What is Fatmax?

Fat burning or fat oxidation (the term preferred by scientists) is of interest to endurance athletes and other athletes. Non-athletes who want to reduce body weight or body fat have also been interested in the topic. In this blog, we will explore what Fatmax actually is.

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Previous research...

When I started to study fat burning during exercise in the 1990s, I was interested in nutrients that could stimulate fat oxidation or at least allow fat oxidation to occur. It was common practice in research projects to study the effects of an intervention on fat oxidation at one fixed exercise intensity. For example, studies had investigated the effects of caffeine on fat oxidation during exercise. Athletes exercised twice at 70% of their maximum. Once with caffeine and once with placebo.

The problem with this approach is that you can only draw conclusions about what caffeine does to fat burning at that particular intensity. You still don’t know what happens at lower and at higher intensities. If you want to know the answers to these questions as well, you would have to conduct completely separate experiments. This is expensive and time consuming.


It was common practice to study fat oxidation at one fixed exercise intensity

The FatMax test

Therefore we developed a test that would give information about fat oxidation at a wide range of intensities (instead of just one intensity). This test was called the FatMax test (because the test can be used to determine the individuals exercise intensity at which fat burning is “maximal”).

The FatMax test lasts about 20-45 min, depending mostly on how trained the athlete is. The test starts at a low intensity (equivalent to walking) and every 3 min the intensity is increased (power on a bike or speed on a treadmill). During the test, expired gases are collected and/or measured from the athlete. This is used to calculate fat burning as well as carbohydrate burning.


We developed a test that would give information about fat oxidation at a wide range of intensities

How are carbohydrate and fat burning calculated?

When fat and carbohydrate are oxidized, different amounts of oxygen (O2) are used and different amounts of carbon dioxide (CO2) are produced. The ratio of the expired volume of CO2 to O2 therefore tells you something about the fuel that is being used. If, for example, only carbohydrate is used, this ratio of CO2:O2 is 1:1. If only fat is used as a fuel this ratio is 0.7:1. Scientists express this in a so called respiratory exchange ratio or RER. RER ranges from 0.7-1.0 but is usually somewhere in between, indicating that a mixture of carbohydrate and fat is used.


How accurate are the numbers?

This method can give us pretty accurate figures for carbohydrate and fat use, although we do make a number of assumptions and scientists must be careful not to violate those! One such assumption is that all CO2 produced and O2 used is derived from the oxidation of either fat or carbohydrate. This is a reasonable assumption in most cases. However, when the intensity is high; when breathing becomes difficult and talking would be an effort; lactic acid is being formed and when this happens the acid will be buffered by a buffer that the body naturally contains: bicarbonate. As soon as this starts to happen bicarbonate will release CO2 and water. Thus we have another source of CO2, that is not directly related to carbohydrate and fat oxidation. In short, the method of calculating carbohydrate and fat burning cannot be used at high intensities.


A number of assumptions are made and scientists must be careful not to violate those

How do we calculate FatMax?

So if we use this technique during a FatMax test we can plot carbohydrate and fat oxidation against exercise intensity. Carbohydrate oxidation increases linearly with exercise intensity. Fat oxidation shows a different pattern, the so called FatMax. In this figure you can see that on average (the data displayed is averaged from a larger group of athletes), fat oxidation increases as the exercise intensity increases from walking to jogging or running at a moderate pace. Then, when the intensity increases from running at a moderate pace to running fast, fat oxidation actually drops. Fat burning peaks at a mo