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Why Carbohydrate is the King for Endurance Performance

BY Ted Munson

Whatever race you’re about to take on— loading and refueling with carbohydrate is key for performance.

Proper nutrition is often the missing link between training and performance gains. There are a lot of nutrition philosophies out there on the topic, but in this article we’ll highlight the different types of carbohydrates and how they work to fuel your body during different activity intensities.

The Science

Carbohydrates travel quite the journey before they finally absorb into the bloodstream via the small intestines. Energy (glucose) can be stored in the liver and muscle (as glycogen) to be used as energy during exercise. Taking on carbohydrate during exercise delivers rapid energy to the working muscles and prolongs your endurance capacity. However, the effects of the energy you receive can differ drastically depending on the type of carbohydrate you use.

Different Sources

Carbohydrates come in a variety of forms. Sugars, including glucose, sucrose and fructose are all carbohydrates that you may have heard of. While they contain similar calories, they are all metabolized differently, affecting performance output. Maltodextrin, an alternative form of carbohydrate, is broken down into glucose, which is the base of SiS GO Energy products.

So what is the difference between sources of carbohydrate?


  • Must be converted into glucose in the liver before they can be metabolized
  • Is oxidized at a much lower rate during exercise
  • Can cause stomach issues


  • Fast, readily available source of energy
  • Has a higher concentration compared to maltodextrin and may require water to aid digestion in high concentrations
  • Increased risk of GI distress


  • Also known as table sugar
  • A chemical combination of glucose and fructose
  • Has been shown to digest quickly


  • Made up of chains of glucose molecules and has a high GI, meaning that energy is available quickly
  • Oxidized quickly during exercise
  • Reduce the risk of developing stomach complaints during prolonged exercise

The carbohydrate source in many energy gels, including SiS GO Isotonic Energy gels, is specifically selected maltodextrin. The particular size of molecule balances the amount of energy delivered versus how quickly it empties from the stomach. This means that you will feel the performance benefits of taking on a isotonic energy gel far more quickly than when a non-isotonic gel is consumed and the risk of upsetting your stomach is much less.

Can We Combine Carbohydrate Sources?

The digestion rate of drinks containing multiple types of carbohydrate is higher than that of drinks with a single carbohydrate source. This means that, for example, drinks containing maltodextrin and fructose are less likely to cause stomach issues and can potentially deliver more energy to the muscles.

The Fat Vs. Carbohydrate Debate

There is a major split as to what should best fuel athletes. Here is a comparison:

At low exercise intensities, you will mainly use fat as your energy sourceCarbohydrate is the main fuel for high intensity exercise
Fat store 9kcal per gram, versus the 4 kcal that can be stored as carbohydrateMuscle and liver glycogen stores can only last for around 90 minutes of aerobic exercise
Fat is oxidized much slower than glycogen, meaning that it does not supply energy rapidlyCarbohydrate, especially that of a high GI provides fast energy to be used by the working muscles
Fat is less available for fueling high intensity exerciseWe can only absorb around 60-90g of carbohydrate per hour during exercise

Fuel For The Work Required

By “fueling for the work required” an athlete can potentially enhance the way they use carbohydrate and fat as a fuel source during prolonged exercise. Some sessions could be performed without carbohydrate (this may even take the form of having breakfast after and not before morning training) whereas for harder effort sessions and very long endurance sessions, carbohydrate intake is essential for performance.  Additionally, athletes should include “train as you race” sessions where a race day nutrition strategy is practiced. This can teach the muscles how to use both fat and carbohydrate as fuels. However, always ensure that harder training sessions are fueled to also train your gut to be able to tolerate the high carbohydrate intakes on race day.

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About Ted Munson

Ted Munson is the Performance Nutritionist at Science in Sport. He comes from a sports science background having worked in elite sport for the past four years. Ted has worked with athletes in football, rugby and tennis, most recently with Hull City FC as a sports scientist. Ted continues to provide sports science support for teams, alongside his MSc in nutrition and physiology, focusing on hydration markers in elite athletes.For more on training and nutrition by SiS Performance Nutritionist Ted Munson, visit us at scienceinsport.com.

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