Regular exercise doesn't erase the higher risk of serious illness or premature death that comes from sitting too much each day, a new review reveals.
And even if study participants exercised regularly, the accumulated evidence still showed worse health outcomes for those who sat for long periods, the researchers said. However, those who did little or no exercise faced even higher health risks.
"We found the association relatively consistent across all diseases. A pretty strong case can be made that sedentary behavior and sitting is probably linked with these diseases," said study author Aviroop Biswas, a Ph.D. candidate at Toronto Rehabilitation Institute-University Health Network.
"When we're standing, certain muscles in our body are working very hard to keep us upright," added Biswas, offering one theory about why sitting is detrimental. "Once we sit for a long time . . . our metabolism is not as functional, and the inactivity is associated with a lot of negative effects."
The research is published Jan. 19 in the online issue of Annals of Internal Medicine.
About 3.2 million people die each year because they are not active enough, according to the World Health Organization, making physical inactivity the fourth leading risk factor for mortality worldwide.
Among the studies reviewed by Biswas and his team, the definition of prolonged sitting ranged from eight hours a day to 12 hours or more. Sitting, or sedentary activities ubiquitous with sitting such as driving, using the computer or watching TV, shouldn't comprise more than four to five hours of a person's day, Biswas said, citing guidelines issued by Public Health Agency of Canada.
"We found that exercise is very good, but it's what we do across our day," he said. "Exercise is just one hour in our day, if we're diligent; we need to do something when we're not otherwise exercising, like finding excuses to move around, take the stairs, or carry groceries rather than use the [shopping cart] at the supermarket."
The biggest health hazard stemming from prolonged sitting, according to the review, was a 90 percent higher risk of developing type 2 diabetes. Among studies examining cancer incidence and deaths, significant links were specifically noted between sedentary behavior and breast, colon, uterine and ovarian cancers.
One study in the review showed that fewer than eight hours of sitting time per day was associated with a 14 percent lower risk of potentially preventable hospitalization.
Dr. Joshua Septimus, a clinical associate professor of internal medicine at Houston Methodist Hospital in Texas, praised the new research, saying it "gives us more data to help counsel our patients."
"The idea that we could exercise for 15 or 20 minutes a day and that could completely erase any harms of a sedentary lifestyle for the other 23 hours a day is just too hopeful," Septimus noted. "This showed us that yes, there is some benefit to physical activity . . . but it's not enough."
Biswas and his colleagues offered additional tips to reduce sedentary time, including:
· Taking a one- to three-minute break every half-hour during the day to stand (which burns twice as many calories as sitting) or walk around,
· Standing or exercising while watching TV,
· Gradually reducing daily sitting time by 15 to 20 minutes per day, aiming for two to three fewer sedentary hours over a 12-hour day.