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Treadmill desks help workers in a Virginia office avoid prolonged sitting, identified as a health risk.

Scientists study how sitting affects life span

Michael Jensen, a researcher at the Mayo Clinic, is talking on the phone, but his voice is drowned out by what sounds like a vacuum cleaner. “I’m sorry,” he says. “I’m on a treadmill.”

David Dunstan, an Australian researcher, uses a speakerphone so he can walk around his office at the Baker IDI Heart and Diabetes Institute in Melbourne.

It’s not that Jensen and Dunstan are hyperactive. Rather, both are exercise researchers looking into the link between sitting down and premature death. And what they have found is disturbing enough that they both make sure they spend most of the day on their feet.

Jensen explains that he and his colleagues at Mayo, in Rochester, Minn., were studying weight control when they discovered that some people “spontaneously start moving round and don’t gain weight” when they have overeaten. These people don’t dash to the gym; they just walk more, hop up from the couch to run errands or find other excuses to get onto their feet.

“This really got us thinking about this urge to move,” Jensen says, “and how important that might be for maintaining good health.”

That led them to a field known as inactivity research, which suggests that inactivity, particularly sitting, can be very bad for your health. It might sound like a statement of the obvious, but the killer point is this: Inactivity is bad for you even if you exercise. Heading to the gym is not a license to spend the rest of the day on your backside.

In 2010, a team led by Alpa Patel of the American Cancer Society in Atlanta analyzed the data from a 14-year study of 123,000 middle-aged adults.

When they compared mortality rates of those who spent six hours a day or more sitting and those who reported three or fewer hours – and when they took into account other factors such as diet – they found something surprising: Extra time on the couch was associated with a 34 percent higher mortality rate for women and 17 percent higher for men in the 14 years after they joined the study. It is not clear why there is such a big gender difference.

No canceling effect

The message is clear: Sitting still for hours at a time might be a health risk regardless of what you do with the rest of your day.

Just as you cannot compensate for smoking 20 cigarettes a day with a good run on the weekend, a bout of high-intensity exercise may not cancel out the effect of watching TV for hours on end. Patel’s study found that people who spent hours sitting had a higher mortality rate even if they worked out for 45 to 60 minutes a day. The researchers call these people “active couch potatoes.”

But it is not just sitting on the couch that worries them. If the harm comes primarily through the inactivity itself – discounting sleep, which brings its own health benefits – the researchers suspect that watching TV, reading a novel or sitting at a desk may be just as harmful.

“The sobering reality,” Dunstan says, “is that across a 14- or 15-hour waking day, we’re getting 55 to 75 percent sedentary time. Moderate to vigorous activity – what people like to call ‘exercise’ – occupies just 5 percent or less of people’s days.”

That’s not the lifestyle to which the human body is adapted.

“From an evolutionary point of view, we are built to be active,” says Audrey Bergouignan, a human physiologist at the University of Colorado at Denver. “Your grandparents were not going to the fitness center. They were active all day.”

Much of Bergouignan’s research involves bed-rest studies funded by space agencies. They are primarily concerned with the effects of low gravity on astronauts, but the results also apply to earthbound inactivity.

The studies reveal that inactivity produces a complex cascade of metabolic changes. For example, unused muscles not only atrophy but also shift from endurance-type muscle fibers that can burn fat to fast-twitch fibers that rely more strongly on glucose. Inactive muscles also lose mitochondria, the cells’ power packs, which can also burn fat. With the muscles relying more on carbohydrates for what little work they are doing, unburned lipids accumulate.

“Your blood is going to become very fatty,” Bergouignan says, which could be why sitting has been linked to heart disease.

Exercise matters

So what can people do to avoid this, other than quitting their desk jobs and taking up nursing, hairdressing, waiting tables or other jobs that require them to be on their feet?

First, it is important to note that exercise still has great benefits: An hour’s workout cannot undo hours of sitting, but it is still good for your health. Patel’s active couch potatoes fared better than people who sat a lot and did not go to the gym.

In his latest experiments, Dunstan has been bringing people into his lab so that he and his team can find out precisely what works. In a study published last year, volunteers visited on three days. The first visit, they simply sat watching TV. On the other two, they watched TV but stood up three times an hour to spend two minutes on a treadmill. One day they went at an easy pace; on the other, they walked more briskly. On each visit, they were given lunch with a sugary drink.

The scientists discovered that short activity breaks reduced the volunteers’ blood sugar and insulin spikes after the drink by roughly 25 percent. Jensen thinks that what makes these short bouts of activity effective is that they’re enough to burn off some of the glucose that has accumulated in your bloodstream.

The next step, adds Dunstan, is to determine the best ways to build activity breaks into the day. Is it better to have frequent short breaks or less-frequent longer ones? Are treadmill desks and adjustable-height workstations even better, allowing people to switch from sitting to standing or walking as they work? At home, the questions are similar.

If you are working on the computer, Dunstan suggests, “take a break and do the dishes.” If you are watching TV, get up and move around every 20 minutes, or whenever there’s a break.

Patel adds that this may actually come as good news to the millions of people who have not been able to get close to the recommended daily exercise levels.

“The nice take-home message,” she says, “is that anything is better than nothing. Just getting up and moving at all is taking a big step in the right direction.”

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