It's well known that too much sitting time is bad for our health. "What we didn't know was whether the sitting time and health relationship was because people were also exercising poorly," said senior author Dr. David Alter of the Toronto Rehabilitation Institute-University Health Network and Institute for Clinical Evaluative Sciences, in a phone interview.
Two distinct factors
It turns out, he and his colleagues say, that sedentary time and exercise time are two distinct factors when it comes to health outcomes.
"Another way of saying it is just because one does their 30 to 60 minutes of exercise per day doesn't ensure their health," Alter said. "These are two distinct factors, we need both, we need exercise and need to be sitting less."
The researchers analysed 47 studies that tracked groups of people as they reported roughly how much time they spent sitting around and not expending much energy, as well as how often they exercised.
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People who were the most sedentary were more likely to be diagnosed with type 2 diabetes, breast, colon, ovarian and other cancers, and cardiovascular disease than people who spent less time sitting.
They were also 24 percent more likely to die during the studies than those who spent the least time sitting.
The pattern tended to be more pronounced for people who also reported less time exercising, the authors reported in the Annals of Internal Medicine. But regardless of physical activity level, prolonged sedentary time was independently associated with bad health outcomes.
A host of diseases
The studies "all seemed to show a similar result," Alter said. "There is a strong and consistent link between sitting time and a host of diseases."
Strategies for encouraging people to sit less are different than those used to promote exercise, he said.
"There are very simple things we can do, every half an hour get up for two to three minutes," he said. "You do that and that's already nearly an hour less sitting per day."
Standing burns twice as many calories as sitting, he noted. People can also stand during commercial breaks while watching TV or during the last 15 minutes of a sporting event, he said.
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These strategies don't replace daily exercise, Alter stressed.
None of the studies in the review were randomised controlled trials, so researchers can't yet say that sitting directly causes disease, Alter said.
Elevated health risk biomarkers
There will need to be considerably more research done to fill in those gaps and help develop guidelines for sedentary behaviour, like there are for physical activity, Neville Owen, programme head of Behavioural & Generational Change at Baker IDI Heart and Diabetes Institute in Melbourne, Australia, wrote in an editorial published with the study.
Even among adults who meet the public health guideline – that is, they walk at least 30 min a day – "those who sit for prolonged periods have elevated health risk biomarkers," Owen told Reuters Health by email. "However, there is insufficient evidence yet to know whether very highly active people who sit for prolonged periods are also at risk."
Regardless, sitting time was most important for non-exercisers, Alter said.
"Those that do not exercise and who sit a lot, the health hazards are accelerated quite markedly," Alter said.
"If there is a priority population that I tune myself into, it's the non-exercisers high-sitting-time people."