Scientists are calling for greater action to prevent obesity after a major study established that overweight and obese women run an increased risk of breast cancer that is not diminished by weight loss.

The study of more than 67,000 women in the United States, who were followed for a median of 13 years, confirms that excess weight is a real risk for breast cancer after the menopause. The paper, published in JAMA (Journal of the American Medical Association), highlights that two-thirds of women in the US, as in the UK, are either overweight or obese and therefore running a raised risk of breast cancer.

The findings “should motivate programmes for obesity prevention”, it says. The results “suggest that prevention of weight gain may be an important public health strategy for reducing breast cancer risk”.

The study, by Marian L Neuhouser of the Fred Hutchison Cancer Research Center in Seattle and colleagues, found that a woman’s risk of breast cancer increased if she gained more than 5% of her bodyweight, even if she was initially of normal weight (a BMI of 18.5 to 25), “but there was no change in risk for women who lost weight”.

The risk rises the more excess weight women carry. Those who are obese, with a BMI over 30, have a 58% higher chance of breast cancer than those of normal weight. Deaths among the most obese were also twice those among those of normal weight.



The findings come from the Women’s Health Initiative, launched in 1993 by the National Institutes of Health in the US to investigate ways to prevent major illness, including heart diseases and cancer, in women. It has also looked at hormone replacement therapy and diet.

In a commentary in the journal, Clifford Hudis of Memorial Sloan Kettering Cancer Center, New York, and Andrew Dannenberg of the Weill Cornell Medical College, New York, note that the National Cancer Institute recently suggested obesity will replace tobacco as the single biggest modifiable risk factor for cancer. “The danger of inaction is before us – a growing public health burden in the form of cancer and other ailments – but which action to take remains far from clear,” they write.

The finding that losing weight does not protect women from breast cancer is frustrating, they say.

“This challenges the simple suggestion that patients who are overweight or obese should just lose weight to reduce their cancer risk. Weight control (when achieved) may be very effective for many weight-associated illnesses and ailments, but the data suggesting that it will reduce an already elevated risk of breast cancer are limited.

“We need clinical trials to determine whether weight loss and body composition changes in overweight and obesity will reduce breast cancer risk.”

Because body fat is a significant source of oestrogen, the breast cancers in overweight and obese women are usually oestrogen-receptor positive. The paper says it is not clear at which stage in a woman’s life weight gain confers the greatest risk. During adolescence and pregnancy, breast epithelial cells undergo rapid division and differentiation, for instance, which might make them susceptible – but there are also changes during the menopause.

Katie Goates from Breast Cancer Campaign and Breakthrough Breast Cancer said: “It’s not yet possible to predict with certainty who will get breast cancer but there are proven steps women can take to reduce their risk, aside from maintaining a healthy weight.

“Limiting the amount of alcohol you drink and being regularly physically active can also help to keep BMI down and lower breast cancer risk – in fact, just 30 minutes of physical activity each day can reduce your risk of developing breast cancer by at least 20%.”