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Physical activity has been associated with reduced breast cancer risk, but studies of occupational activity have produced inconsistent results. The purpose of this study was to evaluate the relationship between occupational physical activity and breast cancer in a prospective study of women with a family history of breast cancer.

We studied breast cancer risk in 47,649 Sister Study participants with an occupational history. Information on occupational activity and breast cancer risk factors was collected during baseline interviews (2004-2009). Physical activity at each job was self-reported and categorized as mostly sitting, sitting and standing equally, mostly standing, and active. Multivariable Cox proportional hazards regression was used to evaluate associations between lifetime occupational activity and incident breast cancer, after adjusting for established risk factors and recreational activity.

During follow-up, a total of 1,798 breast cancer diagnoses were reported. Compared with women who did not spend any time in active jobs, women who spent a high proportion (a parts per thousand yen75 %) of their working years in active jobs had a reduced risk of breast cancer (HR 0.72; 95 % CI 0.52-0.98). Associations were strongest among overweight (HR 0.64; 95 % CI 0.42-0.98) and postmenopausal (HR 0.67; 95 % CI 0.45-0.98) women.

Occupational activity was associated with a reduced risk of breast cancer. Occupational activity is a domain of physical activity that should be further examined in studies of postmenopausal breast cancer risk. Additional research is necessary to better understand the mechanisms underlying the relationships between occupational activity, body size, and breast cancer.