The Facts About Fat Burning and Running

If you’re reading this while sitting down, congratulations—you’re primarily burning fat. However, if you get up and start moving around your home, you won’t burn as much fat. And if you felt really active and broke into a sprint outside, you’ll be burning almost no fat.

While everything we just said is true, it doesn’t mean all you have to do is sit tight and watch your fat stores melt away. The common presentation of how to exercise to lose fat—presented in admittedly exaggerated form above—is misleading.

There’s no special “fat-burning zone” that’s key to getting lean. Here’s what you need to know about burning fat through exercise.

What Fuels Your Running?

Look at wall charts or cardio equipment in the gym, or listen to many personal trainers, and you’ll encounter the fat-burning zone. The standard advice for getting in this zone is to work out at about 60 percent of your maximum heart rate.

That level of exertion is relatively low-intensity. Runners can usually talk in complete sentences at this effort, which is an easy pace like you might run the day before a race or the day after a hard interval workout. Working in this zone, it’s said, will burn more fat, and therefore result in greater long-term weight loss, than doing the same exercise at higher intensities.

On the surface, there’s some substance to part of this claim, according to decades of research. At all times, your body fuels itself primarily by burning a mix of fat and glycogen (the stored form of carbohydrate in your muscles). The less active you are at a given moment, the greater the percentage of that fuel mix comes from fat. 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 carbohydrate, and moves increasingly toward carbohydrate the faster you go.

One reason your body goes through this shift is because your brain runs almost entirely on carbohydrates, and it wants to preserve its limited carbohydrate stores. Although burning fat requires a lot more oxygen than does burning carbohydrate, there’s plenty of oxygen available when you’re at rest or working at a low intensity. As you start exercising harder, however, your body needs fuel more quickly, and turns more to carbohydrates.

This change in the fuel-burned ratio is why you might hit “The Wall” when trying to run a marathon as fast as possible, but you might not during an ultramarathon. A marathon run at faster than your normal training pace can use up all of the glycogen stored in your muscles. When that happens—usually in the final 10K—your muscles turn to fat to fuel your stubborn insistence on reaching the finish line. But burning fat requires a lot more oxygen than burning carbohydrate, so to meet that demand for more oxygen, you have to slow considerably, usually by a minute per mile or more.

In contrast, if you’ve ever run an ultramarathon, it was likely at a much lower intensity, probably slower than your normal easy pace. The percentage of each mile that’s fueled by fat is higher than at the faster pace of racing a marathon. So even though you might be running longer in an ultra, you’re less likely to have that sensation of having to suddenly slow significantly because of depleted glycogen stores. Even the leanest runners have enough body fat to fuel hundreds of miles at a leisurely pace.

There’s Burning Fat, and There’s Losing Weight

So, it’s true that at some workout intensities you’re burning a higher percentage of fat than at other intensities. But running a certain pace so that you burn a higher percentage of fat doesn’t magically melt fat away. And even if it did, the difference in total fat burned in running three miles slowly and doing the same distance faster is perhaps a couple dozen calories. That’s negligible in the grand scheme of things, given that burning a pound of fat entails burning about 3,500 calories.

More important, as Asker Jeukendrup, Ph.D., a leading sport nutrition researcher, puts it, fat burning and weight loss aren’t synonymous. Weight control is a matter of calories in and calories out. Burn more calories than you consume, and you’ll eventually lose weight. Do the opposite, and you’ll eventually gain weight. “If you burn more fat, but you eat more calories than the calories you burn in total, you will not lose weight,” Jeukendrup has written.

Because total calories burned is what matters, perhaps you can see another flaw with the “fat-burning zone” line of thinking: You could spend an hour walking three miles; of the roughly 300 calories you’d burn, a higher percentage would be from fat than if you ran three miles. But in that hour, you might run six miles, burning about twice as many calories.

If you want to get all geeky, the math in the following example (and in the graphic) argues against the fat-burning zone.

Walk 5 Ks in an hour, and of the roughly 300 calories you’ll burn, about 210 of them (70 percent) will be fueled by fat. Run 6-minute Ks for that hour, and of the roughly 600 calories you’ll burn, about 300 (50 percent) of them will be fueled by fat. Also, your metabolism stays revved up longer after vigorous workouts than it does after low-intensity exercise. While this postrun burn is likely only a few dozen additional calories, or less than the amount in a banana, every bit helps if weight loss is one of your goals.

The Real Reason to Consider Fat Burning as a Runner

Running at the gentle effort of around 60 percent of your maximum heart rate isn’t the key to weight loss, but there are still many reasons to regularly run at this pace.

Easy runs help you recover before and after harder workouts, they provide great cardiovascular and mental health benefits, and they’re simply enjoyable. Easy running also allows you to accumulate lots of mileage, and thereby burn more calories, if doing so is one of your motivations to run.

As for fat burning and running, perhaps the most important reason to care about the topic has to do with training performance, not weight loss. As we noted above, at sustained higher-effort levels, such as half marathon to marathon pace, your running is fueled by a higher percentage of carbohydrates than at slower paces. Run far enough at these paces, and you’ll start to deplete your muscle’s glycogen stores, and you’ll have to slow.

One of the main goals of marathon training is to become more efficient at burning fat when running these faster paces. If you can train your muscles to burn a little more fat per Km when running at marathon pace, then your glycogen stores will last longer, and your chances of holding a strong pace to the finish will increase.

The best way to accomplish that is by running not at a gentle jog, but at close to your marathon pace, with wiggle room of about 5 percent per Km faster or slower.

You can do these runs as stand-alone workouts, such as 10 to 15 Km at marathon pace after a 2- or 3-Km warm-up. You can also incorporate them into the latter part of your long runs, such as running easy for an hour and then running another hour at around marathon pace. When done a few times a month, sustained runs at these effort levels will improve your muscle’s fat-burning efficiency.

Starting some medium-length to long runs on an empty stomach, such as soon after waking up, and taking in no fuel during those runs, can also train your body to burn more fat at a given pace. It’s best to save these “fasted” runs for easy to medium-effort sessions.

All of this might seem like a lot to remember about fat burning. The key takeaway: Ignore claims you’ll lose more weight by running in a special “fat-burning zone.” Follow a good training plan, eat a well-balanced diet, and stay healthy enough to run the amount you want to, and you’ll likely settle at what for you is a good running weight.