"There was a time when the optimal exercise speed was however fast you had to run to get away from a sabertooth tiger. Even today in much of the developing world people exercise through activities such as farming and fetching water that are necessary for survival. However in the developed Western world where exercise tends to be an extracurricular activity there is apparently tremendous interest in just how fast you should move in order to improve your health. Consider for example the many posts on The New York Times Well blog on the topic walking versus running httpwell.blogs.nytimes.comisitbettertowalkorrun the right dose of exercise httpwell.blogs.nytimes.comtherightdoseofexerciseforalongerlife walk hard walk easy httpwell.blogs.nytimes.comwalkhardwalkeasyrepeat all of which focus on the relative benefits of walking versus jogging versus running. So is it better to walk To walk fast To run slow To run fast On its face this question is poorly posed since it says nothing about our goals or our constraints. Am I aiming to lose weight To live longer To win road races Am I willing to exercise for three hours a day Twenty minutes Almost never Clearly these considerations matter when trying to determine the optimal speed. Here is how I would think about asking the question instead What is the easiest way to reduce my chance of death To analyze the impact of walking or running researchers need a way to describe the effort exerted. The ideal measure would combine the length of time spent exercising with the amount of energy expended  basically we want to figure out a way to credit people who walk at half the speed for twice as long the same amount as those who walk faster for less time. The way researchers do this is with a measurement known as MET  metabolic equivalent of task  which gives a numeric value to various activities depending on their energy intensity. By multiplying an activitys METs by the time you engage in it you can get an overall measure of how much energy you expend. The goal here is to use these METs and their relationship to health to analyze the value of walking compared with jogging or running. At least in this article I wont say anything about other kinds of exercise  no yoga or SoulCycle  though these have their own MET measures. Lets first look at the results from two papers  here http and here httpjournals.plos.orgplosonearticleid.journal.pone.  that relate energy expenditures from walking in MET hours per day to the risk of death. Doing this analysis is a little complicated. You cant look at the relationship between exercise and the chance of ever dying because ultimately everyone dies. Instead these studies look at whether exercise changes the risk of death at a given time. But since the people in the studies are of different ages researchers cant just look to see whether they have say a  percentage point lower risk of death in a given year since that lower risk would mean a lot more for someone who is  than for someone who is . Results in these two papers  and basically all the others well look at here  therefore report their results in hazard ratios. A hazard rate of . for example means a  percent reduction in the risk of death. If there is a  in  chance youll die in the next year without exercise this means by exercising youd reduce that chance to  in . This research shows that those who walk have a lower risk of mortality relative to people in the comparison group who dont walk for exercise at all they probably do still walk some just not for exercise. This risk of death is lower even with a very minimal energy expenditure. The lowestenergyexpenditure group in each study is walking at about  mph for  to  minutes per day. In other words a mile or two of walking. In exchange their risk of death goes down by  percent. Walking a bit farther  say  to  miles at  mph  gets you an additional death reduction of about  percent. But walking more than that or more than an hour a day at this speed is no better. Other studies of walking find similar magnitudes. A large metaanalysis http found that on average walking  to  miles at about a minutemile pace results in a reduction in death risk of about  percent. Many of these studies do not separate out speed and duration but to the extent they do it seems like walking faster may be better than walking slower. Here is one study httpcpr.sagepub.comcontent.full that shows large mortality reductions for fast walking even for short periods and here is another http showing that very slow walking has fewer benefits than faster walking. At first glance it may seem obvious that the harder you exercise the better. If walking faster is more energyintensive than walking slower and running is more energyintensive than walking it seems like the health benefits of running would be even greater. In some sense youd hope so since most of us find running more difficult than walking. Broadly over some range this seems to be right. The chart below shows results for two http studies  http runners that again relate METs to hazard rates of death. Running  even slowly like  mph  is far more energyintensive than walking. An energy expenditure of . METs per day the lowestenergyexpenditure group in the second study here means about  minutes of running a minute mile three times a week.  Slow running like this appears to have a much larger impact on mortality than walking. A hazard rate of .  compared with a sedentary group  suggests a much larger reduction in the risk of death than a hazard rate of .. What is a bit surprising about this chart is that running faster or farther doesnt seem to reduce death rates any more than running slowly and in fact in both cases is slightly worse. In one of these studies it looks like running very intensely faster than an eightminute mile more than four hours a week increases your risk of death relative to not running at all. But a closer look at the data suggests that this is probably just a statistical artifact This intense running group contains only  people and two deaths versus hundreds of people in the more casual running groups. Even if we dismiss the possibility that running harder is worse it is probably useful to note that running harder or farther doesnt seem to be better. I have occasionally felt guilty about my thriceweekly minute runs especially in comparison to my brother who runs a  marathon and once attempted the Leadville http Trail . It is comforting to know that I can still outlive him. One general problem with all of this research is that none of these studies is randomized. People who exercise are different from those who do not and those who exercise more are different from those who exercise less. This should as usual give us some pause in trumpeting the virtues of exercise. However Id argue that the comparisons the studies make among runners are still valid. To be more concrete In most of these studies the more intense runners are healthier for example less likely to smoke and thinner than the lessintense runners. We may tend to attribute the health of this group to their running habits. The fact that even with this bias we see no large benefit  indeed seemingly no benefit at all  to running more rather than running less reinforces the value of moderate running. If we take this research at face value we learn a few things. First some exercise reduces your risk of death. Second the optimal walkingjogging exercise is light to moderate jogging. The optimal speed is between  and  mph and if you do  minutes about three times a week youre all set. Nothing in the data suggests that running more  farther or faster  will do more to lower your risk of death."