Lifetime Physical Activity and Risk for Premenopausal Breast Cancer

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Summary and Comment

Lifetime Physical Activity and Risk for Premenopausal Breast Cancer

Women who engaged in regular strenuous exercise during adolescence and young adulthood benefited most.

Although only one quarter of breast malignancies occur before menopause, tumors in young women tend to be more aggressive than those in postmenopausal women. During a 6-year follow-up of 64,777 participants in the Nurses’ Health Study, investigators assessed adolescent and adult physical activity as reported from age 12 years to the participants’ current ages (age range at study entry, 33–51). Physical activity was defined as strenuous (e.g., running, racquet sports, swimming laps), moderate (e.g., aerobics), or walking (categorized by pace). Each form of activity was assigned a metabolic equivalent (MET) value; total amounts were calculated as MET-hours per week (MET-hours/week) by multiplying the weekly hours of each activity by the corresponding MET values and summing all the values. Self-reported incident breast cancer was confirmed with medical records.

Women who reported lifetime physical activity of $≥$ 54 MET-hours/week had a 23% lower risk for invasive premenopausal breast cancer than did those reporting <21 MET-hours/week (P for trend, 0.04). The authors noted a threshold effect: Women who reported $≥$ 39 MET-hours/week (the equivalent of running 3.25 hours/week or walking 13 hours/week) also had a 23% lower incidence of premenopausal breast cancer than did those engaging in fewer weekly MET-hours (P<0.05). Physical activity during the youngest age range (12–22) was associated with the greatest protection, even among participants who became less active later in life. Adjusting for age at menarche, other menstrual characteristics, and BMI did not change the association between physical activity and breast cancer risk.

Comment: The authors note that, although other studies in which physical activity and risk for premenopausal breast cancer was assessed have yielded inconsistent results, total activity was not always evaluated in sufficient detail or during adolescence. An important limitation of this study of primarily white nurses is that the results might not apply to women of other ethnicities. Nonetheless, the overall message is clear: Premenopausal breast cancer is less likely to be diagnosed in adolescents and young adult women who engage in regular, strenuous physical activity.

— Andrew M. Kaunitz, MD

Published in Journal Watch Women's Health July 3, 2008