Carbohydrate Dose and Type During Exercise

How much and what type of carbohydrates should we consume during exercise?

The adequate availability of carbohydrates during exercise is essential for performance in endurance sports. When glycogen stores (the storage of carbohydrates in the liver and muscle) are depleted, a drop in blood glucose will occur, and our energy substrate will be fats, a metabolic pathway much less developed in most athletes and which also does not allow for performing exercise at high intensities, so we will encounter the dreaded “wall” or “bonk”.

Therefore, supplying carbohydrates during exercise will be important to avoid that depletion of glycogen stores. However, what type and amount of carbohydrates should we consume? On this subject, Dr. Asker Jeukendrup is probably the world's leading expert. In a study (Jetjens et al., 2004) conducted by his research group, they observed the response of trained cyclists during two hours of pedaling at medium intensity when consuming medium levels of glucose (1.2 g/min), high levels of glucose (1.8 g/min), a mixture of glucose (1.2 g/min) and fructose (0.6 g/min) or water.The results showed that the carbohydrate oxidation rate (and therefore the energy obtained from them) was higher when consuming a mixture of glucose and fructose than when consuming glucose alone, regardless of the level of glucose consumed.

Confirming these results, years later Currell and Jeukendrup (2008) conducted a study in which, after two hours of moderate-intensity effort consuming 1.8 g/kg of glucose alone, a combination of glucose and fructose, or a placebo solution, they performed a one-hour time trial. In this case, the authors observed that the consumption of the carbohydrate mix (+ fructose) increased performance by 8% compared to the consumption of glucose alone, and by 19% compared to the consumption of placebo.In addition, other authors Smith et al. (2013) analyzed the performance benefits (time trial of 20 km after 2 hours at moderate intensity) with the consumption of different doses of a glucose, fructose, and maltodextrin drink, finding that the higher the dose, the greater the benefits up to 80 g/h, but obtaining negative results (partly due to gastrointestinal discomfort) from that dose onward.

In conclusion, carbohydrate intake is very important to avoid fatigue during exercise and improve performance. Consuming carbohydrates that use different transporters to enter the cell (such as glucose, maltodextrin, and fructose) allows for greater utilization of them than consuming glucose alone. Specifically, when consuming carbohydrates with different transporters, we could assimilate up to 80-90 g/h of carbohydrates in long-duration exercises, whereas with the consumption of glucose alone, the mechanisms would saturate at 60 g/h at most, also increasing the risk of gastrointestinal discomfort. In the image, we show some recommendations made by Dr. Asker Jeukendrup based on the evidence on the subject.As we can see, in efforts lasting less than 1 hour it may be sufficient to use mouth rinses with a single type of carbohydrate (e.g., glucose), since this strategy stimulates some changes at the brain level that reduce fatigue. However, as the duration of exercise increases, we will require a higher supply of carbohydrates, reaching up to 80-90 g/h in longer efforts if the athlete can tolerate it at the gastrointestinal level, something that should always be 'trained' before the competition.

Recommendations for carbohydrate intake during exercise by Jeukendrup (2014)

REFERENCES

1. Jentjens RLPG, Venables MC, Jeukendrup AE. Oxidation of exogenous glucose, sucrose, and maltose during prolonged cycling exercise. J Appl Physiol. 2004;96(4):1285–91.
2. Currell K, Jeukendrup AE. Superior endurance performance with ingestion of multiple transportable carbohydrates. Med Sci Sports Exerc. 2008;40(2):275–81.
3. Smith JW, Pascoe DD, Passe DH, Ruby BC, Stewart LK, Baker LB, et al. Curvilinear dose-response relationship of carbohydrate (0-120 g/h-1) and performance. Med Sci Sports Exerc. 2013;45(2):336–41.
4. Jeukendrup A. A step towards personalized sports nutrition: Carbohydrate intake during exercise. Sport Med. 2014;44(SUPPL.1).

AUTHOR

Pedro Valenzuela
Researcher in the Physiology Unit at the University of Alcalá and the Performance Monitoring Unit at the Sports Medicine Center (AEPSAD, Madrid High Performance Center).
Website: www.fissac.com