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Fat-Burning Heart Rate Zone: Myth That Never Dies, By Tom Venuto

Burn the Fat, Feed the Muscle Q & A

Q: Hey Tom, I recently went for metabolic testing and when going over the results, the doctor told me that at around 142 BPM I start to burn more sugar than fat for fuel. If this is true, doesn't that lend credence to the fat burning zone? Can someone explain to me how I would have a point where I start burning more sugar if there is no sweet spot heart rate for fat burning?

A: The fat burning zone is the idea that if you exercise at a lower intensity, you burn more fat for fuel, and if you exercise at a higher intensity, you burn more sugar for fuel. Therefore, the theory continues, you should exercise at a lower intensity if you want to lose the most body fat.

Only half of that theory is true, and overall, the theory fails. It's a myth that working out with less intensity makes you lose more body fat.

It's true that you do burn a greater percentage of fuel from sugar/carbs for energy during the workout as your intensity increases. The converse of that is also true - you burn a greater percentage of calories from fat when you exercise at a lower intensity, during the exercise session.

But there are several huge flaws with the fat burning zone idea.

The first flaw is that there's a big difference between the percentage of calories burned from fat and the total calories burned.

If you intentionally slow down and work out at a lower intensity to burn a greater percentage of calories from fat, and if the duration is the same, you burn fewer total calories. Therefore, you burn fewer calories from fat as well.

If you were to follow the fat burning zone theory to its logical conclusion, then you should sit on your couch all day long to burn more fat because that's about as low in intensity as it gets. Of course, the problem is, you hardly burn any calories lounging on the couch. As you can see, worrying only about the percentage of calories burned from fat to the exclusion of total calories burned makes no sense at all.

Second, there's a difference between burning fat for fuel during a workout and actually losing body fat over time (seeing a decrease in your body fat percentage).

Burning fat for fuel during a brief workout doesn't mean you will lose body fat over time. The fuel source you use during a single workout is not the same thing as fat loss.

Furthermore, looking at what you burn during your workouts is only considering a 30 or 40 or 60 minute window of time. What about what you burn and how much you burn the rest of the 24 hours in the day? The amount of calories you burn the rest of the day far exceeds what you burn in one short workout.

Here's something else most people don't realize about fuel usage: If you burn more carbs during your workout, you tend to burn more fat later in the day. If you burn more fat during your workout, you tend to burn more carbs later in the day. It could end up being a wash and not mattering much at all what fuel you burned for a short 30-60 minute period.

Fat oxidation matters. However, the fuel source used during the workout is less important than 24-hour energy balance and the net effect on your body composition over time. You have to look at long term effects, not just what's happening during the workout.

For example, what if you're in the "fat burning zone" for your cardio workout, but then you eat yourself into a surplus later in the day? You don't lose fat, and the workout being in the fat burning zone - or any zone - was irrelevant. What's relevant is the calorie deficit.

And what if you burn mostly sugar during your workout because it was high in intensity, but you were restrained the rest of the day and you ended the day in a deficit? You do lose fat, even though you were in the "sugar burning zone" during the workout.

Here's something else most people don't know about fuel oxidation: You're always burning a mix of fat and carbs, not one or the other exclusively.

Take a look at this example:

Low intensity exercise - 30 minutes
Fuel mix: 50% fat, 50% carbs Carbs burned: 110 calories
Fat burned: 110 calories
Total calories burned: 220

High intensity exercise - 30 minutes Fuel mix: 33% fat, 67% carbs
Carbs burned: 222 calories
Fat burned: 110 calories
Total calories burned: 332

When you train harder (at a higher intensity), you're still burning a mix of fat and carbs, but you burn more total calories. So even if the most important thing were the type of fuel you burned during your workout, you could actually end up burning just as much fat or more, during the workout, by training at a higher intensity. But for the day, you end up in a bigger deficit because you burned more total calories.

In addition, with the high intensity workout, you may stimulate excess post exercise oxygen consumption, or EPOC, also known as the afterburn effect. So there may be a small, but significant residual effect that gives you an even greater deficit at the end of the day.

Here's the bottom line: 24-hour energy balance is more important than the fuel source burned during workouts. The more total calories you burn every day, week, etc., the more fat you will lose, if all else remains equal.

This means you should not worry about what fuel you are burning during a short workout and start focusing on your calorie deficit for the day.

But counterintuitively, this does not mean you have to track how many calories you burn during workouts. It's more effective to focus your attention on how many calories you eat each day and track that to be sure you have a deficit.

In your cardio training, if you simply track your weekly cardio duration, frequency, and intensity, that's all you need to know. If you want to burn fat faster, increase one, two, or all three of those variables while making sure you don't eat more to compensate. For example, increase from 20 minutes to 30 minutes. Or increase from 3 days a week to 4 days a week. Or increase from level 8 on the stairclimber to level 9. All of those tweaks will result in more weekly calories burned.

One final note: this should not be taken to put intense cardio on pedestal or to discount or discredit longer, lower-intensity cardio sessions. Low intensity exercise can be just as effective for fat loss as high intensity cardio, it simply takes more time to burn the same amount of calories. Depending on the intensity levels, it's entirely possible you could burn as many calories in a high intensity 30 minute session as you could in a 60 minute low intensity session. The difference is efficiency more than effectiveness.

A benefit of low or moderate intensity cardio is that it can help bump up your total weekly calorie burn without tapping too far into your recovery reserves. There's only so much high intensity training you can do each week without teetering on overtraining. Low intensity cardio also may be safer for the lean body builder in terms of holding onto lean body mass when dieting in a deficit.

Every calorie you burn counts and will have an impact on fat loss. If your intensity drops too low however, you simply won't be burning as many calories per unit of time, and it will take a lot longer to get the same results.

There certainly could be a fat burning "sweet spot." I think there is. It's the place where you get maximum total calories burned and maximal fat oxidation, and where the workout stays practical. In my opinion, that sweet spot could be low intensity cardio for around 60 minutes, medium intensity cardio for 40-45 minutes or high intensity cardio for 20-30 minutes. All of these approaches can produce similar results if the total calories burned are similar.

By the way, it's not only not necessary to track the amount of calories you burn when training, it's not mandatory to track heart rate either. Heart rate is one way to track intensity, but you can also get a good idea of whether your workout is low, medium or high intensity by using the rating of perceived exertion scale.

Learn more about cardio training for fat loss in the Burn the Fat, Feed the Muscle book:

Burn the Fat, Feed the Muscle Print Book Edition

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Tom Venuto,
Author of Burn the Fat, Feed the Muscle