Updated: Apr 18, 2020
By Wageningen Beasts
We probably all know the struggle of cutting. Sometimes it is difficult to lose fat, even though you are in a caloric deficit. Cardio is a helpful tool to cut down on body fat. Some speculation exists about the cardio intensity that would be optimal to burn fat. Is it better to do cardio with a low intensity for a long time, or a high intensity for a short time?
Energy systems in the body Before explaining what the ideal form of cardio is for burning fat, you should have a basic understanding of metabolism in the human body.
The body needs energy for a lot of different processes: basic functions to stay alive, repair and growth of tissue, and physical activity. In this situation we are mainly interested in the latter one. The body gets its energy from the conversion of ATP to ADP (I am sure you know what that is by now). To create ATP, several forms of fuel are used, such as carbohydrates (sugars), fats (fatty acids), and ketone bodies (which are only formed and used when being in a fasted state). These fuels are present in blood and stored in the body. After a meal, food is digested and taken up in the blood via the intestines. However, only small amounts of nutrients are present in the blood, because the blood has a strict range of concentrations of nutrients and other compounds. If these concentrations would be much lower or higher, the body cannot function properly. For example, in total only a few grams of sugar are present in the blood, providing the body with about 20-30 kilo calories (kcal). When you cycle for 2 minutes, all of this sugar will be used up. Therefore, the body needs strict regulations of the nutrients: an excess of nutrients will be quickly stored, and a nutrient shortage will be compensated for by releasing nutrients from the body stores into the blood.
Carbohydrates and sugars are stored in glycogen in the muscle and liver. Fat is mostly stored in adipose (fat) tissue, and some of it in the muscles and around the organs. Proteins can also be used as energy fuel, for which they first have to be converted to sugars (and urea). Protein is present in lots of different tissue, but its main storage location is in the muscles.
When the body needs energy (for example during exercise), it does not exclusively use one type of fuel, instead it will use different forms of fuel at the same time. The situation determines how much of a fuel is used. For instance, when you have not eaten for more than 24 hours, your glycogen stores will be low. Your body will then switch to more fat oxidation (the burning of fat to get energy), and the breakdown of protein (and thus muscle) will be increased in order to supply the body with enough glucose. The latter is important, because the brain can only use glucose as energy fuel, and not fat. If there would be no glucose, the brain would stop functioning. When you have just eaten a big meal, your body will switch to predominantly carbohydrates, and will thus burn less fat or breakdown less muscle for protein.
During exercise, the intensity determines how much carbohydrates and fats will be burned. In general, when exercise intensity increases, carbohydrate oxidation increases[1,2]. Also, fat oxidation will increase when exercise intensity increases, but at some point it will decrease again. Therefore, there is an optimal intensity to burn fat. Implementing this knowledge in your (cardio) workout, could help with optimizing fat loss.
Fatmax Studies tried to investigate which exercise intensity is ideal for fat loss. This optimal fat burning point, or the exercise intensity at which the maximal fat oxidation rate occurs, has been named Fatmax. Exercise intensity in cardio can be expressed in Wmax: The maximal amount of Watt produced before hitting failure. Wmax is closely related to VO2max, which is the maximal volume of oxygen the body is able to use. Wmax and VO2max therefore reflect pulmonary (lung) and cardiac (heart) functioning. Wmax and VO2max are therefore higher in trained athletes. A Wmax of 100% means maximal intensity of (cardio) exercise. At that point, the body simply cannot work harder because it cannot use more oxygen than it already does.
One study tried to investigate the amount of fat oxidation during different exercise intensities. The exercise intensities in cardio were expressed in Wmax. This study compared the amount of carbohydrates and fats that are burned during rest and at a Wmax of 40%, 55% and 75%. As can be seen in Figure 1, at Wmax 40%, the body will have a fat oxidation of about 50% (muscle and plasma TG + plasma FFA is about 25 KJ/min which is half of the total energy that is burned(50 KJ/min)). At a Wmax of 55%, fat oxidation is about 46% (30 KJ/min fat oxidation, 35 KJ/min glucose oxidation, total 65 kJ/min), so at this point in absolute numbers more fat is burned, but since glucose oxidation increases more, relatively less fat is burned. At a Wmax of 75%, fat oxidation is about 20% (20 KJ/min of total 80 KJ/min), which is both lower in absolute and relative numbers. Thus, exercising at 40% of max Watt has the highest relative fat oxidation.
