A previous report examining physical activity and breast cancer risk among participants in the Women’s CARE Study showed a modest inverse association (increasing physical activity associated with decreasing risk of invasive breast cancer) among black women and among white women 16
. Here, analyses were limited to postmenopausal women to determine whether this inverse association differed according to a women’s history of HT use. Although statistically significant inverse associations were observed for women who never used HT, had used HT for a relatively short time (<5 yrs), or who were current ET users (with adjustment for duration of HT use), such associations were not observed for longer-term HT users, former HT users or current EPT users. Despite these differences, none of the tests for homogeneity of trends was statistically significant.
The present study examines physical activity and breast cancer risk according to a woman’s history of HT use in greater detail than was done in two prior publications 11–12
. A number of previous investigations observed the interaction of hormone therapy on the relationship between breast cancer risk and physical activity, some of which indicated no effect modification by HT use 2–3, 12, 18–24
while few observed a lower breast cancer risk among active versus sedentary women who had never used HT 13, 25
. However we limited our discussion to the studies most relevant to our methodology and findings which includes cohort studies by Howard et al.11
and Patel et al12
Cohort studies by Howard et al.11
and Patel et al.12
observed that breast cancer risk was inversely associated with physical activity among postmenopausal women who had never used HT, findings similar to the results presented here. However, the first of these studies compared postmenopausal women who had ever used HT to those who had never used HT 11
; the second study compared current, former, and never users of HT 12
. The current investigation subdivided HT users according to duration of use and formulation of current use. An analysis comparing never to ever HT users did not demonstrate any effect modification by HT use of the association between lifetime physical activity and breast cancer risk (data not shown).
Important differences in the extent of physical activity assessment exist between the Women’s CARE Study and the two cohort studies. Howard et al. limited physical activity to that performed in the year prior to study entry 11
. Patel et al. assessed physical activity at 3 different time points, during the year before recruitment, in the year that was 10-years before recruitment and at age 40 years 12
. Although Patel’s study was a more comprehensive assessment than Howard et al., only physical activity during the year before recruitment was associated with breast cancer risk 12
. The current study obtained extensive detail on lifetime (age 10 to reference date) physical activity assessed through in-person interviews during which a calendar of life events was completed to facilitate recall of activity levels throughout life.
In a case-control study of in situ
and invasive breast cancer by Slattery et al. (2007), analyses were restricted to postmenopausal non-Hispanic white and Hispanic/American Indian women whose ages ranged through 79 years; physical activity data were obtained during in-person interviews using a computerized questionnaire to assess activities for the reference year, and at ages 15, 30, and 50 years 13
. Contrary to results from the current study, HT use modified the physical activity-breast cancer risk association with the greatest reduction in breast cancer risk observed among women who had not used HT within the past 2 years 13
. Notably, this category combined never users with past users of HT. The demographic profiles of Slattery’s study and the current study differ in that the Women’s CARE Study restricted age (35–64 years) and birthplace (US-born), included black women and had few Hispanic women (n=96) due to the birthplace restriction. The Women’s CARE study also limited case eligibility to women with invasive breast cancer.
In summary, results from the Women’s CARE Study show that breast cancer risk decreases with increasing lifetime physical activity levels among postmenopausal women who have not used HT, have used HT for less than 5 years, or are current ET users, but no trend is observed among longer duration HT users, current EPT users or past HT users. Despite these differences, this study was unable to demonstrate statistically that HT use modifies the relationship between physical activity and breast cancer. In light of the profound changes in HT use occurring since 2002, it will be important in future studies conducted after 2002, to learn whether or not any association between physical activity and breast cancer among former HT users is a function of time since last HT use.