Early-life physical activity and postmenopausal breast cancer: Effect of body size and weight change

It is not Set known whether early-life physical activity reduces the risk o f developing breast cancer. Subgroup analyses according to menopausal statu s and body mass may help clarify this association. Data from a population-b ased case-control study of female residents of Wisconsin, Massachusetts, Ma ine, and New Hampshire were used to examine associations between body mass and breast cancer risk. Cases (It = 4613) were identified by each state's t umor registry; controls (n = 5817) were randomly selected from population l ists. Frequency of participation in strenuous physical activity when 14-22 years of age, weight at age 18 and 5 Sears before interview, height, and ot her factors were ascertained through structured telephone interviews. Adjus ted odds ratios (ORs) and 95% confidence intervals (CIs) were computed usin g logistic regression. Reductions in postmenopausal breast cancer risk asso ciated with strenuous physical activity mere greatest for women in the four th quartile of body mass index at age 18; the OR for women with the highest activity frequency on average (greater than or equal to once/day) was 0.45 (95% CI = 0.26-0.79), Associations with frequency of activity also varied by weight change. Compared to women with no activity and little adult weigh t gain, frequent physical activity was associated with reduced postmenopaus al breast cancer risk in women who had lost weight since age 18 (OR = 0.19, 95% CI = 0.05-0.70) or had gained little or modest amounts of weight (weig ht gain: first tertile, OR = 0.36, 95% CI = 0.05-0.853 second tertile, OR = 0.31, 95% CI = 0.14-0.66), Weighted MET score analyses yielded similar but less inverse results. These findings suggest that the reduced risk of post menopausal breast cancer associated with frequent, early-life physical acti vity may be greatest in women who, over the adult years, either lost n eigh t or gained only modest amounts.