A major finding from the California Teachers Study (CTS) suggests recreational physical activity plays a protective role against invasive and in situ breast cancer.

This study is unique because it provides the first prospective study data documenting that a woman’s long-term exercise habits are important in determining her future breast cancer risk.

In a study to be published in the Archives of Internal Medicine, researchers analyzed data collected from a cohort of 110,599 women who were current or former California teachers and public school professionals with no prior history of breast cancer, and who were between the ages of 22 and 79 years at the start of the study. The study was initiated in 1995 with detailed collection of information on women’s exercise histories and current exercise habits.

The study paid attention on the impact of strenuous activities, as well as moderate activities, per week from high school through age 54 years. During the 6.5 year follow-up period, 2,649 women were diagnosed with invasive breast cancer and 593 were diagnosed with in situ breast cancer, meaning the cancer was confined to the ducts or lobules of the breast.

This finding was somewhat unexpected; however, if it is repeated in future studies of physical activity and breast cancer risk, in offers a promising complement to our current approaches to preventing breast cancer, namely the use of drugs that block estrogens, like tamoxifen or raloxifene. These drugs seem to be effective in reducing risk of ER-positive cancers, but do not impact the occurrence of ER-negative cancers.