Vigorous Physical Activity Tied to Lower Risk of Postmenopausal Breast Cancer

BETHESDA, Md., Oct. 31 (MedPage Today) -- Postmenopausal women may lower their risk of breast cancer with vigorous physical activity, a study from the National Cancer Institute suggests.

During 11 years of follow-up, women who reported the highest levels of activity had an almost 20% lower risk of breast cancer compared with women who exercised the least, Michael F. Leitzmann, M.D., Dr.P.H., formerly of the NCI Division of Cancer Epidemiology and Genetics, and colleagues reported online in Breast Cancer Research.

Anything less than vigorous activity had no influence on postmenopausal breast cancer risk, and the benefits were limited to lean women, defined as having a body mass index of less than 25 kg/m².

Recent systematic reviews have suggested that physical activity has an inverse relationship with breast cancer risk. However, authors of the reviews called for more research to identify specific characteristics of physical activity that contribute the most toward risk reduction.

Responding to the evidentiary shortcomings cited in the reviews, Dr. Leitzmann and colleagues analyzed data from the Breast Cancer Detection Demonstration Project Follow-up Study. They focused on 32,269 women who completed health surveys from 1987 to 1989, 1993 to 1995, and 1995 to 1998.

All participants completed the first survey, 91% the second survey, and 85% the third survey. The authors said 94% of participants completed either the second or third survey.

The surveys included questions about physical activity during the previous year and offered a list of physical activities categorized as nonvigorous or vigorous. Examples of nonvigorous exercise included washing clothes, lawn mowing, and walking. Vigorous activity included scrubbing floors, chopping wood, and running or fast jogging.

The participants' self-reported daily hours of activity (vigorous and nonvigorous) were converted into weekly averages, expressed as metabolic equivalent tasks (METs).

The investigators stratified study participants into quintiles of average weekly exercise, which ranged from a low of 105 to 244 METs to a high of 395 to 721 METs. Self-reported vigorous activity ranged from 0.9 hours per week to 22.7.

During 269,792 person-years of follow-up from 1987 to 1998, 1,506 cases of postmenopausal breast cancer were identified.

Comparison of the top and bottom quintiles of total physical activity in METs resulted in a relative risk of 0.87 in favor of the top quintile. However, the difference did not reach statistical significance (P=0.21 for trend).

Comparison of the highest and lowest quintiles of vigorous activity yielded an identical relative risk of 0.87, still not significant (P=0.08 for trend).

"The apparent inverse association between total physical activity and postmenopausal breast cancer was virtually entirely contributed by vigorous physical activity," said Leitzmann, currently at University Hospital Regensburg in Germany, and coauthors.

After adjustment for age, however, the relative risk associated with the highest level of physical activity improved to a statistically significant value of 0.81 (P=0.03).

In two different multivariate analyses, the benefit of physical activity decreased to a nonsignificant relative risk of 0.86 (P=0.15, P=0.21).

Additional analyses showed that the benefits of vigorous physical activity applied only to lean women (RR 0.68, 95% CI, 0.54 to 0.85).

Total nonvigorous activity and hormone receptor status also had no influence on the risk of postmenopausal breast cancer.

The authors reported no disclosures.

Additional source: Breast Cancer ResearchSource reference: Leitzmann MF et al. "Prospective study of physical activity and risk of postmenopausal breast cancer" Breast Cancer Res 2008; 10: DOI: 10.1186/bcr2190.