As I’ve reported here several times before, a growing number of studies suggest that although regular aerobic exercise (running, walking fast, biking, cross-country skiing and so on) is essential to good health, it’s not enough.

We also have to spend less time sitting during all those hours between our workouts.

Earlier this year, for example, Australian scientists (who have led much of the research on this topic) found that people who sat for more than 11 hours daily were 40 percent more likely to have died during the next three years than those who said they sat for four hours or less daily — even when everybody’s physical activity was taken into account.

That kind of study, however, is observational, which means it can show only a correlation between two things (prolonged sitting and poor health outcomes), not a causal relationship. Still, as health reporter Gretchen Reynolds noted Sunday in a commentary article in the New York Times, other types of studies have begun to offer some biological support to the idea that prolonged sitting is by itself an unhealthful activity (or non-activity):

In a noteworthy recent experiment conducted by scientists at the University of Massachusetts and other institutions, a group of healthy young men donned a clunky platform shoe with a 4-inch heel on their right foot, leaving the left leg to dangle above the ground. For two days, the men hopped about using crutches (and presumably gained some respect for those people who regularly toddle about in platform heels). Each man’s left leg never touched the ground. Its muscles didn’t contract. It was fully sedentary.

After two days, the scientists biopsied muscles in both legs and found multiple genes now being expressed differently in each man’s two legs. Gene activity in the left leg suggested that DNA repair mechanisms had been disrupted, insulin response was dropping, oxidative stress was rising, and metabolic activity within individual muscle cells was slowing after only 48 hours of inactivity.

In similar experiments with lab animals, casts have been placed on their back legs, after which the animals rapidly developed noxious cellular changes throughout their bodies, and not merely in the immobilized muscles. In particular, they produced substantially less of an enzyme that dissolves fat in the bloodstream. As a result, in animals and humans, fat can accumulate and migrate to the heart or liver, potentially leading to cardiac disease and diabetes.

More positive results

Reynolds also reports on this study, which will be published next month in Diabetes Care:

Scientists at the Baker IDI Heart and Diabetes Institute in Melbourne, Australia, had 19 adults sit completely still for seven hours or, on a separate day, rise every 20 minutes and walk leisurely on a treadmill (handily situated next to their chairs) for two minutes. On another day, they had the volunteers jog gently during their two-minute breaks.

When the volunteers remained stationary for the full seven hours, their blood sugar spiked and insulin levels were out of whack. But when they broke up the hours with movement, even that short two-minute stroll, their blood sugar levels remained stable. Interestingly, the jogging didn’t improve blood sugar regulation any more than standing and walking did. What was important, the scientists concluded, was simply breaking up the long, interminable hours of sitting.

Equally beguiling … were results from experiments at the University of Massachusetts showing that when volunteers stood all day — nothing else; no walking or jogging; just standing — they burned hundreds more calories than when they sat for the same period of time.

Steps you can take

Few people can install a treadmill near their desk or wherever else they may be sitting at their worksite. But, say the experts in this field, there are things almost everybody can do to break up long spells of sitting. Get up regularly — at least once an hour — and move about. Take a short stroll down the hall. Or do some simple standing stretches by your desk. Reynolds offers a couple of other possible solutions: She props papers on a music stand and reads them standing up. She also stands and/or “prowls” her office while talking on the phone.

But don’t give up your regular aerobic exercise, Reynolds adds. That must stay on our to-do list for a healthier lifestyle.

What we must do now is expand that list to include one more item: “Reduce daily sitting time.”

You can read Reynolds’ commentary piece on the New York Times website.