Is There a Fat Burning Zone and Does It Matter?

If you’ve been reading this blog on a consistent basis you probably already know where I’m going with this one. You guessed it. The Fat Burning Zone is another of the urban legends of fitness. Does anyone think that when they are in the so-called fat burning zone that stored bodyfat melts off them like butter?

A little reality therapy is in order. The Fat Burning Zone is a big fat lie. Here’s the truth.

1- The “fat burning zone” supposedly describes a level of exercise that results in a larger number of the calories burned during exercise being derived from fat. This does not mean that stored bodyfat is the selective source. It only describes the relative percentage of utilization of three substrates, fat, carbohydrate and protein.

2- The fat burning zone actually describes what percentage of calories burned are derived from fat as an energy source. Do you know when you are burning the most calories from fat? Sorry. The highest percentage of fat utilization is at rest. The more intense the exercise becomes, the more carbohydrate is used as a source.

Guess what. It doesn’t matter. The reality is that it’s about the number of calories burned, not the number of those calories that come from fat as a source. If the fat burning zone idea actually worked we could get extremely lean by simply sitting still. Guess again. That doesn’t work, does it.

Confused, let’s use a mathematical example.

Lets assume that we have two identical exercisers who are going to exercise for twenty minutes. Exerciser one is doing a slow walk to stay “in the fat burning zone”. Exerciser two is going to run hard for twenty minutes. To keep the example simple we will assume that exerciser one will derive forty percent of his or her calories from fat. Exerciser two will move out of the fat burning zone and only derive 20 percent of his or her calories from fat.

Exerciser one will walk at 3 miles per hour and will cover one mile in twenty minutes. This will result in a caloric expenditure of 100 calories with 40 calories coming from fat.

Exerciser two will run at 7.5 miles per hour and will cover 2.5 miles in twenty minutes. This will result in a caloric expenditure of  250 calories with 50 calories coming from fat.

Hmm, seems interesting. The exerciser in the “fat burning zone” burned less calories and less calories from fat in the same amount of time? The exerciser working harder and leaving the fat burning zone burned 2.5 times as many calories and, 10 more calories from fat.

I rest my case. Figures lie and liars figure. Stop worrying about burning fat and start worrying about working harder

PS- If you read Gary Taubes Why We Get Fat you might reconsider all of this. If you want to get leaner exercise is not the route. There are lots of reasons to exercise but fat loss is 90% diet. In either case working harder still beats working longer.


14 Responses to “Is There a Fat Burning Zone and Does It Matter?”

  1. […] In a nutshell…at different heart rate levels your body will use differing amounts of fat vs carbohydrates for its energy source.  At lower heart rate levels your body will use proportionally more fat than carbohydrates for energy.  So, for example, in the “fat burning zone” your body might be deriving 40% of its energy from fat and 60% from carbohydrates.  At you move out of the “fat burning zone” and into higher heart rate zones your body will start to use proportionally more carbohydrates and less fat, say 80% carbohydrate and 20% fat.  The “logic” behind staying in the “fat burning zone” is that you are using up more fat for your energy.  Yet the problem with this argument is that at lower heart rates you are burning fewer total calories.  The harder you work, the more calories you burn.  So if two people are sitting in a spin class and one spends the entire hour in the “fat burning zone” (sorry, I have to keep putting it in quotations because it is a LIE) and the other spends some of her time in the “fat burning zone” and some of her time in the aerobic and anaerobic zones (gasp!) who do you think is going to burn more calories?  The one who spends time outside the “fat burning zone.”  And if you’re trying to lose weight, isn’t that really the name of the game?  For more explanation and another example, see Mike Boyle’s blog post “Is There a Fat Burning Zone and Does It Matter?”.  […]

  2. mboyle1959 Says:

    Mike- the obvious question is “How do you feel”? Are you getting stronger and gaining muscle mass? If you are, I would not be worried.

  3. Hi Mike, I have read gary taubes and I follow that style of diet right now. High fat, meats and leafy greens. As a bodybuilder and strength trainee, I am worried about the need for carbohydrates for glycogen resoration if i am trying to increase size. I am also grain free.

  4. […] Is There a Fat Burnin&#103&#32&#90one and Does It Matter? « Michael Boyle's … […]

  5. @ Darcy, I think the article Mike wrote was about LSD as a means of losing fat, and not a method of conditioning. The crux of the article is that there really is that focusing on a “fat burning zone” is not really productive to losing fat.

    As to conditioning, I would agree that for many types of athletes “cardio output” training is helpful. Especially for endurance athletes.

  6. Hi Mike, I thought you were going to recommend “table pushaways”, but done in the fat burning zone. Still one of the funnier articles i read from you about how people kept asking you how to do the table pushaways when you said that was the best exercise to lose weight.

    Thanks for trying to deconstruct another fitness myth. Funny enough, I was just reading a comment by some poor woman whose personal trainer kept telling her she needs to exercise in the fat burning zone.


  7. Darcy Norman Says:

    Mike, be sure to call it what it is, “the oxidative zone”. Don’t kid yourself, it is very necessary and can have a huge impact on training depending your goals. Talk to any elite endurance athlete and they will tell you. My comments are not be confused with the slow run neither. Everything we were given coming onto this planet has it’s purpose, otherwise we wouldn’t have it. Lets focus on its purpose rather than what people May think it does. It’s not all or none. The cliche, everything in moderation. To take a quote from you Mike, cliches are cliches because they are true.

  8. […] Mike Boyle: Is there a fat burn­ing zone and does it matter? […]

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