According to all those infographics I saw on Google shmoogle, the “fat burning zone” is something you don’t want to mess up if you wish to lose weight:

The “fat burning zone” states that if you exercise at a specific intensity, you will use more fat for fuel and turn yourself into a “fat-burning furnace” or something. This is true — as you increase exercise intensity, you use more glucose aka carbohydrates than fat as a fuel source:

FAox (fat oxidation) is limited at higher exercise intensities. The point at which FAox reaches maximum and begins to decline is referred to as the crossover point. Exercise intensities that exceed the crossover point (~65% VO2max) utilize carbohydrates as the predominant fuel source for energy supply.

Hyperfocus on “fat oxidation” wording. It’s quite possibly keto zealots’ favorite term in English language. Hits all the pleasure centers (we will talk about keto later, I promise). It refers to the use of fat as the main source of fuel. 

Now, the above study also highlighted this image:

The crossover concept. The relative decrease in energy derived from lipid (fat) as exercise intensity increases with a corresponding increase in carbohydrate (CHO). The crossover point describes when the CHO contribution to substrate oxidation supersedes that of fat. MFO: maximal fat oxidation.

This is the first sign that there is something wrong with the “fat burning zone” concept — sleeping (“rest” in the infographic) gives the highest fat oxidation because when you are sleeping you are burning the greatest proportion of fat to carbohydrate.

So why not take enough sleeping pills to kill a horse and just hit the bed for 14 hours? You will burn a helluva lot of fat. The problem is that sleeping is so low in intensity, it barely burns any calories. 

The same goes for exercise performed in the “fat burning zone” — you use more fat for fuel but you also burn fewer calories compared to higher intensity exercise. 

And this is why the “fat burning zone” is only useful if you live in a weed-infested world — if you burned more total calories and utilized more total fuel in a form of BOTH glucose and fat, that is what will determine greater fat loss

Ben Carpenter, a fitness trainer, has a cool analogy for this — think of fuel during your training session as pieces of a pie. Fat usage will be higher at lower exercise intensity aka fat will be a much larger piece…

Fat oxidation pie

…But on a much smaller pie aka total calories burned:

fat oxidation versus total calories burned

Just because you burn more fat than carbohydrates during your workout doesn’t necessarily mean you will lose more body fat in the long run. This is because fat burning/oxidation is not the same as fat loss — exercise your ass off in the “fat burning zone” until you get a heart attack but if you are not in a calorie deficit, you are not going to lose fat. 

The same can be applied to a keto diet (see? I told you we would talk about keto): you burn a lot of fat because you are eating a lot of fat (you are also storing more fat). But if eating more fat puts you in a calorie surplus, you will GAIN fat even while having increased fat oxidation. 

Oh, and by the way, if fat oxidation is greater during any given time of the day (during cardio in the “fat burning zone, for instance), it’s compensated by greater carbohydrate usage later in the day:

The decreased rate of carbohydrate oxidation observed during the fasting period (from rising in the morning until the first meal at 12:00 h), was compensated by an increased fat oxidation from 09:00 to 12:00 h to cover energy needs.

I’m surprised you made it through the whole post. I thought you might hang yourself there at some point. Since you are still with me, let’s wrap this up.

While the “fat burning zone” is technically true, it does not aid in greater fat loss. If you prefer training at a lower intensity, knock yourself out. If you enjoy HIIT, do that. Whatever you can stick to long-term. 

The most important factor for fat loss is not the fat:carbohydrate ratio oxidized during a workout. You need to oxidize more fat than you store over a long period of time — weeks and months — to lose body fat (a hint — sustainable calorie deficit achieved through both exercise and dietary adjustments). 

So if you are trying to lose fat, ignore the “fat-burning zone” because it’s a textbook definition of pointless.


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Originally published by me on Medium on December 22, 2021

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