IRONMAN Italy: optimising fuel delivery

Asker Jeukendrup’s point of view

The IRONMAN Italy is approaching. And everyone who is taking part on September 21st should remember that a good nutrition strategy could be the difference between a good race and a great race or between finishing and not finishing!

Carbohydrate intake during prolonged exercise (exercise of 90 min or longer) can enhance performance. This is the conclusion of a fairly recent review articles that looked at over 60 studies, performed in almost 600 athletes. In 86% of the cases carbohydrate improved exercise performance. This is convincing evidence. The question is how can we make sure we deliver the right amount and the right type of fuel?

How much carbohydrate?

How much carbohydrate is needed depends on a number of factors: the duration of exercise, the intensity (the power that is produced, the energy expended, the pace).

During shorter exercise there is no need for high carbohydrate intake but with increasing exercise duration higher intakes of carbohydrate are recommended. Studies suggest a dose response relationship between carbohydrate intake and exercise performance.

A higher carbohydrate generally results in better performance but ingesting too much may cause stomach discomfort and this can have a negative effect on performance.

So, it is important to manage the carbohydrate intake well. Another factor that plays a role is that above an intake of about 60 grams per hour, the oxidation of ingested carbohydrate does no longer increase. This is because absorption is limited to about 60 grams per hour. It is possible to increase the delivery of carbohydrates, but this means we have to change the composition of the carbohydrate source.

What type of carbohydrate?

There are many different types of carbohydrate and some carbohydrates are oxidised better than others. Generally, we can divide carbohydrates into slower and faster carbohydrates. Slower ones include galactose and fructose and faster ones: glucose, sucrose, maltose, maltodextrins and some starches. These fast carbohydrates all behave very similar.


So even a starch can behave the same as simple sugar like glucose! But even the faster carbohydrates will not provide more than 240 kcal per hour (60 grams per hour).

For example when 100 grams of carbohydrate would be ingested per hour, only 50-60 grams of carbohydrate would be used as a fuel by the muscle each hour.

Studies have shown that the only way to use more carbohydrate from a drink, gel or energy bar is if we use a combination of two different carbohydrates. Ideally this is about 60 grams of carbohydrate per hour from glucose or maltodextrin and 20-40 grams per hour of fructose.

In other words: when more than 60 grams of carbohydrate is ingested, a combination of glucose and fructose or maltodextrins and fructose can increase the oxidation of that carbohydrate.

These combinations of carbohydrates (>60 grams per hour) can also increase performance (>2.5h) more than just a single carbohydrate. With the use of appropriate combinations of carbohydrate ingestion rates of 90 grams per hour are recommended in some conditions.


optimal fueling

How often we need to take carbohydrate?

Studies suggest that it does not matter too much whether you take smaller amounts of carbohydrate frequently or a couple of larger doses. Essentially the carbohydrate will be stored temporarily in the stomach and gastric emptying will determine the delivery of the carbohydrate. However, studies have also shown that gastric emptying is enhanced when there is a larger volume in the stomach, so for many events it may be good to start eating/drinking soon, or maybe already in the 5-10 minutes before the start.

Does it matter how carbohydrates are delivered?

Carbohydrates can be delivered in many different ways: gels, bars, drinks, jellies, fruits and other natural foods. Studies have shown that gels ingested with water and sports drinks deliver carbohydrate at the same rate.

Solid foods can be delivered at the same rate as well if they do not contain too much fat, fibre and protein (these will all slow down gastric emptying).

These findings have important practical implications because it means that we can mix and match our carbohydrate sources. If an athlete prefers solid food and water that will work, but if an athlete prefers sports drinks and gels that can also work. Whether you use drinks, gels, bars or other foods thus depends on personal preference and strategy. For example, professional cyclists will often use solid foods in the first part of the race and move to drinks and gels later.


In summary

So optimal carbohydrate delivery is dependent on a number of factors and what is “optimal” depends on a balance between delivering enough carbohydrate but without causing gastro-intestinal discomfort.



  • Stellingwerff T, Cox GR. Systematic review: Carbohydrate supplementation on exercise performance or capacity of varying durations. Appl Physiol Nutr Metab. 2014 Sep;39(9):998-1011.
  • Jeukendrup A. A step towards personalized sports nutrition: carbohydrate intake during exercise. Sports Med. 2014 May;44 Suppl 1:S25-33.



Professor Asker Jeukendrup is one of the world’s leading sports nutritionists/ exercise physiologists who spent most of his career as a professor at the University of Birmingham (UK). Dr Jeukendrup authored 8 books and over 200 research papers and book chapters. His expertise stretches from exercise metabolism and sports nutrition to training and overtraining. He is currently a (visiting) professor at Loughborough University and director of his own performance consulting business “Mysportscience” and communicates science through the popular website mysportscience.com. Asker works as Performance Manager for the Dutch Olympic team and is Head performance Nutrition for the Lotto Jumbo Pro cycling team. He also works with FC Barcelona, and other elite football clubs. Asker practices what he preaches and completed 21 Ironman races including 6 time the Ironman world Championship in Hawaii.