The idea of changing to high-intensity interval training has been shown… (Feng Li / Getty Images )
Turn your body into a high-powered fat-burning machine!
If you see those words on a weight-loss website — especially if the site offers "secrets," "tricks," "simple rules" or other gimmicks that promise to make your fat melt away — you should be very skeptical.
I'm not saying it wouldn't be nice if there were some kind of shortcut, but that's just not the way it works.
I'll make it simple: When you exercise, you burn more calories than you do while sitting on the couch. The more intensely you exercise, the more calories you burn. That's pretty much all there is to it.
Despite this reality, people embrace all kinds of alleged miracle methods that promise to increase your resting metabolism and allow you to burn extra calories while doing absolutely nothing.
One of those methods is high-intensity interval training, also known as HIIT. It was popularized by Bill Phillips in his bestselling book, "Body for Life." "Not only does high-intensity training burn fat more effectively than low-intensity exercise," he wrote, "it also speeds up your metabolism and keeps it revved up for some time after your workout."
Phillips makes numerous references to "research" and "studies" in support of this idea, yet when I searched through his book, I couldn't find any of them cited by name. Through an assistant, Phillips told me that he based his claims on a 1994 study in the journal Metabolism that examined exercise intensity and fat loss. But in that study, neither the people in the lower-intensity training group nor the HIIT group lost much weight at all, and measurements of fat loss in the torso were very similar for both groups. Members of the HIIT group had 20% more body fat to start with, and those in the lower-intensity group gained fat in their calves for some reason, which also skewed the data.
Regardless, Phillips' endorsement of HIIT caught on with a vengeance. Personal trainers, fitness magazines and other media outlets often recommend HIIT for people trying to jump-start their weight loss.
I see the results of this pervasive advice firsthand at the gym, where people on treadmills, stationary bikes and elliptical trainers are constantly changing speeds, presumably in an attempt to get some lasting metabolic boost. Various models of these machines even come with "fat-burning" interval programs built right in.
It makes me shake my head.
Don't get me wrong — interval training is useful for improving athletic performance. It boosts endurance by improving your ability to take in more oxygen, and it also increases anaerobic metabolism, which improves short-burst power capabilities. Competitive runners, rowers, cyclists and other athletes do use HIIT to get faster.
What it doesn't do is burn calories any better than exercising at a steady pace.
Fat-loss "gurus" love HIIT for its alleged potential to dramatically boost something called excess post-exercise oxygen consumption, or EPOC. This is the caloric after-burn you get when your metabolism stays elevated — or "revved up," as Phillips puts it — after a workout.
EPOC does exist, but its significance has been overblown.
In a 2006 article in the Journal of Sports Sciences, researchers found that EPOC was indeed higher after intense exercise but only by 6% to 15%. So if you burned 1,000 calories during an intense and prolonged workout, at most you'd burn an additional 150 calories as a result of EPOC.
What's more, the better shape you're in, the less you gain from EPOC because your body's metabolism returns to normal sooner. A 1990 study in the Journal of Applied Physiology found that in nine well-trained people, the average EPOC was 4.8%, and in one case was as low as 1%. I'm in good shape, so let's assume I'd be near the average of that group. If so, then after my usual eight-mile run, EPOC calorie-burning earns me about one-third of a Newcastle Brown Ale.
So if EPOC is of little consequence, even for intense exercise, does interval training somehow change this dynamic?
No, it doesn't. If you do the same amount of exercise "work" (i.e., distance traveled) using either a steady pace or via HIIT, you burn close to the same number of calories — including the minimal EPOC.
In 1997, the researchers who measured EPOC in those nine people took another look at EPOC and how it is affected by interval training. Using eight men, they compared 30 minutes of running at 70% maximal oxygen uptake against doing 20 rounds of one-minute sprints at maximum effort. The sprints were interspersed with two-minute rest periods, during which I assume participants muttered things like, "Please let me die."
Because of the rest intervals, the HIIT runners took twice as long to complete the exercise sessions. But it paid off just a teeny little bit.