Figure 1. Quantification of glucose and fat oxidation during different exercise intensities. Muscle glycogen and plasma glucose are part of glucose oxidation, muscle and plasma TG and plasma FFA are part of fat oxidation. %Wmax= percentage of the maximal exercise intensity displayed in Watt. Copied from van Loon et al (2001).
This does not mean that exercising at a Wmax of 40% is best for fat loss. Once again, it depends on the situation:
When you have a lot of muscle mass and you are trying to lose some body fat to get to a low body fat percentage, it is important to minimize muscle loss. When the body is low on carbohydrates (during a cut), it will break down proteins and thus muscle mass to produce sugars. Loss of muscle muss is therefore minimized when fat oxidation is relatively high, and glucose oxidation relatively low. This is the case for a Wmax of 40%: less fat is burned than at a Wmax of 55%, but also much less glucose is burned. In this situation a Wmax of 40% might be ideal.
When you don’t have a lot of muscle mass, and/or when you just want to lose a lot of fat, minimizing muscle mass loss is less important than losing fat. In this case, a Wmax of 55% might be more ideal. In this situation, energy balance is much more important: you just have to burn more than you eat. Therefore, you might also exercise at an intensity of 75% Wmax. You burn less fat and much more glucose in this case, but this will indirectly result in greater fat loss because fat stores will be burned to supply the body with enough energy. However, when having little carbohydrates/glycogen in the system, exercising at a high intensity is very heavy, and it might therefore be a better option to exercise at an intensity of 55% Wmax.
According to literature, the optimal fat burning point could be different for persons, as gender, age, training status, diet and body composition might play a role. An explanation for this could be that the bodies of trained athletes and people who consume low carb diets (either by fasting or high fat diets in the absence of carbs) are more efficient by being better able to switch to fat oxidation. Furthermore, there is some variation in Wmax, as these might differ per day. This variation is estimated to be around 3 to 7%. It is therefore difficult to implement the Fatmax concept with 100% accuracy in your training strategy. It might be that you need a slightly higher or lower intensity than what is recommended to have optimal fat oxidation. However, it might still be a good approach for cutting.
How to implement Fatmax in your training To start exercising at a certain intensity, you should know what your Wmax is. To determine Wmax, the following (simplified) protocol on a cycle ergometer could be used:
Warm up for 5 minutes at 100W for males or 75W for females. Maintain at least 60 rotations per minute during the whole test.
Increase the intensity with 35W every 2 minutes, until exhaustion.
Exhaustion = the point at which you cannot maintain 60 rotations per minute for more than 20 seconds
Write down the maximal work load at exhaustion.
Now you know your Wmax, you can implement the concept of Fatmax into your cardio workout schedule to optimize fat loss.
Conclusion The concept of Fatmax might be useful with regard to weight loss programs. The concept is however based on assumptions of physiology. I could not find any randomized controlled trials that investigated the validity of Fatmax for weight loss in comparison to another exercise regime. Therefore, it is not sure if optimal fat oxidation results in significantly greater weight loss compared to other fat burning strategies. I would therefore recommend to primarily stick to the key concept in weight loss, which is a negative energy balance. If you are able to implement Fatmax in your weight loss schedule, you might give it a try.
In short, Fatmax can be implemented in your training by:
Finding your Wmax with a cycling test.
Exercise at 40% of Wmax if you want to minimize muscle loss during a cut.
Exercise at 55% of Wmax if you want to maximize fat loss during a cut.
Look at your total exercise energy expenditure. A lower %Wmax also means that the exercise duration should be longer in order to burn the same amount of calories.
References  Jeukendrup, A., & Gleeson, M. (2010). Sport nutrition: an introduction to energy production and performance (No. Ed. 2). Human Kinetics.  van Loon, L. J., Greenhaff, P. L., Constantin‐Teodosiu, D., Saris, W. H., & Wagenmakers, A. J. (2001). The effects of increasing exercise intensity on muscle fuel utilisation in humans. The Journal of physiology, 536(1), 295-304.  Ghanbari-Niaki, A., & Zare-Kookandeh, N. (2016). Maximal Lipid Oxidation (Fatmax) in Physical Exercise and Training: A review and Update. Annals of Applied Sport Science, 4(3), 0-0.  Kuipers, H., Verstappen, F. T. J., Keizer, H. A., Geurten, P., & Van Kranenburg, G. (1985). Variability of aerobic performance in the laboratory and its physiologic correlates. International journal of sports medicine, 6(04), 197-201.