Running may be the single most effective exercise to increase life expectancy, according to a new review and analysis of past research about exercise and premature death. The review found that, compared to non-runners, runners tended to live about three extra years, even if they run slowly or sporadically, and smoke, drink or are overweight. No other form of exercise that researchers looked at showed comparable effects on life span.

The findings come as a follow-up to a study done three years ago, in which a group of distinguished exercise scientists scrutinised data from a large trove of medical and fitness tests conducted at the Cooper Institute in Dallas. That analysis found that as little as five minutes of daily running was associated with prolonged life spans.

After that study was released, the researchers were inundated with queries from fellow scientists and the general public, said professor of kinesiology Lee Duck-chul at Iowa State University and a co-author of the study. Some people asked if other activities, such as walking, were likely to be as beneficial as running for reducing mortality risks.

High-mileage runners wondered if they could be doing too much, and if at some undefined number of kilometres or hours, running might become counterproductive and even contribute to premature mortality.

And a few people questioned whether running really added materially to people's life spans. Could it be, they asked rather peevishly, that if in order to reduce your risk of dying by a year, you had to spend the equivalent of a year's worth of time on the trails or track, producing no discernible net gain?

So for the new review, which was published last month in Progress In Cardiovascular Disease, Professor Lee and his colleagues set out to address those and related issues by reanalysing data from the Cooper Institute and also examining results from a number of other large- scale recent studies looking into the associations between exercise and mortality.

Overall, this new review reinforced the findings of the earlier research, the scientists determined.

Cumulatively, the data indicated that running, whatever someone's pace or mileage, dropped a person's risk of premature death by almost 40 per cent, a benefit that held true even when the researchers controlled for smoking, drinking and a history of health problems such as hypertension or obesity.

Using those numbers, the scientists then determined that if every non-runner who had been part of the reviewed studies took up the sport, there would have been 16 per cent fewer deaths overall, and 25 per cent fewer fatal heart attacks. (One caveat: the participants in those studies were mostly white and middle class.)

Interestingly, the researchers calculated that, hour for hour, running statistically returns more time to people's lives than it consumes.

Figuring two hours per week of training, since that was the average reported by runners in the Cooper Institute study, the researchers estimated that a typical runner would spend less than six months actually running over the course of almost 40 years, but could expect an increase in life expectancy of 3.2 years, for a net gain of 2.8 years.

In concrete terms, an hour of running statistically lengthens life expectancy by seven hours, the researchers report.

Of course, these additions "are not infinite", Prof Lee said. Running does not make people immortal. The gains in life expectancy are capped at about three years, he said, however much people run.

The good news is that prolonged running does not seem to become counterproductive for longevity, he continues, according to the data he and his colleagues reviewed. Improvements in life expectancy generally plateaued at about four hours of running per week, Prof Lee said. But the figures did not decline.

Meanwhile, other kinds of exercise also reliably benefited life expectancy, the review found, but not to the same degree as running.

Walking, cycling and other activities, even if they required the same exertion as running, typically dropped the risk of premature death by about 12 per cent. (To make my own biases clear, I run but I also love cycling and I walk my dogs every day.)

Why running should be so uniquely potent against early mortality is uncertain, Prof Lee said. But it is likely, he said, that it combats many of the common risk factors for early death, including high blood pressure and extra body fat, especially around the middle.

It also raises aerobic fitness, he said, and high aerobic fitness is one of the best-known indicators of an individual's long-term health.

Of course, the findings in this new review are associational, meaning they prove that people who run tend also to be people who live longer, but not that running directly causes the increases in longevity. Runners typically also lead healthy lives, Prof Lee said, and their lifestyles may be playing an outsize role in mortality.

But even taking that possibility into consideration, he said, the data suggests that running could add years to our lives.

NYTIMES