If new research is to be believed, overweight and obese women have a lower risk of breast cancer prior to menopause.
The researchers admit the finding has puzzled them and agree that it does contradict conventional wisdom.
The researchers from Harvard Medical School are at a loss to explain why weight should protect premenopausal women from breast cancer, and do point that out in fact and also note that obesity actually increases the breast cancer risk after menopause, which is when the disease is most often diagnosed.
Karin Michels, an associate professor of obstetrics, gynaecology and reproductive biology and lead researcher in the study, says it is not a good public health message, and the last thing they want to do is to give women an excuse to be overweight.
The findings were based on medical data from 113,130 premenopausal registered nurses who were part of the Nurses' Health Study from 1989 to 2003, during which time, 1,398 cases of invasive breast cancer were diagnosed.
The researchers found that women with a body mass index of 30 or above (obese by the U.S. Centres for Disease Control and Prevention), had a 19 percent lower risk of premenopausal breast cancer.
This appeared to be particularly so in young adults, as those with a body mass index at age 18 of 27.5 or higher, had a 43 percent lower risk of breast cancer before menopause than women of normal weight at the same age.
The World Health Organisation considers obesity to be an ever-increasing problem in high-income nations as well as increasing numbers of low- and middle-income nations and being overweight is associated with a wide range of health risks.
Apparently some experts suggest that the reduced premenopausal breast cancer risk could be the result of such women not ovulating as much due to their larger body size and it is not uncommon for overweight women to have irregular or long menstrual cycles, or develop a condition called polycystic ovary syndrome in which ovaries malfunction.
Such conditions are linked to disruptions in ovulation that lower levels of certain hormones and this might explain the diminished breast cancer risk.
Michels team however say they accounted for such factors and concluded that they did not appear to be the cause and are planning further research to explain by which kind of mechanisms a larger body size might protect women from breast cancer.
Michels offers the speculation that obese women are less likely to be screened for breast cancer, and that is harder to detect tumours in these women and the diagnosis is delayed until post-menopausal years.
The research is published in the Archives of Internal Medicine.