Women who gain a substantial amount of weight at any age, but especially after 50, may have an increased risk of breast cancer, new study findings suggest.

On the other hand, researchers found, young and middle-aged women who lose weight may lower their risk of the disease.

The findings offer yet more incentive for women to avoid excessive weight gain as they get older, the researchers report in the August 1st issue of the American Journal of Epidemiology.

Their study, of nearly 2,000 women with or without breast cancer, found that weight changes over a lifetime appeared to influence a woman’s risk of the disease - particularly the odds of developing hormone-sensitive breast tumors, which rely on estrogen and progesterone to fuel their growth.

This, according to the study authors, supports the theory that excess body fat may raise the risk of breast cancer by elevating estrogen levels.

Moreover, Dr. Sybil M. Eng and her colleagues conclude, the study suggests “women can still modify their breast cancer risk later in life by avoiding weight gain.”

Eng, who is currently with drugmaker Pfizer, Inc., was at Columbia University in New York City at the time of the study.

She and her colleagues based their findings on data from a study of Long Island women ages 20 to 98, about half of whom had been diagnosed with breast cancer. All of the women were interviewed about potential risk factors for breast cancer; as part of the survey, they estimated their weight at age 20 and for each decade of age after that.

The researchers found that women who gained a significant amount of weight after the age of 20 were more likely to develop breast cancer than women whose weight remained fairly stable. Those who gained more than 33 pounds throughout adulthood had a 60 percent greater risk than their peers who remained within 6 or 7 pounds of their age 20 weight.

The greatest risk seemed to come from weight gain after age 50, according to Eng’s team. Compared with women whose weight was stable at this time, those who put on 25 pounds or more were 62 percent more likely to develop breast cancer.

In contrast, women who lost more than a few pounds throughout adulthood were about half as likely to develop breast cancer as those whose weight stayed largely unchanged - though weight loss after 50 did not seem to lower a woman’s risk of the disease.

Further analysis showed that the relationship between weight change and breast cancer was true only of women who had never used Hormone replacement therapy (HRT), which itself may raise the risk of breast cancer.

It’s possible, according to Eng and her colleagues, that HRT “masks” any effect of body fat on breast cancer risk, because hormone therapy would likely raise estrogen levels more than excess body fat would.

SOURCE: American Journal of Epidemiology, August 1, 2005.