But mainstream researchers in the field of exercise were a little slower to buy in. Government standards for physical activity, based on advice from the American College of Sports Medicine, still favored more vigorous activities such as running. It prescribed at least half an hour of heavy exercise at least three times per week. "I wrote the guidelines, and I still hold to them," said Michael Pollock, a prominent exercise physiologist, in the middle of the walking hype. The debate in academia would last for almost a decade.
Finally, in the mid-1990s, government agencies softened up in deference to the power walkers. Now Americans were advised to do at least half an hour of something less intense than running—a brisk 4- or 5-mph promenade, for example—but they were advised to do this lighter work more often. In other words, their total dose of exercise would be the same, but they would be taking it in a less concentrated form.
The present version of the guidelines makes this logic more explicit. The Centers for Disease Control and Prevention equalizes workouts of varying intensities according to a standard exchange rate of 2-to-1: Every two minutes we spend hoofing around in Rockport ProWalkers equates to a single minute spent on the jogging trail. That means people can mix and match their workouts until they approximate a recommended weekly total: Either 75 minutes' worth of sweaty, vigorous workouts, or a double helping (150 minutes) of something moderate—or any custom combination of the two.
Williams calls this the "exchangeability premise," that any form of exercise can be subbed in for any other, and that their effects on health will be identical once corrected for the amount of energy that goes into them. Obesity experts often talk about the same idea, using the phrase "calories-in/calories-out." Either way, they're subscribing to a model of personal health where quantity trumps quality. It doesn't really matter what we choose to eat—fats or carbs or proteins—or which machines we end up using at the gym. All that counts is the total energy we absorb from food, minus the total energy we expend in action.
Several of Williams' recent findings support this notion. Most recently, he found that running and walking have about the same effect on risk for high blood pressure, high cholesterol and diabetes as long as they're matched up for total energy expenditure. (That is to say, someone who walks for two hours a day will see the same benefit as a matched control who runs for half that time.) In two other papers from the past few months, he showed that running and walking produce equivalent reductions in the risk of osteoarthritis, hip replacement, and cataracts.
An Australian research group has just released a study confirming this idea. For that paper, published March 28, the scientists followed about 11,000 middle-aged women over a 12-year stretch and compared both their total exercise and types of exercise to signs of hypertension and depression. They found that, in general, women who never bothered with vigorous activity were at no greater risk than the others, even when they were matched for total levels of exertion.
Yet Williams' papers and the Australian one suffer from a common flaw: With this type of research, it's impossible to know what hidden factors might be biasing the data. In Williams' group, for example, the runners could have been eating less overall, or they could have been more inclined to exaggerate their workouts. The sorts of long-term, case-controlled studies that could prove the exchangeability premise once and for all would be almost impossible to carry out.
So it's possible that running and walking aren't quite the same, after all. The calories-in/calories-out model does have some notable detractors. The endocrinologist Robert Lustig and journalist Gary Taubes have made a case for saying that food quality does matter, and some calories are worse than others. Could the same critique be applied to exercise? Might some forms of exercise be like eating fruits and vegetables, while others are like eating meat and dairy?
Williams notes that runners do seem to get some extra benefit from their exercise. In short-term studies, he says, they show improvement at making up for bouts of overeating. (If they get a big meal at one sitting, they'll eat less at the next.) They also experience a heightened metabolism that extends beyond the period of working out.
But runners have another, more important edge: They get more done in less time. A vigorous workout is quicker to finish, so it fits more easily into a busy schedule. That may be why the runners in Williams' cohort ended up getting more exercise, over all.
With that in mind, I asked Williams—a runner himself—if he'd recommend the more vigorous activity. He declined to answer. "I'm not really an exercise advocate," he said. "I don't really care if people exercise or not. But I do care about good science."