Dear Doctor: I recently read an article that said even a little bit of weight loss -- just 5 percent of your total body weight -- can lessen your risk of breast cancer. Why is that? Does it hold true for women who are of normal weight?
Dear Reader: The link between being overweight and breast cancer risk is somewhat complex. Having more fat tissue is associated with higher levels of the hormone estrogen. This, in turn, has been connected to an increase in the risk of breast cancer. That extra weight can often result in higher levels of insulin, which has also been linked to an increase in breast cancer risk. Weight gain is also associated with a rise in inflammation, though whether this plays a role in cancer is still being studied.
Meanwhile, some research has connected this rise in breast cancer risk to excess weight that was gained in adulthood but finds that it may not apply to women who were overweight or obese as children. And to top things off, as these studies become deliberately more inclusive and diverse, it appears that ethnicity and race also play a role in whether or not excess weight adds to an individual's breast cancer risk.
The study you're referring to comes from City of Hope, a cancer treatment and research center here in Southern California. The impetus was to learn if weight loss might reverse the increased risk of breast cancer in women who were overweight or obese. The scientists also wanted to know whether the timing of that weight loss would matter.
They drew from data compiled by the Women's Health Initiative, a long-term study of health outcomes in older women overseen by the National Institutes of Health. The 61,000 women in the breast cancer study, all post-menopause, had normal mammograms at the start of the 11-year period of the study.
The researchers compared the health data of women who lost (and maintained the loss) of at least 5 percent of their total body weight with the health data of those whose weight remained the same. One of the takeaways, as you mentioned, was that the 5 percent weight loss was associated with a reduced breast cancer risk.
Unfortunately, this study doesn't answer your question about weight loss and a reduction of breast cancer risk among women who are not overweight. The women in the study who lost weight started out with an average body mass index, or BMI, of 29. That's deep into the overweight category, which is a BMI between 25 and 29, and bumping up against the lower threshold of obesity, which is a BMI of 30 and above.
However, body weight isn't the only factor to consider. The presence of abdominal fat, independent of body weight, has been linked with an increased risk of several types of cancer, including colon, rectal and pancreatic cancers. It's also a risk factor in a number of metabolic diseases.
Our advice is to reduce your weight and your middle with a healthful, whole-food diet and regular exercise. And, if you're regulars here, you pretty much know what's next: Please, no smoking.