The most important source of energy for supplying intensive sports performance consists of the carbohydrates. The carbohydrates are in fact capable of providing most energy per unit of time. If the effort is less intensive, fat burning plays a more important role.

A 400-meter runner receives his energy through the burning of carbohydrates; the 400-meter is extremely intensive.

Cycling and marathon running are sports which are all about endurance and fat burning is mainly addressed. The cyclist in the final of a race or during an escape switches to carbohydrate burning; because of the intensity the fat burning is not sufficient to supply energy. On this principle the work of the servants in cycling is largely based.

The servant’s role is to keep his leader away from the wind, so he uses less energy and consequently may start less tired than his competitors the final stage of the race. The servant with the leader in the shelter of his wheel, uses carbohydrate oxidation for his energy supply but at the end of the race his carbohydrate supply is exhausted.
The leader on the other hand, in the wheel of his servant, needs to make efforts less intense. His energy is created by burning fat. In this way, the leader can save his carbohydrate reserves for the final of the race where he can fight for the victory.

The amount of carbohydrates in the body in trained cyclists is approximately 500 – 600 grams. This stock is sufficient to maintain a sub-maximal effort for 60 to 90 minutes.
If the carbohydrates are not supplemented during these 60 to 90 minutes a low glucose level in the blood will emerge. This is the moment of ‘getting the bonks’ or the experiencing of ‘a fall of energy’.    The fats and proteins become, after the depletion of the glycogen depots, the major energy supplier sources. This conversion is accompanied by an irreversible decrease in performance.

Some remarks about fats as an energy source.

The average daily diet is too fat. The absorption of fats through our diet, therefore, should be limited. The fat reserve is in most athletes 10 to 15 kg. This amount is theoretically enough for 15,000 minutes or 350 hours of walking or 4,000 minutes or 67 hours of marathon running.

Fun runners with overweight who want to lose weight should do sports well at a low intensity. In that case, the burning of fat is maximally stimulated with weight reduction as a result

Metabolic profile shows that fat oxidation dominates during exercise at a low intensity and with intense effort the burning of glycogen supplies energy.

As has been previously described intensive effort is associated with carbohydrate burning and less intensive effort with fat burning. Through training, it is possible to realize a shift resulting in: a longer use of fat burning, as a supplier of energy, at a higher intensity of the effort and only at a later time the body switches to the burning of glycogen. Through this mechanism the glycogen stock is spared. That ensures that more glycogen is available for the final of the race.

After a training period arises an increase in the maximum performance capacity. The trained athlete is capable of, up to 80% of its maximum performance ability, to perform with fats as an energy source. If he makes efforts between 80 and 100% of its maximum performance the energy supply comes via burning of carbohydrate. Untrained people can only rely,  up to 50% of their maximum performance, on fat burning. They will therefore have to switch much earlier to carbohydrate burning.


About protein as energy source the final word has not been spoken yet.  Until recently, there was the idea that proteins play no role in energy supply during exercise. Recent research shows that proteins certainly play a role. In endurance sports 5 to 15% of the delivered energy coming from protein. This percentage may be further increased in several strenuous workouts in succession or if the duration of the effort further increases, such as during a marathon or triathlon

A large use of proteins as an energy source is unfavourable. The proteins used are, inter alia, derived from the muscles. The muscles are, so to speak, eaten, so that the performance will certainly be negatively affected. The recommendation that an endurance athlete should eat as sufficient 0.75 to 1 gram of protein per kg per day is also revised, and at the time the standard need for proteins in endurance athletes is from 1.5 to 2 grams of protein per kg per day. An endurance athlete of 70 kg body weight needs per day 105 to 140 grams of protein. Milk protein are preferred. It contains all the essential amino acids in significant quantities, is easily digestible and also easy to concentrate.


The combustion of carbohydrates, fats and proteins provides different amounts of energy:

  • 1 gram of carbohydrate yields   171 kJ = 4.1 kcal
  • 1 gram of fat provides                 6 kJ = 9.5 kcal
  • 1 gram of protein provides        7 kJ = 5.7 kcal

At first sight fat seems to provide the most energy. Nevertheless, this is not always true because different quantities of oxygen are required for the combustion of these three fuels. In case of intensive effort, the amount of oxygen per unit of time to reach the muscle is more important than the amount of fuel that is present in the muscle. In other words, the oxygen supply is the limiting factor.

When burning carbohydrates one litre of oxygen provides:      21.1 kJ = 5.0 kcal

In the combustion of proteins one litre of oxygen delivers:      18.7 kJ = 4.5 kcal

In the combustion of fats one litre of oxygen produces:            19.8 kJ = 4.7 kcal

The burning of carbohydrates yields most energy  per litre of oxygen.
During exercise, the body will operate mainly from that fuel which produces most energy with the given oxygen uptake. Therefore the body burns fat at low intensity efforts, because each gram of fat supplies most calories and because oxygen supply is not a limiting factor at that low intensity.

If the effort is more intense, the oxygen supply will be a problem. Then the carbohydrates, which provide more energy per liter of oxygen, are preferred.
Proteins should make a long way in the body before they are available as fuel. Therefore, these are not the most economical fuel, despite the favorable fuel / oxygen / energy ratio.

Carbohydrate combustion:     17.1 kJ            1 liter O2 21.1 kJ
Protein combustion:                23.7 kJ            1 liter O2 18,7 kJ
Fat burning:                              39.6 kJ            1 liter O2 19.8 kJ

Carbohydrates need relatively less oxygen (O2).  Therefore carbohydrates be consumed during intense workouts.
Fats need relatively more oxygen, less intense workouts consume fat. Proteins take a long, energy-consuming way.