Prolonged sitting can in some cases lead to blood clots because the normal circulation of blood through the legs and feet is impaired, said researchers from the University of Minnesota in the US.
"Results suggest that even individuals who regularly engage in physical activity should not ignore the potential harms of prolonged sedentary behaviours such as TV viewing," said Yasuhiko Kubota from the University of Minnesota.
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The study, published in the Journal of Thrombosis and Thrombolysis, focused on the risk of developing a common and potentially fatal blood clot in the vein called venous thromboembolism (VTE).
One type of VTE is known as deep vein thrombosis, where the blood clots form in the deep veins of the legs, the researchers said.
Another serious problem is when VTE become dislodged and travel through the bloodstream to block off another vein somewhere else in the body.
If VTE ends up in the lungs, they cause blood clots in the lungs known as pulmonary embolism (PE).
The researchers analysed data of 15,158 people aged between 45 and 64. The participants were initially asked about their health status, whether they exercised or smoked, and whether they were overweight or not.
The findings show that participants who watched television very often had more than 1.7 times the risk of suffering from a VTE compared to those who never or seldom watched TV.