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Avoid GI Issues: How to Fuel Racing and Training

Updated: Jun 1, 2021

One of the toughest challenges faced by athletes is determining what to consume during exercise.

There is so much information out there.

The reality is there is no 'one size fits all'.

There are a number of guiding principles that you can use to help you pick a strategy.

In this video and article we are going to look at the following:

  • Explain the different types of carbohydrates in sports products

  • Show what the science and research says about using carbohydrates

  • Give some practical guidelines on how to fuel

During exercise we use fat and carbohydrates as sources of energy.

Depending on exercise intensity we utilise our fat and carbohydrate stores at different rates.

During long slow steady swims, cycles and runs we burn fat with a little carbohydrate.

For higher intensity work we will use more carbohydrate as fuel. The graph below shows this relationship. Here we see the impact of fuel utilisation as power output


At easy intensities the body uses more fat than carbohydrate as a fuel source, usually typical zone 2 training .

The breakdown of fat as fuel is a slower process than that of carbohydrates.

As intensity increases your body needs more energy fast, this is where carbohydrates come in.

Once you reach your FTP you will use mainly carbohydrates to fuel your performance.

Our body stores roughly 500g of carbohydrate (2000 kcal) in the liver and muscle as glycogen. During exercise you convert these stores into energy as the body requires it.

Fat stores are a lot more abundant. At the right intensity you can tap into up to 20hrs worth of energy using fat as the primary fuel source.

We have a limited supply of carbohydrates and an abundance of Fat.

Our fuelling requirements should always be guided by the intensity and duration of training.

The figure below shows how one individual athlete uses fuels at different intensities. Ironman intensity is in and around the Aerobic Threshold. You will see that even at this easier intensity they will burn roughly 65g of carbohydrate per hour of exercise.

This is very individual and you may burn more or less at the same intensity based on your own physiology.

When it comes to racing fast over long duration carbohydrates are king.

Yes we want a high fat oxidation, but we also need to include carbohydrates into the mix if we want to perform at our best.

Carbohydrates are macronutrients made up of long chains of smaller units called saccharides, or sugars.

Simple sugars are broken down into the following:

  • Monosaccharide: one ring structure of carbons and hydrogens

  • Disaccharide: two ring structures of carbons and hydrogens

  • Oligosaccharides: a few rings joined together

You will find that these simple sugars are the main ingredients to the majority of sports products.

As carbohydrates link together in more intricate structures (hence the term “complex carbohydrates”), they become starches (rice and potatoes), glycogen (i.e., the fuel stored in our liver and muscles), and various types of soluble and insoluble fibers.

Sports nutrition products will mainly contain the following simple sugars:

  • Maltodextrin

  • Glucose

  • Fructose

  • Sucrose (Glucose + Fructose)

You will see these pop up on the back of every sports nutrition product.

Scientists have spent decades researching the impacts that carbohydrates have on performance.

The research suggests that athletes can only oxidise or break down a single carbohydrate source (i.e. Maltodextrin) during exercise at a rate of 1g/min or 60g/h.

For example if you have a gel or drink with maltodextrin as it’s only source of carbohydrate. You will only be able to use 60 g/h.

It is all the body can tolerate. The figure below shows how carbohydrate uptake happens:

  • You ingest carbohydrate

  • Single sources of simple sugars like maltodextrin and glucose use a transporter called SGLT1 (sodium-dependent transporter)

  • These transporters deliver carbohydrate to the bloodstream and muscles for use

But what happens if you want to or do consume more than 60 g/h?

If you are aiming to have 90 g/h of a single source of carbohydrate you will only be able to absorb 60 g/h.

The remaining 30 g/h is not absorbed and will remain in the intestine. Because this remains undigested over long periods of time this could lead to GI distress.

But what happens if you need to digest up to 90 g/h based on the duration of your event?

A study by Currell and Jeukendrup in 2008 answered just that question. In this study they took 3 groups of cyclists and asked them to perform a 40K time trial after 2 hours of exercise.

Each group was given different amounts of nutrition for the ride.

