Carbohydrates are the main source of fuel for runners and are typically classed as either simple or complex.

Simple carbohydrates are composed of one or two sugar molecules whereas complex carbohydrates are formed from long, complex chains of sugar molecules – starchy foods such as bananas and sweet potatoes.

What Is The Glycemic Index?

The glycemic index (GI) is a figure representing the relative ability of carbohydrates to increase blood glucose levels (blood sugar levels). The figures range from 0 to 100, with 100 being the carbohydrate sources that are rapidly absorbed, digested and metabolised leading to fluctuations in blood sugar levels (think sugar rush and sugar crash).

Table sugar
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The glycemic index of table sugar (typically sucrose) is 63

It gives you an idea of how much your blood sugar levels would rise if you were to take the same amount of carbohydrate from each food and digest it. You could say the higher the value, the quicker all the sugar molecules in the carbohydrate source are converted into pure glucose.

Why Does Glycemic Index Matter For Runners?

Using the glycemic index, you can try to eat carbohydrates more mindfully, thinking about how it will affect your performance, recovery and if they are needed. You can plan to eat low to medium glycemic index foods on rest days when you are not doing much exercise as your body will not require as much glucose and eat high glycemic index foods on training and race days. Too much glucose in the body is toxic and can cause all sorts of health issues such as dehydration, tiredness, blurred vision and increased risk of diabetes.

Sugar Molecules In Carbohydrates

Foods with a higher glycemic index are composed of more simple sugars which can be more readily metabolised such as fructose, galactose and glucose – these are monosaccharides (single sugar molecules).

Disaccharides are sugar molecules such as sucrose, lactose and maltose where two monosaccharides have combined together in a condensation reaction.

Finally, complex carbohydrates are polysaccharides where long chains of sugar molecules are joined together. By definition, these are molecules that contain ten monosaccharide units, but typically have anywhere between 40 up to 3000 monosaccharide units joined together. Cellulose (found in plants), starches (composed of two polysaccharides – amylopectin and amylose) and glycogen (the source of energy stored in your muscles which is created from glucose) are all forms of complex carbohydrates.


The range of sugar molecules joined together affects how our body digests these sugars. Simple sugars are better to consume immediately prior to an event if you are feeling hungry or need a bit of a ‘pick-me-up’ as they will metabolise in a very short amount of time whereas having a banana 30 minutes beforehand probably won’t do you much good since it will still be there in your stomach whilst you’re running. Whilst some of the sugars in a banana will be able to have been digested, the majority will still remain undigested. Complex carbohydrates are actually best to have at least two hours beforehand so they can be fully digested before you train, race or run and help keep your blood sugar levels stable.

Post-training Or Racing

After training or racing, it can be a good idea to eat foods with a high glycemic index in order to replenish used up glycogen in the muscle stores. This is particularly useful if you have multiple races in one day or double training days where you want to perform well.

Pre-training Or Racing

Low to moderate glycemic index foods have sugars which are released more slowly into the bloodstream making them more ideal for endurance exercise. These can help stabilise your blood sugar levels for longer periods of time, preventing fluctuations in your blood sugar levels which will help prevent sugar cravings.

Daily Eating

For most of your daily training, you’ll be better off eating low to moderate GI foods as these will help keep your blood glucose levels stable throughout the day, provide you with many minerals, vitamins and enough fibre to meet your nutritional needs. Even if you are not eating a lot of carbohydrates, your body will break down fat into glucose which will be converted into glycogen to ensure your muscle and liver glycogen stores are topped up for your next workout. The only time when this is less likely to be true is if you are training more than once a day or racing intensively multiple times in a day, in which case it is a good idea to eat higher GI foods. Having higher glycemic index foods at the end of a meal rather than at the start will help to reduce the effect on increasing blood sugar levels and reduce the rise in insulin secretion which is determined by the food insulin index.

Try experimenting with low to high GI foods and find out what works best for you. Everyone’s body works different and will respond uniquely to various carbohydrate sources. Where you can, try to replace some high glycemic carbs with healthy carbohydrate swaps! I recommend checking these out for some good options on what carbohydrates you could eat and which ones you could try and swap out.

Here you can see the glycemic index for a range of foods.

What GI foods do you tend to eat? Let me know down below!