Physical activity has been associated with a reduced risk of breast cancer. However, little is known about the association between recreational physical activity and mammographic density. We examined the association between recreational physical activity and mammographic density using mammograms from 375 white and African American women without breast cancer who served as controls in the Los Angeles component of the Women's Contraceptive and Reproductive Experiences Study. We used data from 5 time periods of activity in the statistical analysis: from menarche to mammogram screening, the first 3 and 10 years after menarche, the most recent 10 years and the 3 years prior to mammogram screening. Lifetime history of recreational physical activity was obtained through interviews using a structured questionnaire. We used multiple linear regression to estimate least-squared mean values of absolute and percent mammographic density within categories of physical activity. Overall, we found no statistically significant evidence that physical activity reduced absolute or percent mammographic density. We observed a modest positive association between lifetime physical activity and percent mammographic density (p for trend = 0.04) among younger women, and between recent physical activity and percent density among both younger (<50 years, p for trend = 0.09) and older (> or =50 years, p for trend = 0.06) women, but these associations diminished after additionally adjusting for body mass index (BMI) (all p > or = 0.10). However, among women younger than 50 years, we found some evidence for a protective effect of "strenuous" physical activity in the first 3 years after menarche, with a nonstatistically significant inverse association with both absolute (p for trend = 0.07) and percent (p for trend = 0.08) mammographic density after adjustment for BMI. Our results suggest that physical activity is not a strong predictor of mammographic density.
Copyright 2006 Wiley-Liss, Inc.