Mind & Body

Why the "Fat-Burning" Heart Rate Zone Won't Help You Burn Fat

If you've ever scrolled through the pre-programmed workouts on a treadmill, elliptical, or exercise bike, you've probably seen the one designed to keep you in the "fat-burning zone." You may have even chosen it — after all, who doesn't want to burn fat when they exercise? Well, it turns out that while there is technically a heart-rate zone that burns the largest percentage of fat, exercising in that zone is actually one of the worst ways to burn fat overall. Here's why.

Get in the Zone

The general wisdom is that to burn fat most effectively, you need to exercise at an intensity that raises your heart rate around 60–70 percent of its maximum. That's a pretty low intensity — a walk or a leisurely bike ride would get you there easily. Technically, this is the heart-rate zone where your body uses the highest percentage of fat as fuel. But if your overall goal is to have less fat on your frame, that's not necessarily what you want.

To understand why the fat-burning zone is a misconception, you first have to understand the different fuel sources your body uses and how it mixes and matches them depending on the exercise. Your body can burn fat, carbohydrates, or even protein for energy (although the amount of protein an overall healthy body uses for fuel is so small as to be insignificant). The lower your heart rate, the higher percentage of fat your body uses in its mix of fuel. The more intense your activity, the more carbohydrate takes over in that fuel ratio.

For example, right now, when you're sitting still reading this article, your body is burning fat almost exclusively. If you were to get up and walk around the block — hitting that "fat-burning zone" while you do it — about 70 percent of the calories you'd burn would come from fat. If you were to break out into a run, the calories you'd burn would get closer to 50/50 fat/carbs.

But hold on a minute. If you burn the most fat at rest, why would you ever want to hit the gym? Well, you don't burn the most fat at rest — the highest percentage of the calories you burn at rest comes from fat. That's a subtle, yet important difference. Why? Because as your exercise gets more intense, you burn more calories.

Think of it as money. If someone gave you the choice between a 70 percent return on $100 or a 50 percent return on $200, you'd definitely take the 50 percent return, right? Even though the percentage is lower, the actual amount of money you'd get is higher. Likewise, a one-hour walk might burn 225 calories, with 70 percent of that coming from fat, but a one-hour run might burn 500 calories, with 50 percent fo that coming from fat. Even though you're technically burning a higher ratio of fat during the walk, in real numbers, you're actually burning more fat overall during the run.

Related Video: Why Is It So Hard to Start Working Out?

More Brawn for Your Buck

That brings us to the most important reason you shouldn't care about the fat-burning heart rate zone: The amount of fat you burn during your workout doesn't really matter in the long term. Losing weight — including fat — is about burning more calories than you eat. Exercising in the so-called fat-burning zone burns fewer calories per minute than exercising at a moderate or hard intensity. So if your goal is to burn the most calories in the shortest amount of time, you may want to pick up the pace on that run or bike ride. (Not to mention that higher-intensity exercise has the afterburn effect going for it, so you keep burning calories after you're done).

But that's not to say low-intensity exercise is worthless. If you're just starting a workout routine, it's a great way to get comfortable and not overdo it. If you're a fitness junkie, low-intensity days are a great way to recover from more strenuous workouts. Just like the best exercise, the best exercise intensity is the one you know you can do regularly. Because doing the workout is the thing that will get results — regardless of the zone you're in.

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For more fitness mythbusting, check out "Which Comes First, Cardio or Weights?: Fitness Myths, Training Truths, and Other Surprising Discoveries from the Science of Exercise" by Alex Hutchinson. We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.

Written by Ashley Hamer April 17, 2019

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