Having a sedentary life style doesn't just make you unfit or gain weight. A recent study by Schmid and Meitzmann and published in the Journal of National Cancer Institute meta-analysed data collected from 4 million individuals including 68,936 cancer cases concluded that there is an increased risks of chronic disease and mortality rate associated with sedentary behaviors such as watching TV.
The study found that each 2-hour per day increase in sitting time outside your normal occupation is associated with an 8% increased risk in colon cancer, a 10% increased risk in endometrial cancer and 6% increased risk in lung cancer. The increased risks in these cancers appear to be independent of physical activity. This means spending a large amount of time sitting down could be detrimental to health even to those who do regular exercise. Sitting at work seems to be a lot healthier than watching TV, given that you don't over do it. A 2-hour per day increment in sitting at work has been found to increase the risk of obesity by 5%, whereas each 2-hour per day increase in TV time is associated with 23% increase in obesity risks.
The mechanism of which sedentary behaviors cause cancer is unclear. However, the authors of the study speculated that unhealthy eating habits, vitamin D deficiency due to a lack of sun exposure, weight gain from low energy expenditure and an increase in pro-inflammatory markers in blood due to prolonged sedentary life style maybe the main culprits. Colon cancer and endometrial cancer are obesity related cancers. Therefore, the increased risks of cancer caused by sedentary life style may work through similar pathways, even if you are not obese.
People today spend on average 50 - 60% of their time in sedentary pursuits and we are in the middle of an obesity epidemic. On top of that, it's forecasted by the US National Cancer Institute that 1 in 2 men and 1 in 3 women will bear the risk of developing some form of cancer by 2050 in the US, doubling the current rates. Doing regular exercise can reduce the risk of mortality and chronic diseases. The world health organization (WHO) recognizes this and recommends adults to do a minimum of 150 minutes of moderate intensity exercise each week. What the WHO guideline doesn't address is the amount of time spent by people in sedentary pursuits, which will cause an increased risk in cancer independent of physical activity levels. It is therefore recommended that one should consciously limit the time spent watching TV and other screen-based entertainments. It is also recommended that children and adults should breakup their sitting-down periods, let it be at work or watching TV, traveling on a plane or during long distant driving with interspersing intervals of standing or short exercises. We as humans are not built to cope with sedentariness, start moving and life will prosper.