KEY POINTS:
Overweight and obese women have a lower risk of breast cancer before menopause, researchers say in a finding that both puzzles them and contradicts conventional wisdom.
The researchers admit they do not know why the extra kilos may protect pre-menopausal women from breast cancer, but note that obesity
actually greatly boosts breast cancer risk after menopause, when the disease is more often diagnosed.
"It is so puzzling. And it is not a good public health message," said Karin Michels, associate professor of obstetrics, gynaecology 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, in this day and age, is to advise people to gain weight," Professor Michels said.
The findings, published in the Archives of Internal Medicine, were based on medical data from 113,130 pre-menopausal registered nurses tracked from 1989 to 2003.
During that time, 1398 cases of invasive breast cancer were diagnosed.
Women with a body mass index of 30 or above - considered obese by the US Centres for Disease Control and Prevention - had a 19 per cent lower risk of pre-menopausal breast cancer than women in a normal weight range (BMI between 20 and 22.4), the study found.
The lower risk was especially evident in young adults. Those with a BMI at age 18 of 27.5 or higher, which makes them overweight or obese, had a 43 per cent 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 Organisation describes obesity as a growing problem in high-income nations as well as increasing numbers of low- and middle-income nations.
Professor Michels said some experts had suspected the reduced pre-menopausal 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," Professor 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 it is harder to detect tumours in these women.
- REUTERS
