Insulin could be one of many factors in the development of hormone-negative cancer, say investigators on two major breast cancer studies, the California Teachers Study (CTS) and the Women’s Intervention Nutrition Study (WINS).
The CTS found that moderate-to-strenuous exercise is a far more potent deterrent to hormone-negative cancer than it is to hormone-positive. Insulin sensitivity might be why, says Leslie Bernstein, PhD, one of the researchers on the study.
“Women who exercise regularly have lower levels of insulin in their blood,” says Bernstein, director of the Department of Cancer Etiology at the City of Hope National Medical Center. “They are also more likely to maintain normal body weight.”
Rowan T. Chlebowski, lead WINS researcher, says insulin might be one reason women with hormone-negative cancer benefit more from a low-fat diet than women with hormone-positive. “Reducing dietary fat reduces insulin and insulin resistance,” says Chlebowski, M.D., Ph.D., of the Los Angeles Biomedical Research Institute.
While the WINS research focused on diet, Chlebowski speculates that weight loss and exercise may be the real keys in reducing risk of recurrence, and both are related to insulin. He is now working on a clinical trial to study the effects of walking a minimum of three hours a week and losing at least moderate amounts of weight.
Neither study connects breast cancer and diabetes and neither suggests that insulin sensitivity causes hormone-negative breast cancer. The basic message here is that everything in the body is connected in some way, and that an imbalance in one area can cause an eruption in another, with cancer as one possible—but not inevitable—result of this internal gurgling.
All aspects of a healthy lifestyle are important in limiting cancer risk, Bernstein says, but the real key is physical exertion. “I think it’s the exercise and I think it works alone,” she says. ‘If it works through weight, it is the exercise that does it.”
Bernstein and her colleagues studied more than 100,000 women over a ten-year span as part of the California Teachers Study . The longer and more strenuously a woman exercised, the bigger the reduction in hormone-negative breast cancer risk, with moderate activity such as brisk walking at least 3.8 hours a week showing significant benefit. The greatest benefit came with more than five hours weekly of strenuous activity such as running.
Roughly 40 percent of the the WINS 2,500 participants followed a diet that kept fat at about 20 percent of their daily calories, an average of 33 grams; 60 percent followed a normal diet, with an average of 51 grams of fat. Cutting dietary fat reduced the risk of recurrence of hormone-receptor-negative by 42 percent after five years. The women, who were all postmenopausal, did not change their activity level; most lost weight.