There is some evidence that women who are more fit - more active and not overweight - have a lower chance of developing breast cancer.

A recent study from the Keck School of Medicine at the University of Southern California shows that women who exercise on a regular basis have a lower risk of getting breast cancer. The results also show that being more active help both white and black women. This study, which was headed by Leslie Bernstein, Ph.D., involved interviews with over 9000 women (both black and white) between 35 and 64 years old. Roughly half of the women had previously been diagnosed with breast cancer, the others had not.

The researchers found that the women who exercised an average of at least 1.3 hours a week since they were 10 had around a 20 percent lower risk of breast cancer than women who were not very active. In cases where there was a family history of breast cancer activity levels did not seem to make a difference.

Keep in mind however that these types of studies are correlational only and could be skewed for a number of reasons - including inaccurate memory of past activity levels. It also does not attempt to discover a possible mechanism for how activity levels might affect breast cancer risk (if one does in fact exist). The issue with a family history of breast cancer does not imply that regular exercise doesn’t help those who have breast cancer in their family. It does suggest however that genetic factors may in some cases have a greater influence on the development of cancer.

Another study (by Marilie D. Gammon Ph.D.) from the Long Island Breast Cancer Study Project has some evidence that if you gain more than 24 pounds after you turn 50 your chances of getting breast cancer go up 62%

Again, just correlational. But considering other known health benefits of staying active and keeping your weight down it does seem like keeping fit sounds like a good thing to do! And there may very well be a direct connection - only future research can say for certain!