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Tuesday, June 15, 2010

How little exercise can you get away with? Good question.

How little Exercise Can You Get Away With?
By GRETCHEN REYNOLDS
New York Times Well Blog.
Decembre, 30th, 2009.


Recently researchers trawled through a vast database of survey information about the health and habits of men and women in Scotland, hoping to determine how much exercise is needed to keep the Scots from feeling gloomy (or in technical terms, experiencing “psychological distress”). The answer, according to a study published in this month’s British Journal of Sports Medicine: a mere 20 minutes a week of any physical activity, whether sports, walking, gardening or even housecleaning, the last not usually associated with bringing out the sunshine. The researchers found that more activity conferred more mental-health benefits and that “participation in vigorous sports activities” tended to be the “most beneficial for mental health.” But their overall conclusion was that being active for as little as 20 minutes a week is sufficient, if your specific goal is mental health.

The question of how much exercise is enough gains special piquancy at this time of year, when many of us dust off last year’s New Year’s resolutions and promise to be more diligent about working out in the coming year. Unfortunately, figuring out an ideal exercise dosage is not simple, in part because the amount of exercise needed depends on the benefits you hope to gain. Twenty minutes a week of vacuuming or other activity may, according to the Scottish study, increase your contentment, but it certainly won’t do much for your cardiovascular fitness and is unlikely to lessen your risks for a multitude of diseases and, ultimately, of premature death, benefits that a greater amount of exercise may provide. It also won’t help much with weight loss. That said, anyone resolving to increase the amount of housecleaning they do in the New Year is welcome to begin their regimen at my home.

In general, a wide reading of the latest sports science makes it clear that the “amount of physical activity necessary to produce health benefits cannot yet be identified with a high degree of precision,” according to the authors of the 2008 Physical Activity Guidelines for Americans report, which was produced by the Department of Health and Human Services and was based on the recommendations of an advisory committee scientists. These experts waded through dozens of studies on the health effects of exercise, looking at the impacts that exercise can have on people’s risks for heart disease, obesity, diabetes, cancer, depression and, in general, premature death. In some studies that were cited, exercise actually seemed to confer little if any disease-fighting benefits. In others, the benefits kicked in only if the exercise was quite strenuous; in yet others, a gentle stroll a few times a week was enough to lessen the risk of early death.

Despite the inconsistent results, caused in some part by even more inconsistent methodologies between the different studies, the advisory committee did ultimately reach some conclusions about how much — or, really, how little — exercise we each should be doing. That minimum amount of exercise required to see a significant lowering of your risk of dying prematurely was, they concluded, 500 MET minutes of exercise a week. Of course, unless you’re an exercise scientist, there’s a good chance you don’t know what a MET minute is. A single MET, or Metabolic Equivalent of Task, is the amount of energy a person uses at rest. Two METs represent twice the energy burned at rest; four METs, four times the energy used at rest; and so on. Walking at three miles per hour is a 3.3-MET activity, while running at 6 miles an hour is a 10-MET activity. The committee concluded that a person needs to accumulate a weekly minimum of 500 MET minutes of exercise, which does not mean 500 minutes of exercise. Instead, 150 minutes a week (two and a half hours) of a moderate, three- to five-MET activity, such as walking, works out to be about 500 MET minutes. Half as much time (an hour and 15 minutes per week) spent on a 6-plus MET activity like easy jogging seems, according to the committee, to have similar health effects.

Interestingly, they did not find that exercise beyond a certain point conferred significant additional health benefits. Instead, the “dose response” for exercise, the committee found, is “curvilinear.” In other words, people who are the least active to start with get the most health benefit from starting to exercise. People who already are fit don’t necessarily get a big additional health benefit from adding more workout time to their regimens. Which is not to say that if you are, for instance, a devoted runner or cyclist, you should reduce your workout time in 2010 to 500 MET minutes per week. You’re already well ahead in terms of health benefits. According to the Physical Activity Guidelines report, “It has been estimated that people who are physically active for approximately seven hours a week have a 40 percent lower risk of dying early than those who are active for less than 30 minutes a week.”

Whether there might be an upper limit to the advisable amount of exercise is an issue that was not addressed by the group. It hasn’t been directly studied much by science, either, in part because of logistical and ethical barriers; you can’t run people until they drop. But there have been intimations that you can be too avid. As I reported in this column in October, laboratory mice that were made to run to exhaustion were more likely than mice that ran moderately to succumb to the flu. Similarly, a few small but provocative studies of the coronary health of long-time, competitive marathon runners have suggested that their efforts may not, in every case, be doing their hearts good, as I also reported this year.

So what does all this mean as you plan your 2010 exercise routines? First, because “activity affects so many organs and pre-disease states,” according to Frank Booth, a professor in the department of biomedical sciences at the University of Missouri at Columbia, who has extensively studied the health effects of exercise, “any activity is better than no activity.” For those contemplating their first regular exercise routine, consult a doctor before starting, of course. Then, get out and walk, working your way up to least 150 minutes a week. Although not all of the studies under review found health benefits from such relatively light aerobic exercise, enough of them did to support the recommendation of regular brisk walks or other moderate activities. (Moderate exercise, by the way, is defined by the Department of Health and Human Services as activities of between three and six METs, equivalent to about 45 to 64 percent of your maximum heart rate — or in simpler terms, activities during which it is possible for you to talk to a companion but too hard for you to sing the words to your favorite song.)

You do not necessarily have to divide your exercise time into daily allotments, either. Existing “scientific evidence does not allow researchers to say, for example, whether the health benefits of 30 minutes on five days a week are any different from the health benefits of 50 minutes on three days a week,” according to the activity guidelines. Do what suits your schedule. But, Mr. Booth says, do something. “Inactivity is looking more and more like one of the underlying causes of many chronic diseases,” he says. If, he adds, “you want to live to be 100,” which happens to be my New Year’s resolution, “then don’t just sit all day.”

Correction: An earlier version of this post misstated the name of the city in which the University of Missouri is located. It is Columbia, not Columbus.

nytimes.com

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