How sitting hurts. When people sit or lie for hours at a time, the muscles—particularly the large ones in the leg and back—go dormant, setting off some dangerous metabolic consequences. "Sitting shuts down electrical activity in the legs," says Toni Yancey, a professor of health services at UCLA's Fielding School of Public Health. "It makes the body less sensitive to insulin, causes calorie-burning to plummet, and slows the breakdown of dangerous blood fats, lowering 'good' HDL cholesterol."
In fact, just one day of inactivity reduced insulin action in a small group of men and women by about 40 percent in a 2011 study from the Department of Kinesiology at the University of Massachusetts–Amherst and the Pennington Biomedical Research Center in Baton Rouge, La. Vernikos likens hours of continuous or uninterrupted sitting to the weightlessness that astronauts experience in space. "The absence of gravity even for a few days accelerates astronauts' degeneration, inducing harmful changes we typically associate with aging," she says.
The human body evolved to function with the constant downward pull of gravity. Without it, the heart shrinks, blood volume plunges, muscles atrophy, fat replaces muscle, and bone mass decreases. Even the immune system is weakened, with wounds taking longer to heal and infections more likely. When astronauts return to Earth, however, gravity helps them rebound by stimulating the bones, muscles, and other body systems. The more you work against gravity, the greater these systems respond. The key to optimal functioning, Vernikos says, is making as much use of gravity as possible. "That means increasing the number of small, low-intensity movements you make throughout the day as you go about the business of living," she says. "Humans today use much less gravity than our ancestors did throughout history."
Research bears out Vernikos's advice. A 2008 study from the Cancer Prevention Research Centre at the University of Queensland in Brisbane, Australia, showed that people who took the most breaks from sedentary time had on average a smaller waist size, a lower body mass index, and reduced triglycerides, a particularly unhealthy type of blood fat. The latest study shows that interrupting sitting time with two-minute bouts of walking every 20 minutes significantly improves glucose metabolism. "Importantly, walking at a light pace and walking at a moderate pace yielded almost identical responses," says lead author David Dunstan, associate professor at the Baker IDI Heart and Diabetes Institute in Melbourne, Australia. "That suggests that the frequency of physical movement is the most important consideration, not the intensity of the movement." Standing up to take a stretch, getting a drink of water, or marching in place for a few minutes all count toward better health.
But don't ditch your running shoes or gym membership, experts say. Standard exercise guidelines—30 minutes a day of moderate to vigorous exercise, five times a week—still apply. Indeed, this prescription has been shown to lower blood pressure, improve cholesterol levels and metabolism, and lower the risk of some cancers. "The goal is to minimize sitting, especially continuous sitting, and follow a regular exercise program," says Timothy Church, an investigator at the Pennington Biomedical Research Center.
That's borne out by the recent American Cancer Society research. While sitting time increased risk of early death, irrespective of exercise levels, both being physically inactive and spending too much time sitting was far worse, says ACS researcher Alpa Patel. "Women who were the least active and spent the most time sitting were 94 percent more likely to die early compared to women who were the most active and sat the least," she says. "For men there was a 48 percent increased risk." The bottom line, Church says: "A fitness program and movement throughout the day are both important—it's all good."