Relation between intensity of physical activity and breast cancer risk reduction

Purpose: To examine the influence of frequency, duration, and intensity of physical activity on risk of breast cancer and to compare breast cancer ris ks associated with self-reported versus assigned intensity levels of activi ty. Methods: A population-based case-control study of 1233 incident breast cancer cases and 1241 controls was conducted in Alberta between 1995 and 19 97. The frequency, duration and intensity of occupational, household, and r ecreational activities were measured throughout lifetime using the Lifetime Total Physical Activity Questionnaire and cognitive interviewing methods. Unconditional logistic regression analyses were used to estimate odds ratio s and a full assessment of confounding and effect modification was undertak en. Odds ratios for self-reported and compendium-based assigned levels of a ctivity were compared for lifetime total activity and by type of activity. Results: Breast cancer risk reductions were comparable when self-reported a nd assigned intensity values were used, although the results and trends wer e more evident with the assigned intensity data. Moderate-intensity occupat ional and household activities decreased breast cancer risk, whereas recrea tional activity, at any intensity level, did not contribute to a breast can cer risk reduction. Conclusion: This study found that moderate-intensity ac tivities were the major contributors to the decrease in breast cancer risk found in this study and that risk reductions were more evident when the fre quency and duration of activity alone were modeled. Of the three types of a ctivity considered, the greatest risk reductions observed were for occupati onal and household activities.