Certainly, I hoped it was. It's a daily struggle to make the time to exercise, and the current federal health guidelines call for at least 150 minutes a week of moderate exercise — a lot of time that somehow manages to seem like even more, magnified by the "should" it adds to so many days. There are hundreds of other reasons to exercise, and the one that works best for me is wanting to feel at my best on that very day. But it would be very comforting, I thought, if I knew that all of that time would come back to me.

"Really?" I wondered. I knew vaguely that being active lengthens life expectancy, but was the return on time spent really 1 to 1?

At a recent dinner party, a geeky friend of mine was cheerily justifying the piles of money he spends on a personal trainer. He's feeling so great that it's worth every cent, he exulted, "And the best part is the return on the time! Every minute you spend working out comes back to you, because you'll live that much longer!"

Not only do you get the time back, it comes back to you multiplied — possibly by as much as seven or eight or nine.

Let me cut to the happy conclusion: It seems that it does. And then some. If you play with the data of a recent major paper on exercise and longevity, you can calculate that not only do you get the time back; it comes back to you multiplied — possibly by as much as seven or eight or nine.

To quote Tom Anthony, a regular CommonHealth reader with a Harvard physics degree who kindly helped me with the math, "I wish I could get these paybacks in the stock market."

This is all a bit of a public health parlor game, of course, resting on averages and approximations. You, personally, could work out ten hours a week and still die flukishly young. But the math looked so striking that I asked for a reality check from Dr. I-Min Lee of Brigham and Women's Hospital, a Harvard professor and senior author of that recent paper, "Leisure Time Physical Activity of Moderate to Vigorous Intensity And Mortality: A Large Pooled Cohort Analysis."

Yes, she confirmed, she had not calculated out the question before, but according to her data, a middle-aged person who gets the recommended 150 minutes per week of moderate exercise — defined as the level of brisk walking — can expect a 1-to-7 return: seven extra minutes of life gained for each minute spent exercising.

Some background:

The paper on exercise and longevity broke ground by calculating for the first time the gains in life expectancy from various levels of activity. In the past, researchers had found that in general, being active gains people from two to four years of life, though some calculations concluded that it was a wash, that active people gained only as much time as they spent exercising.

(Of course, Dr. Lee noted, it's not just how long you live, it's how well, and exercise is key to quality of life, particularly in older age: "My mentor said it best: It's not the years you add to your life, it's the life you add to your years.")

Dr. Lee's paper drew on pooled data from six large studies that included more than 650,000 people followed over ten years, and showed that people who exercised at the recommended level gained 3.4 years of life after age 40. According to its numbers, she said:

Say you start with someone 45 years old who begins to follow the 150-minute-a-week recommendation. Average American life expectancy is 78. So: "If you start exercising at 45 and you die at 78, that means that you exercise for 33 years, at 150 minutes a week. I calculated that over 33 years you would need to spend basically 4,290 hours in exercise, which is 179 days of exercise, which is less than half a year. So that's half a year, and you gain almost three and a half years, so it is worth exercising. That's an approximate scenario using reasonable assumptions, and you're getting a 1-to-7 return."

And, I asked, what if you exercise more vigorously than the brisk-walking level?

In general, she said, more strenuous exercise has approximately double the effect. So, for example, 75 minutes of jogging has roughly the effect of 150 minutes of brisk walking. "So instead of gaining seven times the time spent, you'd be gaining 14 times."