Leisure-Time Spent Sitting and Site-Specific Cancer Incidence in a Large U.S. Cohort.

BACKGROUND Time spent sitting is distinctly different from accumulating too little physical activity and may have independent deleterious effects. Few studies have examined the association between sitting time and site-specific cancer incidence. METHODS Among 69,260 men and 77,462 women who were cancer-free and enrolled in the American Cancer Society Cancer Prevention Study II Nutrition Cohort, 18,555 men and 12,236 women were diagnosed with cancer between 1992 and 2009. Extended Cox proportional hazards regression was used to estimate multivariable-adjusted relative risks (RR) and 95% confidence intervals (CI) of leisure-time spent sitting with total and site-specific cancer incidence. RESULTS Longer leisure-time spent sitting, after adjustment for physical activity, BMI, and other factors, was associated with risk of total cancer in women (RR = 1.10; 95% CI, 1.04-1.17 for ≥6 hours vs. <3 hours per day), but not men (RR = 1.00; 95% CI, 0.96-1.05). In women, sitting time was associated with risk of multiple myeloma (RR = 1.65; 95% CI, 1.07-2.54), invasive breast cancer (RR = 1.10; 95% CI, 1.00-1.21), and ovarian cancer (RR = 1.43; 95% CI, 1.10-1.87). There were no associations between sitting time and site-specific cancers in men. CONCLUSION Longer leisure-time spent sitting was associated with a higher risk of total cancer risk in women, and specifically with multiple myeloma, breast, and ovarian cancers, but sitting time was not associated with cancer risk in men. Further research is warranted to better understand the differences in associations between men and women. IMPACT For women, these findings support American Cancer Society guidelines for cancer prevention to reduce sitting time when possible.