During exercise, depending on training status and intensity, carbohydrates are the main fuel for the resynthesis of ATP.
Per liter of inhaled oxygen carbohydrates form 16% more ATP than fats.

A well-trained athlete does the whole triathlon with an intensity of 60-70% of VO2 max. At this intensity level both fats and carbohydrates can be used as fuel. The lower the intensity, the greater is the proportion of fats for energy. So muscles burn both carbohydrates and fats. As the effort grows more intensive, more glycogen and less fat is burned.


The amount of glycogen in liver and muscles, under normal conditions, is 400 to 500 grams. A kilogram of carbohydrates is good for ± 4000 kcal. So the carbohydrate stores in the body produce 1600 to 2000 kcal. The glycogen store in the muscles is the fuel for muscular work. This stock is adequate for an average of 60 to 90 minutes sub maximal exercise. The speed at which muscle glycogen is consumed depends on the intensity of the effort. The more strenuous the effort the sooner the glycogen stock is depleted. The depletion of the glycogen results in a low blood sugar level. This is the moment of ‘getting the bonks ’ which is caused by a fall of energy.  Other key features of low blood sugar levels are: performance drop, hungry, sweaty and clammy, limp powerless legs and light-headedness. The nervous system is not functioning properly with uncoordinated movements. Think of the athletes who comes uncoordinatedly drifting over the finish line. By eating in time and regularly during exercise you can prevent depletion of glycogen stores.

Coordination problems caused by lack of fluid and depletion of glycogen.


A well-trained athlete with a body weight of 75 kg and a body fat percentage of 10% has a fat supply of 7.5 kg. Fat per kg contains ± 7000 kcal. In total, this athlete has about 50,000 kcal in his fat stores. Practically speaking, there is always plenty of fat in stock.



The carbohydrates consist of single and multiple sugars. The single sugars or monosaccharides are glucose and fructose. Glucose and fructose can be directly, without digestion, absorbed into the blood stream through the intestine. If one wants, during or after a race, to reach a rapid replenishment then glucose and fructose in dissolved form are the most suitable. The multiple sugars are the disaccharides or polysaccharides. Disaccharides are sucrose and maltose, polysaccharides are starch and glycogen.

The multiple sugars are broken down by digestive enzymes into small glucose particles. Only after that absorption is possible via the intestine into the blood. This digestion process takes time. Therefore, multiple sugars are absorbed slowly. During prolonged exercise a combination of single and multiple sugars is recommended. The fast and the slow uptake ensure a continuous supply of carbohydrates to the blood.

In the body, glucose is stored in the liver and in the muscles in the form of glycogen. Thus, there are two types of carbohydrates present in the body; the blood glucose and glycogen in liver and muscle tissue. A control mechanism under the influence of the hormones insulin and glucagon ensures that the glucose content in the blood remains relatively constant.

A high level of glucose in the blood, for example, immediately after a meal, stimulates the insulin production. Glucose is stored with the aid of insulin as glycogen in the liver and muscles.

The glycogen in the liver has mainly the aim to maintain the glucose content in the blood. This is important because the nervous system can only use glucose as a fuel and thus is constantly dependent on the glucose present in the blood.


The parts and distances of the entire triathlon are: swimming (3.8 km), cycling (180 km) and running (42.2 km). The labor intensity is in a well-trained triathlete during the entire triathlon at 60 to 70% of the VO2 max. Real toppers can complete the entire triathlon approximately at 80% of their VO2 max, while someone who needs 15 hours uses about his 40% VO2 max.

With a weight of 75 kg, the well-trained athlete will consume about 10,000 calories during the whole triathlon. Of these 10,000 kcal required 50% is provided by burning fat and 50% by carbohydrate burning. The fat supply is more than sufficient to supply the 5,000 kcal. Carbohydrate stock is always insufficient for the entire triathlon. Maximally one can store about 2000 kcal carbohydrates while a total of 5000 kcal are necessary. To cover the total requirement about 3000 kcal carbohydrates must be absorbed through food, which corresponds to 750 grams of carbohydrates. From this little sum it appears that the whole stock of glycogen is consumed at the end of the triathlon. To prevent this, the triathlete must ingest more than the 3000 kcal that are required. In order to prevent depletion of the glycogen content, he must ingest once more extra 2000 kcal, in the form of carbohydrates. In total he needs to ingest 3000 + 2000 = 5000 kcal or 1250 grams of carbohydrates.



10.000 kcal

5000 kcal fat 5000 kcal carbohydrates
50.000 kcal fat in stock 2000 kcal glycogen in stock
no intake fat 3000 kcal carbohydrates to cover the need

need for whole triathlon 5000 kcal through food which corresponds to 1250 gr carbohydrates

uptake by food:
40 bananas. 1 banana = 150 gr = 32 gr carbs.
1250 gr soluble carbs


Total distance triathlon: swimming 3.8 km, cycling 180 km running 42.2 km


During the marathon the labor intensity is higher than during the triathlon. Toppers run the marathon at 80-94% of their VO2 max. Because the intensity of the effort has increased during the marathon, so does the proportion of carbohydrate burning strongly. By increasing the intensity, more carbohydrates and less fat is used as fuel for energy. If, after 60 to 90 minutes, during the marathon no carbohydrates are ingested from food or drinks the glycogen reserves will be depleted. Then low blood sugar levels will arise, resulting in a decrease of performance. The running pace cannot be maintained because of switching from carbohydrate to fat burning. These low blood sugar level makes the athlete feel hungry, clammy, sweaty and limp.

The time of depletion of glycogen stores is usually around 30-35 km of the marathon distance. Depletion can be prevented by taking small quantities of carbohydrates during exercise, well before the 30 km. Also a carbohydrate rich diet can be useful, a few days before a marathon or a triathlon to fill the additional carbohydrate reserves. As a result, the moment of depletion shifts to a later point in time.