(By Richard A. Lovett, New Scientist, 15 July 2013)
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 sex difference. In
another study, a team at the University of Queensland in Australia analyzed
data on the television viewing habits of 8,800 Australians. They calculated
that each hour of television correlated with 22 minutes off the average life
expectancy of an adult older than 25. In other words, people who watch six
hours of television a day face the prospect of dying, on average, about five
years younger than those who don’t watch any.
Many other studies have reached similar conclusions. In a review of all
the evidence, Dunstan’s team concluded that there was a “persuasive case” that
excessive sitting “should now be considered an important stand-alone component
of the physical activity and health equation.”
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 by 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. In a typical
study, healthy and previously active volunteers are confined to bed for
anything from a day to three months. “They develop metabolic features very
close to what we observe in obese people and people with Type 2 diabetes,”
Bergouignan says.
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. Other changes involve insulin resistance, a
diabetes-like condition in which glucose accumulates in the bloodstream even
when the body produces insulin to sequester it. All of this happens very
quickly in the astronaut studies. “In three days we have insulin resistance,”
Bergouignan says. Similar effects, she
adds, occurred in a study in which normally active people were asked to curtail
their exercise, in essence spending a few weeks imitating their sedentary
friends.
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. That’s a message exercise advocates don’t
want to get lost. “We know that if you exercise 40 to 60 minutes a day, you’re
going to have a health benefit,” says Inigo San Millan, director of the Human
Performance Laboratory at the University of Colorado Hospital’s Sports Medicine
Clinic in Denver. Dunstan agrees. “We
shouldn’t throw out the well-documented benefits of vigorous physical
activity,” he says. Rather, we should think of extensive sitting as a risk
factor that should be addressed separately.
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 separate 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. “That is a good thing,” Dunstan
says. “We want to avoid those big spikes.” Even more interestingly, ambling on
the treadmill was just as effective as more energetic walking.
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. “Your bloodstream isn’t that big,” he says. “In the whole body
it’s only five liters.” For non-diabetics, that translates to less than 10
grams of glucose in the bloodstream. “If you just burn off four grams — 16
calories — that’s a lot of glucose you’ve taken out of the bloodstream.” It’s easy to burn 16 calories simply by
pacing around the room. That’s also a really good way to clear the mind.
“People who get up and move around for five minutes every hour are every bit as
productive as people who sit there for hours at a time,” Jensen says. 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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