Group 1 were given a placebo (water)

Group 2 were given 90g of Glucose per hour

Group 3 were given 90g a 2:1 ratio of Glucose (60g) + Fructose (30g) per hour

The results are displayed in the figure below


The placebo group were the slowest

Group 2 were 9.1% faster than the placebo group.

Group 3 were 7.6% faster than group 2.

This demonstrates that if you mix your carbohydrate sources your body is able to absorb more. Higher absorption resulted in faster times.

How is this possible?

Researchers found that if you saturate the glucose transporter with 60 g/h you can use a different transporter for fructose (GLUT5).

This is known as multiple transportable carbohydrates.

The figure below shows how this process works in the digestive system

  • You ingest multiple transportable carbohydrate 90g of a 2:1 ratio of Glucose (60g) + Fructose (30g)

  • Single sources of simple sugars like glucose and maltodextrin use a transporter called SGLT1 (sodium-dependent transporter)

  • Fructose uses a transporter called GLUT5

  • These transporters deliver carbohydrate to the bloodstream and muscles for use

Examples of multiple transportable carbohydrates are combinations of:

  • Glucose + Fructose

  • Glucose + Sucrose (Glucose + Fructose)

  • Maltodextrin + Sucrose (Glucose + Fructose)

The tables below contains some popular nutritional products. It highlights which contain single sources of carbohydrate or multiple transportable.

If you mix your carbohydrate sources you will be able to consume more carbs during exercise. More importantly they will get to the muscles that need them.

For exercise over 2.5 hours in duration picking a carbohydrate combination of Glucose/Maltodextrin and Fructose will allow you to consume up to 90 g/h.

Let's use an example:

If you choose a product that has Maltodextrin only.

This means that the maximum absorption is 60 g/hr.

If you want to consume 90 g/hr you might decide to top this up with a gel or chew.

If you do you need to make sure that the gel or chew has either sucrose or fructose in it.

On the other hand you might like a product that includes multiple transportable carbohydrates. Then you can use this product for all your energy requirements.

However there are a couple of caveats to this. As you increase the amount of carbohydrate so do you increase the potential to GI distress.

GI distress is a major cause of DNF and forced slowing down in long distance triathlon.

You will have to practice this in training and train your gut if you intend to go that high.

You will also need to trial and see which products work best for you. The taste and texture is important.

Also consider that with a lot of products you may need to use between 300 to 600ml of water to help the body digest the products.

Carbohydrate intake will also be dependent on the intensity of the race.

You can measure this using a metabolic cart or INSCYD test to show exactly what fuel sources you use at different intensities.

Based on what was discussed above here are some we can offer some practical carbohydrate intake

Exercise 30 to 60mins

There is no need to take on board nutrition. However, it has been shown that a small dose or mouth rinse (take into mouth and rinse then spit out) of carbohydrate solution is beneficial. Ultimately, this depends on exercise intensity.

Exercise 1 to 2 hours

This will depend on the intensity of the exercise. If this is just easy to steady endurance you may not need to take on any carbohydrate.

If it is an all-out effort or race then 30g of carbohydrate per hour has shown to improve performance.

Exercise 2-3 Hours

Exercise between 2 and the 3 hour mark is where you need to increase the amount of carbohydrate intake. 60 grams per hour is what is recommended here.

Again, this will depend on the intensity of exercise. If it is an easy endurance session then you may need less than this.

Exercise above 3 Hours

Exercise above 3 hours the suggested amount in the infographic is 90 grams per hour, but there is a catch.

Our bodies are only able to ingest and use single sources of carbohydrates at 1g/min (60g/Hr).

This means that if your goal 90g/HR then you will need to combine 2 different sources of carbohydrate i.e Glucose and Fructose.

Key Takeaways

  • You can digest a maximum of 1 g/min or 60 g/hr of a single source of carbohydrate

  • 90g of a 2:1 ratio of Glucose (60g) + Fructose (30g) per hour can be absorbed

  • Exercise duration and intensity should dictate your carbohydrate intake

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