The Importance of Carbohydrates

Carbohydrates are no doubt the energy powerhouses for fitness. High carbohydrate foods (such as whole grains, cereals, fruits and vegetables), all supply energy to your working muscles during exercise. They also offer a variety of important nutrients like vitamins C and A, and antioxidants, crucial for maintaining healthy cells in the body.

No matter what type of exercise you do, your body will always use some glucose for energy. Glucose is formed from the breakdown of carbohydrates (sugars and starches) in your diet and is stored as glycogen in the muscles and liver. However, the body can store a limited amount of glycogen, and to keep it topped up you need to refuel with carbohydrates within two hours of exercise as well as eating a carbohydrate-rich diet overall. How much carbohydrate you need really depends on the amount of training you do – the more glucose you use the more you need to eat to replenish your stores.

Your carbohydrate needs

The amount of carbohydrate you need depends on the amount of exercise you do. Research has shown that a diet high in carbohydrate, obtained either from simple sugars or complex carbohydrate, is equally effective in improving exercise performance. If you’re physically active, the optimal diet is one that contains 60-70% energy from carbohydrates. However, it’s difficult enough to guesstimate the proportion of carbohydrate containing foods in one meal, let alone drinks and food for the rest of the day!

Which carbohydrate?

Most carbohydrate foods such as pasta or sugars are eventually broken down into glucose, so one type is not intrinsically better than the other. Having said that, what is important is how quickly the carbohydrate is converted to glucose - and that’s where the glycaemic index (GI) is useful.

The GI of a food is a measure of that food’s effect on blood glucose levels. It is calculated by comparing the rise in blood glucose after eating a food containing 50g of carbohydrate with the blood glucose rise after eating 50g of a reference food (usually glucose). The faster the rise in blood glucose, the higher the GI. The table below gives the GI category of some everyday foods. Unfortunately, there is no easy way to tell what the GI of a food is. Some sugars have a high GI (glucose) and others a low GI (fructose). Some complex carbohydrates have a low GI (pasta) whereas others have a higher GI (rice).

The glycaemic index (GI)

High -GI above 70 Moderate-GI of 50 – 70 Low - GI below 50




Mars bar


Jelly beans


Sponge cake

Sports drink





Fruit cake




White rice

Brown rice


Baked potato

Boiled potato

Baked beans







When to eat carbohydrates

Just before, during and immediately after exercise, try to eat high and moderate GI foods to help stimulate glycogen synthesis.

Before: Remember to allow about two to three hours after a meal before exercising, to avoid stomach upset. Then five to 30 minutes prior to your workout have a 50g moderate- to high-GI carbohydrate snack such as toast, cornflakes or a currant bun. This will help you maintain your glucose levels so that you can train more efficiently. It is probably advisable to avoid the more bulky (fibre-rich) carbohydrates here, as these can cause abdominal discomfort. Try different snacks to find which ones suit you best.

During: If you exercise continuously for more than an hour, it is likely that you will need to consume carbohydrates during your workout to avoid fatigue. One of the best ways to achieve this is by drinking sports drinks, which provide water as well as sugar and therefore help hydration – see the section on fluids.

After: Most people don’t feel hungry immediately following exercise, so this is where sports drinks can be useful, as they are better tolerated and provide both carbohydrates and fluid - all helping speed up the recovery process.

In between: In between exercise sessions, try to include a mixture of low to moderate GI foods in your diet. Although be careful not to overload your bread, potatoes and pasta with lots of butter and cream as that would be a high-fat diet. Gram for gram fat has twice as many calories as carbohydrate so be careful!