Harvard Women's Health Watch

What we do and dont know about exercise

What we do­—and don't—know about exercise

Experts' advice has changed as we've learned more about the connection between physical activity and health.

If exercise could be packed into a pill, it would be the single most widely prescribed and beneficial medicine in the nation.

When Dr. Robert Butler, the founder of the National Institute on Aging, made that statement in the 1980s, he was on to something. Decades of research have determined that regular exercise is one of the most important factors in warding off cardiovascular disease, many types of cancer, diabetes, and obesity.

But while the benefits of "the exercise pill" have been firmly established, the exercise prescription has changed considerably over the years. "Knowledge evolves over time. Just because recommendations change doesn't mean that the experts are wishy-washy," says Dr. I-Min Lee, professor of medicine at Harvard Medical School.

Dr. Lee notes that the first national exercise recommendations, made by the National Heart Association in 1975, called for 20 minutes of vigorous exercise every day. That advice was based on studies conducted primarily in men in their 20s. While "no pain, no gain" may have made sense for young men seeking to become more fit, it was daunting to older people who just wanted to stay healthy.

In 1995 new guidelines were issued, this time with input from physicians and public health experts. By that time it was acknowledged that any physical activity—not just structured exercise—was beneficial to health and that 30 minutes of walking or other moderate activity on most days would help to lower the risk of most degenerative diseases. "It wasn't that the earlier recommendations were wrong; the newer ones just extended them," Dr. Lee explains.

The latest set of guidelines, published in 2008, is in a sense a combination of the two earlier ones. It advises 150 minutes of moderate exercise or 75 minutes of vigorous exercise, or an equivalent mix, spread out over the course of the week. These guidelines give us a lot of leeway in filling our exercise quota. The only stipulation is that each bout of exercise last at least 10 minutes.

What about other types of activity?

The 2008 guidelines were based on studies involving tens of thousands of people. In general, the participants completed questionnaires noting how often they exercised. The researchers then observed how they fared over the years—and matched the levels of exercise reported with the health outcomes of the participants.

Studies in progress, in which people are outfitted with activity trackers, are designed to measure the effects of light exercise, such as gardening. Future research will also help better determine the effects of sitting. "We know sitting for long periods isn't good, but we don't know how long it's safe to sit if you get a lot of exercise otherwise. These studies should give us this information," Dr. Lee says.

What's the bottom line?

The current guidelines aren't likely to be the last. We can expect future directives on light exercise and sedentary time. For now, you can't go wrong following today's exercise prescription and getting out of your chair every 30 minutes or so

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