).

Breaking it down even further, the researchers found that women whose BMI increased the same amount (5 kg/m2) between the time they were 20 and 50 years old had an 88% increase in likelihood of developing breast cancer, compared to women whose BMI was constant. Even women who gained weight after the age of 50 were affected: their risk for breast cancer was upped by 56%, which is less than for women who put on weight throughout life, but still represents a serious increase in risk.

The authors conclude that “BMI gain both before and after age 50 independently contribute to increased risk of postmenopausal breast cancer.” Though it’s important to maintain a healthy weight for a variety of other reasons, this study provides even more evidence that doing so can help one reduce her risk of serious illness.