Evidence-based exercise: What’s the optimal amount?

When a doctor prescribes a medicine, he gives a precise dose calculated to maximize the health benefits while minimizing any side effects.  Similar to medicine, we know that exercise is good for our health; it reduces the risk of developing chronic disease and prolongs our life. But exactly how much exercise is optimal?

The 2008 Physical Activity Guidelines for Americans recommends two-and-a-half hours of moderate exercise or one hour and fifteen minutes of vigorous exercise each week. But until recently, researchers haven’t examined a large body of evidence on exactly how much exercise is best for our bodies.

A new analysis published in the Journal of the American Medical Association looks at that exact question. The report pools data from six prospective, ongoing studies in the United States and Europe where participants self-reported their physical activity levels. The analysis includes more than 660,000 men and women.

Researchers began by grouping participants by how much time they exercised each week; they then compared those group to death records over a 14-year period. They found, not surprisingly, that people who did not exercise at all were at the highest risk of dying. Those who followed the recommended guidelines for exercise – two-and-half hours of moderate exercise per week – lowered their risk of dying by 31 percent over the 14 year period.

But more exercise proved to be better. Participants who worked out moderately for 450 minutes per week, or a little more than an hour per day, were 39 percent less likely to die prematurely than people who never exercised.

At that point, the benefits plateaued. Participants who exercised more – up to 20 hours per week – had about the same reduction in mortality risk as people who simply met the guidelines. Those extra hours of sweating did not result in any increased health benefit. But, just as important, more exercise did not pose a danger to the health of participants.

The take-home message here? About an hour of moderate exercise each day is the ideal amount to prolong your life. But if you enjoy working out more than that, you’re not increasing your risk of an early death.