Glycaemic index is a measure of the ease with which the body uses carbohydrates and changes them into glucose, the precursor to glycogen, the fuel used by the body's muscles. The ease is affected primarily by the size and complexity of the carbohydrate molecules, the largest and most complex being found in such things as unrefined, whole grain cereals. The simplest are sugars, especially refined sugars. Foods with a high GI are converted very quickly and, conversely, foods with a low GI are converted slowly.

When the food is in the gut the chemicals there break down the carbs into glucose and deposit it into the bloodstream where it can be distributed to the muscles that need it. Energy drinks such as Lucozade Sport are designed to practically eliminate this delay by containing glucose in a kind of form that is very easy for the gut to pass straight into the blood stream. This is excellent when you are partaking in sport where a quick energy supply is needed. However if the glucose is not needed by the muscles the body has an inbuilt mechanism for removing the glucose by creating proportionate amounts of insulin and distributing this into the blood stream which removes the glucose from the blood stream and deposits it as fat. With the consumption of high amounts of high GI foods however the body seems not to account for the energy needed to create the insulin and therefore over-reacts with the amount of glucose removed. Because glucose in the bloodstream is the main determining factor in one's perceived energy levels a person is likely to perceive a large rise in apparent energy very soon after consuming a meal consisting of high GI foods followed by a very pronounced low, as too much glucose is removed. This may trick the person into believing that are hungry again and require foods that will raise the glucose levels in their bloodstream to more normal levels, thus compounding the problem.

In contrast, because they are comprised of large, complex carbohydrate molecules that are difficult for the gut to break down, low GI foods are converted into glucose at such a slow rate that the body can easily, and more accurately, measure the amount of glucose in the bloodstream against the required amount and remove as much as is needed. Fat is a requirement of the body system and there should not be alarm in the suggestion that even healthy carbohydrates can end up as fat. However, because the glucose levels in the body stay at a more consistent level a person consuming a meal consisting of low GI foods will not suffer a rise and then drop in apparent energy levels and will therefore be less likely to be hungry following their meal.

My opinion is that if you are eating to prepare for exercise the following day (if you are going for long run for example) then foods with a low GI are likely to be the most beneficial, and that, in general, foods with a low GI tend to mirror the standard image of a healthy diet. High GI foods do have their place but only where a fast form of energy is needed and, in most situation, will be eaten in conjunction with a diet that consists mainly of low GI foods.

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