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Knowledge Gains: Complex & Simple Carbs - Take Your Pick

Carbohydrates are the primary source of fuel that the body utilises during exercise. We ingest carbohydrates in our diet, which are broken down into glucose to then be used by cells in the body to generate energy in the form of a molecule called ATP. Any un-needed glucose at that point in time is converted into glycogen, which is stored in the muscles and liver, ready to be broken back down in to glucose when the body requires it.

There is no doubt that carbohydrates are a key aspect of the diet, particularly for individuals who enjoy exercising/competing in sporting events. For example, marathon runners use 75% of their fuel source in the form of carbohydrates. Track running also relies mainly on carbohydrates. So, spanning across the different time scales of an event, carbohydrates are needed. If we didn’t fuel ourselves correctly, we would run out of glycogen in 1-2 hours. So, the need to fuel ourselves appropriately is very important.

But there are different types of  carbohydrates  which have different effects on the body. This article will give you an insight into which carbohydrates you should be eating before, during and after events and which have higher demands on the body to produce energy.


Carbohydrates can be broken down into two main categories, simple/high glycaemic index (GI) carbohydrates and complex/low glycaemic index (GI) carbohydrates.


Simple/high GI carbohydrates are foods/drinks that have carbohydrates in its simpler forms – glucose, fructose, galactose, sucrose, maltose and lactose. These are the simplest forms of carbohydrates and can be absorbed and converted into glucose very easily and absorbed into the blood stream quickly.



Complex/low GI carbohydrates are foods/drinks that are made up of starch and require much more breaking down in the stomach and small intestine before they are turned into glucose and absorbed into the blood.

They are great for weight control and blood sugar levels as they give you a longer sensation of being full and the spike in blood glucose is not as high and remains relatively low during digestion.

However, for fuelling performance, these types of carbohydrates are not the best. They do not get the greatest uptake into the muscles and liver as glycogen, meaning they won’t provide as much energy. We will now go through the pre, during and post stage of events requiring carbohydrates and let you know which food and drink sources are best, and how much is the right amount.





Carbohydrate oxidation is more effective than fat oxidation to produce energy during exercise, so we want a lot of carbohydrates available at the start of our event to ensure we can fuel the highest level of performance for the longest time possible. Anywhere between 8-10g per kg of body mass is ideal 1-2 days before the event to fully saturate your glycogen stores. Rice, potatoes, cous cous, muesli, cereals, pitta bread and porridge are just a few great examples of good high GI foods you can consume before your event/session to elevate those glycogen levels.


This is a key time where nutrition can play a major role in your performance. Again, high GI/simple carbs can be consumed. But they need to be easily available and not hard to consume. The option we feel are best for this are:

  • Dextrose, Maltodextrin & Fructose powder mixed with water
  • Energy Gels
  • Sports Drinks
  • Fruit Juices

Now, the length of your session/event can influence your levels of fatigue. So follow this carbohydrate consumption guide for the length of your work:

  • 30-60 mins = small sips
  • 1-2 hours = 30g/hour
  • 2-3 hours = 60g/hour
  • 3 hours plus = 90g/hour

If you have food/drinks/supplements containing high GI carbohydrates and you have the right quantities for the length of your event, you should give yourself the best chance to perform at your optimal level for longer periods of time.


Again, the research shows it, glycogen store resynthesis rates are highest when consuming high GI foods. This is where you go back to the same types of foods you were consuming before the event. This will then help to get your glycogen stores back to normal within 24 hours. This should be around 10g/kg/body mass of carbohydrates in 24 hours after the exercise and consuming around 2g/kg of body mass within the first hour after the session as this gives the greatest levels of immediate glycogen resynthesis.

Hopefully this has given you a greater understanding of the different carbohydrates, what types to consume, when and how much for events which place a high demand on the body’s energy systems.

 - The Letterbox Gains Team