In order to provide energy, dietary carbohydrates must first be converted to glucose.
Different forms of carbohydrates have different arrangements of molecules; some are more quickly converted than others, so these tend to raise blood glucose more quickly.
The glycaemic index (GI), provides an indication of the relative speed with which a food is converted to glucose and absorbed into the bloodstream.
Pure glucose is used as the reference measure and is given a GI of 100. All other carbohydrates are indexed in comparison to this measure. Roast parsnips for example, have a relatively high GI of 97, in comparison to kidney beans, which have a relatively low GI of 27.
It’s understood that a pre-exercise meal should be made up of low GI foods.
The reason being that if a high GI carbohydrate meal is consumed before training or competing, the relatively rapid rise in blood glucose will cause a release of insulin and a rapid fall in glucose levels, possibly even before the exercise session has begun.
The conclusion seems to be that athletes perform to similar levels on either a high or low GI meal, providing the timing before exercise is right.
Lower GI foods (beans, milk, porridge etc.) should be eaten 1.5-2 hours before exercise.
Higher GI foods (sugar, white bread, cereal etc.) should be eaten closer to the start of a session.
Knowledge of the GI should allow us to make dietary choices to suit specific circumstances. For example, if you are diabetic you should target low GI foods that limit the fluctuation in your blood sugar levels, or if you’re a more active person you may select a high GI option to aid performance and recovery.
GI response is likely to vary from person to person and it can be exaggerated in people with diabetes.