Overweight women’s breast cancer risk lower: study

WASHINGTON (Reuters) - Overweight and obese women have a lower risk of breast cancer prior to menopause, researchers said on Monday in a finding they said both puzzles them and contradicts conventional wisdom.

The researchers admitted they do not know why the extra pounds (kg) may protect premenopausal women from breast cancer, but noted obesity actually greatly boosts breast cancer risk after menopause, when the disease more often is diagnosed.

“It is so puzzling. And it is not a good public health message,” said Karin Michels, associate professor of obstetrics, gynecology and reproductive biology at Harvard Medical School and lead researcher in the study.

“I don’t want women to use this as an excuse to be overweight. Therefore, it’s even more important for us to find out what the mechanisms are. I mean, the last thing we want is, in this day and age, to advise people to gain weight,” Michels said in a telephone interview.

The findings, published in the Archives of Internal Medicine, were based on medical data from 113,130 premenopausal registered nurses tracked from 1989 to 2003. During that time, 1,398 cases of invasive breast cancer were diagnosed.

Women with a body mass index (a weight-for-height scale) of 30 or above — considered obese by the U.S. Centers for Disease Control and Prevention — had a 19 percent lower risk of premenopausal breast cancer than women in a normal weight range (body mass index between 20 and 22.4), the study found.

The lower risk was especially evident in young adults. Those with a body mass index at age 18 of 27.5 or higher, which makes them overweight or obese, had a 43 percent lower risk of breast cancer before menopause than women of normal weight at the same age.

Being overweight is linked to a broad range of health risks. The World Health Organization describes obesity as a growing problem in high-income nations as well as increasing numbers of low- and middle-income nations.

OVULATION FACTORS

Michels said some experts had suspected the reduced premenopausal breast cancer risk was the result of these women not ovulating as much due to their larger body size.

Some overweight women have irregular or long menstrual cycles, or develop a condition called polycystic ovary syndrome in which ovaries malfunction. These are linked to disruptions in ovulation that lower levels of certain hormones.

The suspicion had been that these lower hormone levels might explain the diminished breast cancer risk. But the researchers weighed these factors and concluded that they did not appear to be the cause.

“Now we’re back to square one in trying to explain with which kind of mechanisms a larger body size might protect women from breast cancer,” Michels said, adding she plans further research.

She speculated the findings might be explained by the fact that obese women are less likely to be screened for breast cancer, and that is harder to detect tumors in these women.

“If we just detect the cancer later and therefore delay the time of diagnosis of the cancer into their post-menopausal years, then that might be an explanation,” Michels said.

Michels said the link between weight and breast cancer risk varies by age. High weight at birth and then after menopause is linked to a heightened risk, while high weight in young adulthood is associated with a reduced risk, she said.