Carbohydrate Timing During Exercise: Fewer or More Intakes?

We have already talked on numerous occasions about the proven benefits of carbohydrates during exercise, especially in endurance sports. As we have seen in previous articles (you can read more here, here and here), consuming carbohydrates helps us maintain stable blood glucose levels and prevent the depletion of glycogen stores in reservoirs such as the liver, thus preventing one of the main mechanisms of fatigue: hypoglycemia. In addition, by promoting the use of carbohydrates as an energy source, we can perform at a higher intensity, since compared to fat metabolism, carbohydrate metabolism allows us to maintain higher intensities.

In the last decade, numerous studies have been conducted regarding what the optimal dose of carbohydrates to consume is according to the type of exercise being performed. However, it is less clear whether the time or timing of carbohydrate intake can influence performance. For example, it is common to see athletes carrying gels or bars to events like a marathon, and most tend to wait until they have covered a large part of the event to start consuming them. Thus, two athletes may consume the same dose of carbohydrates (for example, 60 grams), but one does so by taking a gel after already exercising for an hour, and the other does it through smaller but more frequent intakes. Will both athletes have the same benefits?

Influence of tempo on carbohydrate intake

To answer this question, prestigious researchers such as James Bett and Javier González conducted a study on a group of runners who exercised to exhaustion at a fixed speed.¹ The participants performed the test on two occasions, ingesting 75 grams of carbohydrates in a single dose after 75 minutes of exercise, or in 15 doses of 5 grams every 5 minutes from the start of the race.

Interestingly, although the amount of carbohydrates was the same in both cases, starting to consume them earlier and spreading out the dose made the runners use up less of their muscle glycogen stores in those first 75 minutes, and the time to exhaustion improved by 10 minutes compared to consuming carbohydrates in a single dose (106 minutes versus 96 minutes).¹ In fact, although at the end of the test all subjects showed the same level of muscle glycogen depletion, glycogen levels at minute 75 of the test were correlated with the runners' performance.

These results seem to confirm the findings of another already classic study conducted almost 30 years ago by one of the leading researchers in exercise physiology, Mark Hargreaves.² In that study, a group of cyclists pedaled for 2 hours at a constant intensity, and then performed a 15-minute test. During the 2 hours of continuous pedaling, the participants had to consume either a placebo, or a drink with a low amount of carbohydrates but frequently throughout the test, or a drink with a higher amount of carbohydrates but ingested after they had already been exercising for 90 minutes. Again, the results showed that the intake of a smaller dose but carried out more frequently was the condition under which the cyclists were able to perform better during the final test.² Similarly, another study evaluated cyclists who performed 2 hours of continuous pedaling at submaximal intensity while consuming either a placebo or a carbohydrate drink before facing a 10 km test.³ This drink contained a certain amount of carbohydrates, and according to the assigned condition, they were supposed to take it either distributed throughout the test, only at the beginning, or only at the end. In this case, both the intake of carbohydrates distributed during the exercise and ingested before the final test were the conditions that most improved performance, while if they ingested the entire drink right at the start of the exercise, no significant benefits were observed.³

Conclusions

The debate between 'few or many intakes' of carbohydrates during exercise does not have a single answer for all contexts, although the Evidence suggests that, when consuming a certain amount of carbohydrates, it may be advisable to spread the intake in small doses throughout exercise. In addition, if the exercise is going to be very prolonged, it may be recommended not to delay starting the first intakes if we want to avoid glycogen depletion.. It is important to take into account that other studies have not seen differences between spreading out the intakes or accumulating them in a single dose. For example, a study in football players did not observe differences between ingesting a certain dose of carbohydrates (1 g/kg) before the first half, or spread between before the first and second half.⁴ Therefore, factors such as the athlete's preferences or the context of the test (for example, the possibility of taking in the intakes at certain points) must also be relevant in our decision.

AUTHOR

Pedro Valenzuela
Researcher at the University of Castilla la Mancha
Web: www.fissac.com

References

1.          Menzies C, Wood M, Thomas J, Gonzalez J, Betts J. Frequent Carbohydrate Ingestion Reduces Muscle Glycogen Depletion and Postpones Fatigue Relative to a Single Bolus. Int J Sport Nutr Exerc Metab. 2020;30(3):203-209.

2.          McConell G, Kloot K, Hargreaves M. Effect of timing of carbohydrate ingestion on endurance exercise performance. Med Sci Sports Exerc. 1996;28(10):1300-1304. doi:10.1097/00005768-199610000-00014

3.          Heesch MWS, Mieras ME, Slivka DR. The performance effect of early versus late carbohydrate feedings during prolonged exercise. Appl Physiol Nutr Metab. 2014;39(1):58-63. doi:10.1139/apnm-2013-0034

4.          Mizuno S, Kojima C, Goto K. Timing of carbohydrate ingestion did not affect inflammatory response and exercise performance during prolonged intermittent running. Springerplus. 2016;5(1):4-11. doi:10.1186/s40064-016-2108-6