The 'fat-burning zone' a myth; what matters is calories burned

If you're the kind of exerciser who constantly checks your heart rate to ensure you're in the "fat-burning zone," you can stop. There's no special fat-burning zone that's key to getting lean.

And this is not new information. Here's what you need to know about the myth and about the real relationship between exercise and weight loss.

Yes, we know. If you look at wall charts or cardio equipment in a gym, or listen to many personal trainers, you'll be indoctrinated about the fat-burning zone. The standard advice for getting in this zone is to exercise at 60 percent of your maximum heart rate. That is relatively low intensity; most people can talk in complete sentences. But working in this zone, it's said, will burn more fat and result in greater long-term weight loss, compared with doing the same exercise at higher intensities.

There is substance to part of this claim.

The body primarily fuels itself by burning a mix of stored fat and carbohydrates. The less active you are at a given moment, the greater the percentage of that fuel mix comes from fat. As intensity of activity increases, the percentage of carbohydrates in that fuel mix also increases. At rest, fat constitutes as much as 85 percent of calories burned. That figure shifts to about 70 percent at an easy walking pace. If you transition to a moderate-effort run, the mix becomes about 50 percent fat and 50 percent carbohydrates, and it moves increasingly toward carbohydrates the faster you go.

So it's true that at some workout intensities, you're burning a higher percentage of fat than at other intensities. But you aren't burning more calories.

First — although it might sound better for weight loss to burn a higher percentage of fat — the real-world effect of that intensity on your body composition is next to nil. "The idea that all of a sudden when you hit this zone the fat is just being sucked out of your system is simplistic," says Christopher Breen, an exercise physiologist and online coach in Long Island, N.Y. "That completely ignores that losing or maintaining weight is basically a matter of calories in versus calories out."

If the key determinant of weight loss were the percentage of fat you burn, then your best bet would be to remain still, because that's when you're burning the highest percentage of fat relative to carbohydrates. But, as Breen says, total calories burned is what matters, and that fact leads to the second big problem with the fat-burning zone.

"If you're exercising at this lower intensity, you're burning fewer calories per minute," says Christine Brooks, a University of Florida adjunct instructor and the coaching science coordinator for USA Track & Field. "The average person walking for an hour is going to burn only a couple hundred calories."

In that time, you could burn more than twice as many calories running, cycling or using an elliptical machine at a moderate intensity.

DO THE MATH

Also, if you want to get all geeky, the math argues against the fat-burning zone. Walk two miles in an hour, and you'll burn about 200 calories, with roughly 140 of them fueled by fat. Cycle moderately for that time, and you'll burn about 500 calories, with roughly 250 of them fueled by fat — so you'll burn more calories and more fat. "When I worked with people in a gym, I would tell them, 'Ultimately, it's a matter of calories; the fat burn will take care of itself,'" Breen says.

Another chit for more vigorous workouts: You get an after-burn effect. "You maintain a higher metabolic rate after higher intensity exercise," Brooks says. "The reason is that more damage is being done to various systems, so you have an increased heart rate while the body is making its necessary repairs."

"I have a real beef with the way this fat-burning idea is promoted," Brooks says. "It's a very strange way to talk about exercise." She and Breen agree that the myth persists because it makes good graphics. "It's a way of making exercise machines more appealing — if I'm working at this speed, I'll burn more fat than at another speed," Breen says.

GOOD USE OF TIME

None of this is to suggest low-intensity exercise is a waste of time. Even the top athletes in the world regularly and purposefully work out at a light effort. A gentle jog or easy spin is a great way to clear your head, get re-energized, improve your health, spend time with friends and family, and, yes, burn some calories.

"Mix it up," Breen says about structuring your workouts. "Have some harder, high-intensity days, followed by easier, low-intensity recovery days." Also aim for different durations. When you have the time, do longer workouts at a comfortable level of effort. When you're pressed for time, work a little harder.

Heart-rate training can help you construct a well-rounded exercise program, because harder work raises heart rate. Breen suggests:

■ 20 minutes at a heart rate less than 60 percent of your maximum for recovery days;

■ 20 minutes at 60 percent to 70 percent for easy days when you socialize while working out;

■ 20 to 90 minutes at 70 percent to 80 percent to improve endurance without exhausting yourself;

■ 20 to 30 minutes — or two to three 10-minute segments — at 80 percent to 90 percent with two-minute rest breaks to build ability to work hard;

■ Two- to 5-minute intervals at 90 percent to 95 percent to build aerobic capacity.

Don't forget warmups and cooldowns.

Variety will keep you fresher physically and mentally than if you do the same thing day after day after day. That freshness will make it more likely that you exercise consistently. And that's the zone where the long-term weight loss happens.

Scott Douglas is a contributing writer for Runner's World magazine and the author of several books, including Running Is My Therapy.

Celia Storey added some information to this report.

Style on 12/24/2018

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