Ironman Race Fuelling

I recently completed the Ironman World Championship in Kona Hawaii for the second year in a row (see link to report). But this year was a lot different from a nutrition perspective. For the first time in an Ironman race I didn’t suffer any gastric complaints. Gastric issues during a race can be caused by many things, but is often caused by malabsorption of carbohydrates. This year I decided I needed to change things up a bit in order to perform at my best. So I did some research on the best form of carbohydrates to fuel with during a race.

Carbohydrates are acknowledged by most experts (with some exceptions) as the best type of fuel during a race. They are important because glucose is the main source of energy for muscle cells. During prolonged exercise, muscle glycogen directly provides the energy to the working muscle cells, breaking into single glucose molecules that enter the mitochondria of a cell. Liver glycogen is also converted to glucose to maintain the supply of blood glucose. The muscles and liver store glucose as glycogen, however in limited supplies (500g), and not enough to last for more than a few hours. Therefore, during longer training sessions and racing, athletes must supplement these stores with carbohydrates found in food and sports nutrition products. The goal for the athlete is to consume enough carbohydrates to maintain energy expenditure without causing gastrointestinal distress, and it’s often a fine line. This is made complicated by the different types of carbs available, such as sucrose, glucose, maltodextrin, fructose, and debate about what type is best to take.

A bit of background to carb absorption.

Carbohydrate digestion begins in the mouth and continues in the stomach and small intestine. During digestion, complex carbohydrates (such as maltodextrin, sucrose, and starch) are broken down into glucose, fructose and galactose molecules that are absorbed in the upper small intestine.

Each of these monosaccharides uses a different type of molecule to transport it from the intestine to the blood. Glucose moves through the intestinal wall via a sodium dependent transporter called SGLT1, followed by water through the process of osmosis (hence the need for adequate amounts of fluid, and another reason for gastric issues if you don’t drink enough).


Figure: Glucose absorption in the gut via the SGLT1 transporter

Unfortunately, the glucose transporter has a limited transport capability. It can transport up to 60 grams of glucose per hour before it becomes a bottleneck. For some athletes (and especially in longer training or Ironman races) this amount is insufficient to maintain adequate energy supplies – i.e. it is simply not enough to replace expenditure.

Fructose has its own transporter in the gut, GLUT-5, independent of the glucose transporter. This means that glucose and fructose can be absorbed at the same time without interfering with the others absorption. Once fructose is absorbed, it is transported to the liver and quickly converted to glucose. Studies have shown that the rate of carbohydrate absorption can be increased up to 90g per hour using a mixture of glucose and fructose (Wallis et al 2005).

The complex carbohydrate, maltodextrin, is made up of multiple glucose molecules that are released in the gut and absorbed via the SGLT1 transporter. It’s a great form of glucose because, as it is a large molecule, it isn’t as sweet as glucose but provides the same energy.

Several studies support the use of a glucose/fructose combinations.

Currell and Jeukendrup (2008) showed that using a glucose/fructose mixture in a 2:1 ratio in trained cyclists led to an 8% improvement in time to complete a time trial compared to glucose alone.
Triplett et al. 2010 also showed that using glucose/fructose mixture led to a 9% improvement in speed in competitive cyclists during a simulated 100km cycling time trial, compared to glucose alone. Participants using glucose/fructose experienced no gastric disturbance whilst many of the participants using glucose only reported gastric problems.
Rowlands et al. (2012) showed a significant improvement in race time with the use of maltodextrin/fructose using a 2.5 hr mountain bike race, compared to maltodextrin/glucose. The study also showed a significant reduction in gastric disturbance with maltodextrin/fructose combination.

Putting the theory into practice

Based on this research, I decided to try the combination of maltodextrin/fructose. There are quite a few products out there with this combination, but the one I liked the most was Torq gels. They contain maltodextrin and fructose in the ratio supported by the studies, contain only natural flavours, and don’t use colours, not even natural ones. I liked the latter, because under exercise stress particularly an Ironman, the digestive system is under significant strain, so I didn’t want to put it under more stress with unnecessary ingredients. Another thing I really liked about Torq was the flavour range – they have flavours unlike any other gel I’ve ever had. Orange & banana, rhubarb & custard are just a couple of examples – when you’re on long training session or in an Ironman, variety is important!

After trialling the Torq gels a number of times in training with good outcomes in terms of energy maintenance and lack of gastric disturbance, I decided to use them in the big race.

I couldn’t have hoped for a better outcome during the race. I was easily able to consume 3 gels (90g) an hour, without any gastric problems. Moreover I felt I had so much energy on the run, and for the first time in an Ironman I did not lose more than 2kg and end up in the medical tent!

Changing up my nutrition to maltodextrin/fructose was a big win for me – I recommend trying it out.

Stuart Harsley
Pushys Sponsored Triathlete
Ironman U Certified Coach


Ironman University Nutrition recourses

Currell K,Jeukendrup AE. Superior endurance performance with ingestion of multiple transportable carbohydrates.Med Sci Sports Exerc. 2008 Feb;40(2):275-81.

Rowlands DS, Swift M, Ros M, Green JG. Composite versus single transportable carbohydrate solution enhances race and laboratory cycling performance.
Appl Physiol Nutr Metab. 2012 Jun;37(3):425-36. Epub 2012 Apr 3.

Triplett D, Doyle JA, Rupp JC, Benardot D. An isocaloric glucose-fructose beverage’s effect on simulated 100-km cycling performance compared with a glucose-only beverage. Int J Sport Nutr Exerc Metab. 2010 Apr;20(2):122-31.

Wallis GA, Rowlands DS, Shaw C, Jentjens RL, Jeukendrup AE. Oxidation of combined ingestion of maltodextrins and fructose during exercise. Med Sci Sports Exerc. 2005 Mar;37(3):426-32.

Wallis GA, Hulston CJ, Mann CH, Roper HP, Tipton KD, Jeukendrup AE. Postexercise muscle glycogen synthesis with combined glucose and fructose ingestion. Med Sci Sports Exerc. 2008 Oct;40(10):1789-94.



Categories: Nutrition

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s