The Glycaemic Index

The glycaemic index or GI is a ranking on a scale of 1 to 100, of carbohydrate foods based on the speed at which the body breaks down a particular carbohydrate rich food and converts it into blood glucose.

Foods with a high GI (greater than 70) such as cornflakes, honey, soft drinks and crackers are broken down into glucose very quickly which means they provide a fast energy fix and a rapid rise in blood sugar levels.  While this can be useful for athletes who need to replenish blood sugar after strenuous exercise it’s not such good news for the rest of us as they cause the release of insulin which encourages the body to lay down fat. Foods with a low GI (less than 55) such as apples, pears, beans and pulses are converted into glucose more slowly which brings a number of health benefits.

Glycaemic load (GL) is calculated by multiplying the GI of a food by the amount of carbohydrate contained in a typical serving of that food and many nutritionists believe it’s a more reliable predictor of how a food will affect blood sugar levels.  Foods/meals that have a GL of 20 or more are considered high GL, 11-19 is medium GL and a GL of 10 or less is low GL.

Good nutrition is more than just GI.  The GI of a food is only an indication of how that food, eaten in isolation, will affect blood glucose levels. It does not consider other foods eaten at the same time, and used alone it does not result in the selection of a healthy balanced diet. For example, compare watermelon, broad beans, carrots and wholemeal bread (high GI foods) with sausages and chocolate milk (low GI foods).
By focusing on one element of a food’s composition, the GI system can imply that high GI foods have no other nutritional benefits. Bread is a low fat food containing B-vitamins, calcium and iron.  According to GI tables, food like bread, broad beans, carrots, bananas and most fruits are all high GI foods. Therefore focusing on GI values alone would lead to unhealthy dietary choices. No single food contains all the essential nutrients the body needs to be healthy and function efficiently. A healthy balanced diet is based on choosing foods from all the major food groups including carbohydrates, protein and fat.

In relation to diabetes, scientific evidence suggests that high fibre, low GI foods are a means of improving blood glucose and weight control.  A diet based mainly on low GI foods may improve insulin sensitivity (insulin is the hormone which helps control blood glucose).  Scientific evidence also indicates that when combined with a reduced calorie intake and regular physical activity, GI can play a role in weight management by helping to control appetite and insulin levels.