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Women: if you sit at work all day, and then you sit 6 or more hours during your leisure time, then you are at significantly greater risk of developing any cancer (compared to women who sit for fewer than 3 hours a day during leisure time). And the odds for certain cancers (multiple myeloma, ovarian cancer, and invasive breast cancer) are greatly increased. That 30 minutes at the gym doesn't cancel out the negative effects of sitting all day. But interestingly, this pattern didn't apply to men. From Medscape:

Leisure Time Sitting Increases Cancer Risk in Women

Women who sit 6 or more hours a day during their leisure time have a 10% greater risk of developing any cancer compared with women who sit for fewer than 3 hours a day. In addition, they are more likely to develop certain site-specific cancers, such as invasive breast cancer, ovarian cancer, and multiple myeloma. However, no similar pattern emerged for men.

"So when we think about independent risk factors for many types of cancer, and definitely for invasive breast cancer, you want to tell women to maintain a physically active lifestyle, to maintain a healthy weight, to limit their alcohol consumption, and now you also want to tell them to reduce their time spent sitting," she said.

The findings come from an analysis of data on some 69,260 men and 77,462 women enrolled in the American Cancer Society Cancer Prevention Study II Nutrition. All participants were cancer-free on enrolment. Between 1992 and 2009, 18,555 men and 12,236 women were diagnosed with cancer. On average, men were followed for 13.2 years, and women were followed for an average of 15.8 years.

"In women, leisure-time spent sitting was associated with a statistically significant higher risk of total cancer incidence...after adjustment for physical activity, [body mass index (BMI)], and other potential confounders," the authors report. Sitting 6 or more hours a day during leisure time was also associated with a 65% greater risk for multiple myeloma, a 43% greater risk for ovarian cancer, and a 10% greater risk for invasive breast cancer compared with women who sat less than 3 hours a day during leisure time. The association between longer sitting times and endometrial cancer was statistically significant before adjusting for BMI, but was attenuated when adjusted for BMI.

The same pattern was not seen in men in this study. Leisure time spent sitting was not associated with cancer risk in men, with the exception of an 11% higher risk associated with sitting time among obese men.

"There are a lot of individuals whom I would describe as 'an active couch potato.' " Dr Patel said. "People are going to the gym and maintaining a healthy weight, but they spend the majority of the rest of their time in sedentary activities — sitting at work, sitting in the car, sitting at home — so you really have to think not just of that 30 minutes a day where you are intentionally engaging in physical activity, but what does the rest of your day look like?"