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

The researchers acknowledged that they do not know why the extra pounds 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 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.