Women who reported high levels of physical activity during either period of life were about 10 percent more likely to get breast cancer than less active women. But these findings were not statistically significant, so the researchers could infer neither risk nor benefit. Compounding the problem of interpreting the data from various studies are differences in the specific questions women were asked about their activity levels. Rockhill acknowledges that the questions asked in the Nurses Health Study II about adolescent physical activity were posed in a way that might not optimally reflect important variations in exercise patterns. Nevertheless, responses to these questions do correlate with objective measures such as body mass index, suggesting they are capturing some real variation.
Nobody knows the answers.
Future research, the authors write, should focus on improving assessment of lifetime physical activity as well as clarifying whether there is an optimal time period, frequency, or intensity of physical activity associated with decreased breast cancer risk. Even if no link to breast cancer can be firmly established, Rockhill says, there are still plenty of reasons to exercise. Its benefits for cardiovascular and musculoskeletal health, for example, are unquestioned.