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?”

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