Heart rate and fat burning

Many fitness trainers, athletes and even doctors keep talking about the ideal heart rate that should be observed when exercising to achieve maximum fat burning. Although there is probably little argument against this theory, you may wonder whether it will benefit you much from a fat reduction perspective. An explanation.

Energy supply of the human body

One of the principles of cardio training is that one must keep the rhythm of one’s heart rate within a certain zone during exercise in order to burn as much fat as possible.
Our body has two main ways of storing energy:

  • Everyone is aware that fats can be stored in the body in almost unlimited quantities;
  • but our body can also store carbohydrates in the muscles and the liver. The disadvantage of storing carbohydrates, however, is that the energy supply that can be stored in this way is very limited at 300-400g.

Although very few carbohydrates are available to the body, they play a special role in energy supply. When carbohydrates are burned, much more energy can be supplied per unit of time than when energy is supplied from fat.

The higher the load, the higher the consumption of carbohydrates

The body almost always uses both energy supplies (carbohydrates and fats) simultaneously. The greater the load, the faster we cycle, for example, the greater the amount of carbohydrates used for energy supply. And eventually the body will only seek refuge in carbohydrates during a sprint. The heart rate can now be used to measure the individual degree of strain. And if we know the degree of load, we can in turn determine whether more fat or more carbohydrates are used for energy supply.

The ideal heart rate for fat burning

The theory that one must train in a certain heart rate zone to burn fat effectively, focuses on the use of different energy sources in his argument. If you cycle too fast, mainly carbohydrates are consumed and hardly any fat. So you have to train in the heart rate zone in which you burn the most fat.


An example will make this clear. Two men both walk for an hour:

  • person A at a leisurely pace where the energy supply from fats is greatest;
  • Person B, on the other hand, does not consider fat burning important and runs considerably faster.

Let us now look at the energy consumption:

  • person A consumed a total of 800 calories, half of which came from fat (44 grams = 400 kcal) and half from carbohydrates (400 kcal = 100 grams);
  • because person B ran much faster, he expended more energy, namely 1200 calories. However, 80% of the energy consumed (960 kcal = 240 grams) came from carbohydrates. Only 20% of the energy consumed consisted of fat (240 kcal = 27 grams).

Although person A ran much slower, he broke down 17 grams more fat than person B.

Fat burning does not equal fat reduction

A mistake that is often made now is that only the combustion during exercise is examined. But instead, the whole day should actually be involved in the research. And then it is often a lot less relevant how much fat is consumed during training.

The effect of a hearty meal after exercise

Because what happens when the two athletes in the example arrive in the canteen after training? Both eat a large portion of spaghetti and fruit ice cream for dessert. And as a drink they take apple juice. To keep the calculation simple, we assume that the food contains 240 grams of carbohydrates and 27 grams of fat.
What does the picture look like for our two men after this meal?

  • for Mr. B the picture is simple. His carbohydrate reserves have been emptied to such an extent that all 240 grams can be stored again. The 27 grams of fat are also completely replaced. Mr. B has therefore neither gained nor lost weight;
  • Mr. A has spared his carbohydrate supply by his slow walking, so that there is only room for 100 grams. The remaining 140 grams (560 kcal) remain. These are now converted into fat. The 560 calories correspond to 62 grams of “fat yield”. Mr. A stores a total of 89 grams (62 + 27) of fat through his diet. Although he consumed 44 grams more fat during training than Mr. B, this is not enough to prevent fat storage. After the meal, Mr. A gained 45 grams of fat.


Only the total energy balance counts!

The attentive reader will have noticed that the 45 grams in question correspond exactly to the 440 fewer calories that person A has consumed during exercise. It is therefore completely irrelevant for fat reduction from which source the body obtains its energy. Because carbohydrates can always be converted back into fat, only the total energy balance is important. After all, absorbed energy does not disappear. It is either consumed or converted into fat.
One may therefore wonder whether it makes sense to aim for a certain heart rate zone for fat burning, or whether it might even be counterproductive.

Scroll to Top