This study investigated whether long-term or short-term physical activity before diagnosis of breast cancer improves subsequent survival. The study included 3,539 originally cancer-free women in the California Teachers Study who were eventually diagnosed with invasive breast cancer. The women had provided information during 1995-1996 concerning both long-term (high school through age 54) and recent (past three years) participation in recreational physical activities. Level of physical activity was grouped into low (half an hour or less per week of any activity), intermediate (0.51 to 3.0 hours per week of moderate or strenuous activity but no activity greater than 3.0 hours per week), or high activity (greater than three hours per week of either moderate or strenuous activity). Women with intermediate or high levels of long-term physical activity before diagnosis were found to have lower risk of death from breast cancer than women with low activity levels regardless of estrogen receptor status and breast cancer stage. However, the results held only for overweight women.

Why should exercise make survival more likely for overweight women in particular?

The study authors appear to present the conclusion that long-term exercise provides a survival advantage only for overweight women somewhat reluctantly. However, it is worth examining the possible reasons why such a finding could be valid. It is possible that physical activity could benefit overweight women by reducing circulating levels of insulin and other hormones that tend to be higher in overweight women and that might promote more aggressive cancers:

• A study designed to investigate the possible associations of metabolic syndrome with triple negative breast cancer found that triglyceride, HDL cholesterol, and glucose levels (but not hypertension or body mass index), were associated with increased incidence of triple negative breast cancer, which is relatively aggressive
• A study designed to investigate the mechanisms by which insulin activates the expression of leptin, an obesity hormone that has been shown to promote breast cancer progression, concluded that high circulating insulin levels might induce breast cancer progression through leptin-dependent mechanisms
• A prospective study that examined the association between high fasting levels of insulin and breast cancer progression found that women with the highest insulin levels were found to be four times more likely to experience metastatic disease and disease recurrence. Although many of the women in the study were obese, insulin level was found to be a risk factor for breast cancer independent of obesity.

Bottom line: Numerous studies have found that regular sustained exercise is associated with reduced risk of breast cancer, especially for women who start exercising at a young age. The present study suggests that such physical activity is beneficial even for women who eventually develop breast cancer. How exercise during breast cancer treatment and afterwards affects prognosis is still an open question. Light or moderate exercise may be helpful, but there is some preliminary evidence that heavy exercise could have an unfavorable impact on survival.