Imagine two women working out on treadmills: One is red-faced, doing sprint intervals, and the other is simply walking on a slight incline. Which one is burning more calories? The sprinter, of course. But there’s more to it. “High-intensity cardio causes you to work anaerobically, which means that most of the calories you burn will be in the form of carbohydrates. Your body needs fuel—fast—and carbs are its go-to energy source,” explains McCall. With less-intense workouts, in which you keep your heart rate in a lower, more comfortable zone, you may be burning fewer carbohydrates, but the duration of workouts tends to be longer. This means more total calories will be burned—which is key to losing weight.
And the health benefits extend beyond weight loss. A 2014 study led by researchers at Iowa State University found that just 5 to 10 minutes of slow jogging a day was enough to significantly reduce the odds of dying from any cause, including cardiovascular disease—even if the participants had risk factors like smoking or being overweight. Of course, the Centers for Disease Control and Prevention and other major health organizations recommend a tad more exercise than that. But the researchers found that even an hour a week—total—was enough to get the benefits. Working out longer than that didn’t make a notable difference. In other words, more intense—or more, period—isn’t necessarily better.
In fact, a 2015 study published in The Journal of the American College of Cardiology suggests that slow may even be healthier than hard-core. The researchers followed people who did similar amounts of slow, moderate, and strenuous running over the course of 12 years and found that the light exercisers had a much lower mortality risk than the intense group. The moderate exercisers fell somewhere in between. Experts aren’t sure why slow may be superior, but some believe the stress that super-hard workouts place on your system may be a factor. “Intense exercise causes a cascade of hormones that create a lot of free radicals and inflammation in the body—which, over time, can damage your immune system and lead to chronic health issues,” says McCall. “I’m not saying you should never do another spinning class, but what we now know is that slow and steady is often better than hard and fast.”
McCall recommends low-intensity interval workouts that alternate between a fairly easy pace and one that’s slightly more challenging—a training style that burns more calories than working at one steady pace for the same amount of time. In fact, research shows that interval walking can expend up to 20 percent more energy than walking at a single, moderate pace—because your body expends extra energy to speed itself up and slow itself down. And you can do it with any type of cardio. Here’s the how-to: Start out at a difficulty level of 4 on a scale of 1 to 10 (with 10 being the hardest you could possibly manage) and stick with that pace for a certain amount of time—it could be two minutes, the length of one block…you decide what works. Then pick up the pace to an effort of 6 for the same amount of time. “Use the talk test to make sure your heart rate doesn’t get too high,” says McCall. “You want to walk fast enough that it feels challenging but you can still chat comfortably. If you’re too out of breath to have a conversation, slow it down—you’re working too hard.” Repeat these intervals for the duration of your workout, aiming for 20 to 30 minutes.