• Boris Clark

The ‘fat burning zone’ myth and truth

Updated: Jan 31, 2020

A lot of people talk about ‘the fat burning zone’ and how they have to keep their heart rate under ‘XYZ’ amount (usually some absurdly low percentage of max heart rate) or they will switch to burning carbohydrate.


This couldn’t be further from the truth.


First of all, your body never burns fat in isolation. In fact fat burns in a carbohydrate driven flame. Secondly, at extremely low intensities you will burn a higher percentage of fat, but at low intensity you aren’t burning a lot of anything. A high percentage of not a lot is still not a lot!!!


So the ‘fat burning zone’ as many people know it is a myth. But there is obviously a level of intensity where the most fat is burned, which is called ‘fatmax’. In this article we will explain how fatmax works and how to improve it, and why this may or may not matter for you.


Putting a high percentage of fat on the fire doesn't burn a lot if the fire isn't very big (I.e if the exercise isn't very hard!)

The fat burning process

At the start of any good explanation of how the body uses substrates is anaerobic glycolysis. The process of splitting glucose into pyruvate, mixing in some oxygen, and making a bunch of ATP to let us move our muscles.


Notice I said ‘at the start’. At low intensity we are still performing this anaerobic, carbohydrate utilising process, just not at a very high rate. The ATP gained from the anaerobic glycolysis reaction itself and ATP from oxidising the pyruvate doesn’t give us enough energy to do what we are trying to do at this intensity, so we have to look elsewhere. This energy shortfall will be made up from fat.


As we exercise more intensely, we ramp up the anaerobic glycolysis process, we get more ATP from anaerobic glycolysis and the oxidation of the extra pyruvate this creates, but because we are exercising more intensely, our total energy demand is higher, so there is an even larger shortfall which needs to be covered, which of course gets covered by fat, around this level is where fatmax exists.


So the harder we go the more fat we burn right? Wrong.


As we exercise even harder, the rate of anaerobic glycolysis ramps up a lot more. Now there is so much pyruvate being produced that we don’t have enough oxygen available to oxidise it all straight away, the pyruvate turns to lactate until oxygen becomes available. Pyruvate is the preferred energy source of the body, so if we have more than we can handle then there is no shortfall to be covered, and no fat usage.


A quick note is that the point at which the pyruvate production and combustion are equal (where any excess pyruvate would turn to lactate) is the ‘lactate threshold’. There is emerging evidence that a very small proportion of fat may be able to be burned at intensities higher than this, particularly in athletes who follow a very low carbohydrate diet, but this is yet to be firmly established.


So now we know what causes fatmax, how do we improve it?

Dietary changes are one obvious way, eating less carbohydrate in particular. It is the removal of carbohydrate from the diet rather than the addition of extra fat that causes fat burning to be increased. If you have lower carbohydrate availability you will have less carbohydrate available for anaerobic glycolysis, you won’t produce as much pyruvate, you will need to burn more fat to cover the shortfall, and your body will make adaptations to allow it to burn more fat should this diet continue or happen again.


Want to burn more fat? Well don't rush to the olive oil too qucikly! Dietary manipulations which increase fat burning are more to do with the reduced carbohydrate content than the added dietary fat

But what about from a training standpoint?


There are two main things we can do.


The first is to increase the aerobic capacity. Burning fat requires a large amount of oxygen, if we have a larger aerobic capacity, we can oxidise all the pyruvate that gets produced up to a higher intensity, and therefore cover the energy shortfall at a higher total energy demand with fat, which obviously burns more of it.


The second thing we can to is DECREASE the anaerobic capacity. A lower trained anaerobic system will use anaerobic glycolysis at a lower rate at every intensity. Therefore lower pyruvate production, and a bigger energy shortfall, which means a greater amount of energy needing to come from fat.


So does this matter?

Maybe.


If you want to lose weight (from fat), not really. Total energy burned is still the most important aspect here. Even if you burned mostly carbohydrate when you went and exercised this will mean that any carbohydrate you eat now will be stored as glycogen rather than fat. If you burned mostly fat when exercising, sure you got rid of some fat, but when you eat carbohydrate again your glycogen stores are still quite full, so you will either use the carbohydrate you just ate for energy then and there (acute carbohydrate intake results in higher carbohydrate usage), or will store it as fat. So what substrate fuelled your ride wasn’t really important for this.


If you are an endurance athlete competing several hours at a time, having a higher fatmax probably is important. You can exercise harder for longer, with much less chance of ‘hitting the wall’ since you are less likely to run out of carbohydrate, and can stay topped up on carbohydrate easier.


If you are an athlete competing in short events less than an hour or two. Fatmax is probably less important, the event is likely so intense that you aren’t burning much fat anyway, and you are unlikely to run out of carbohydrate stores in such a short time period if proper nutritional practices were followed in the lead up to the event.


Hopefully this gives you an insight into what fatmax is, and as always, feel free to contact us if you have any questions!