Fat burning is always a hot topic and we seem obsessed with it. Treadmills tell you when you are in your "fat burning zone", some heart rate monitors indicate when you are "bruning fat". In a previous blog I discussed why we are so obsessed with fat burning (I gave 3 main reasons) and in another blog I discussed FatMax, a test to assess fat burning in an individual. I also explained "how fat burning works". I have been interested in this topic for many years and we did several research studies to investigate which factors affect fat burning. One of the most important factors is the exercise intensity.
How do we determine the exercise intensity at which you burn most fat and how do we determine your fat burning zone? We developed the so called FatMax test to give us some answers.
A FatMax test is a graded exercise test that is usually performed till exhaustion (although this is not absolutely necessary). The test will give you a curve like the one displayed above and represents the relationship between fat oxidation and exercise intensity. It allow you to work out at what exercise intensity you burn most fat. This intensity is called FatMax.
A FatMax is a test that needs to be conducted in a well-equipped exercise physiology lab with carefully calibrated equipment and skilled personnel. I have seen many test results that did not make sense because mistakes were made with the measurement of oxygen uptake and/or carbon dioxide production. The test can be performed on a treadmill or a cycle ergometer. During the entire test the participant wears a mask or a mouthpiece and a nose clip so that all inspired and expired air can be measured. To analyse oxygen uptake and carbon dioxide production a metabolic cart is usually used. From these measurements it is possible to calculate carbohydrate and fat burning. It is important for anyone performing the test to be very aware of the limitations and the assumptions made by his methodology. This is why I stress the importance of skilled personnel. More details of the test can be found in one of the first publications on the FatMax test. This paper also describes all the validation work as part of the development of the test.
Now let’s have a closer look at the graph. On the horizontal axis the exercise intensity is displayed. From low intensity to the left to moderate/high intensity on the right. On the vertical axis fat oxidation is displayed. Fat oxidation is expressed in grams per minute. Typically fat oxidation increases from low to moderate intensity (from walking to running ) and decreases from moderate to high intensity (from running to running fast/sprinting). Somewhere between low intensity and high intensity we will find maximal fat oxidation.
It is also obvious that fat oxidation is fairly high around FatMax and from a practical point of view it probably matters very little whether your fat oxidation is 5-10% lower or higher. So the fat burning zone that is often talked about can be quite a wide range of intensities. Looking at the first figure in this blog, however, it must be noted that fat oxidation curves can vary tremendously and it is very difficult to make general statements about fat oxidation. The only way to find your FatMax and your fat burning zone is by performing a test!
We have shown that ON AVERAGE fat oxidation peaks around 65% of maximal heart rate but this is an average! Every person is different and we have also shown that FatMax can vary from very low intensity (50% maximal heart rate) for some, to very high (80% of maximal heart rate) for others. The first figure in this blog clearly shows the wide variety of curves that can be observed.
This has a number of implications. First it is now obvious that if you get on a treadmill in a gym and this treadmill tells you that you are in the fat burning zone… there is no way that that treadmill can know if you are even close to your optimal fat burning. The same is true for various devices that are on the market. At best it works for the "average" person. The only way to really know your FatMax is by performing the laboratory test.
There are numerous factors that influence the FatMax curve, including diet, and I will address these in future blogs.
Achten J, Gleeson M, Jeukendrup AE. Determination of the exercise intensity that elicits maximal fat oxidation. Med Sci Sports Exerc. 34(1):92-7, 2002.