For example 30g of carbohydrate as bread may not have the same effect as 30g of carbohydrate as fruit or as pasta. The GI is effectively a way of ranking carbohydrate-containing foods according to the extent to which they raise blood sugar levels after eating.
They are ranked on a scale 0 to 100, 100 being the response to a food of reference like glucose or white bread. At the same carbohydrate level, foods with a high GI result in marked fluctuations in blood sugar levels whereas low-GI foods produce lower rises in blood sugar levels (1).
What influences GI For many years scientists believed that starchy carbohydrates were digested and absorbed slowly whilst sugary carbohydrates were digested and absorbed quickly - but this is not the case!
Soft drinks and mashed potato for example have a similar GI. So, what is it? The physical form of a food, how it is processed, the type of fibre, type of starch, type of sugar and how it is cooked, all have an impact on a food's final GI. In general, the structure of the food is as important as the type of carbohydrate it contains. Whole wheat grains for example are relatively resistant to digestion, but once ground and baked, they become easy to digest.
This is why both white and fine textured wholemeal bread have a high GI whereas multigrain breads have a medium GI. Some foods like oats, apples and beans contain soluble fibre, which thickens and slows the passage of food in the digestive tract. These foods have a low GI.
The type of sugar in a food is also relevant; glucose has the greatest impact on blood sugar, followed by sucrose (table sugar). In practice most carbohydrate foods are not consumed alone but as part of a snack or meal, and this has an impact on the blood sugar profile.
For example bread may be eaten with butter or oil, and potatoes with meat and vegetables. The addition of foods rich in fat and protein significantly slow down the rate of digestion of the meal and reduce the overall GI.
How to apply GI for better health? Knowing the GI of foods can be helpful for people with diabetes who need to manage their blood sugar levels. The inclusion of low GI foods can help them spread the release of glucose into the blood more evenly throughout the day (2).
The GI concept may also be applied to the sports field.
Foods with a low GI may help to increase endurance whereas high GI foods and drinks lead to an enhanced replenishment of muscle glycogen after exercise (1). There is emerging evidence that diets with an overall lower GI may be beneficial in the prevention of late-onset diabetes and coronary heart disease possibly because they decrease the demand for insulin and improve the cholesterol profile in the blood (2).
Research is also heading towards the use of GI in relation to obesity and it has been suggested that low GI diets may help people lose weight because they are so filling (3).
However, it is not clear whether this is due to GI or other related aspects of low GI foods. Good long-term studies are needed to determine this.
Manage your diet - the glycaemic load concept The GI is not the only factor that determines to what extent eating a food raises your blood sugar levels. If you eat similar amounts of two foods with the same GI (e.g. Swede and croissant), you'll get a lower increase of blood sugar with the food that has a lower carbohydrate content (e.g. Swede, see table).
Also, if you eat two foods of similar GI (e.g. Croissant and couscous), your usual serving of one of them may be smaller (e.g. Croissant, see table) and your blood sugar will rise less.
This is why the concept of the Glycaemic Load (GL) has been introduced. It builds on the GI concept to provide a measure of total glycaemic response to a food or meal (GL= GI/100 x grams of carbohydrate per serving).
In practice, it means that you can directly compare the blood sugar effects of two foods by weighing the GL of a serving of one food against the GL of a serving of the other food. The table below shows that, for a similar GI, one serving of different foods can generate very diffent GLs.
|Food||GI*||Serving size (g)*||Carbohydrate (g)*||GL|
Mix up your GI foods and aim for an overall low GL The GI is a useful tool for classifying the impact of carbohydrates on the body, however it is not useful to get hooked on the GI values of individual foods, as the overall impact of a meal on GI is difficult to predict (4). Including an appropriate mix of nutritious high and low GI foods within the context of a healthy diet is probably the best scenario for most people.
For those who want to lower the overall GL of their diets, good advice is simply to boost intake of low GI foods like pulses (beans, peas and lentils) fruits, oat cereals and pasta, or swap some high GI foods for lower GI alternatives (e.g. use breakfast cereals based on oats, barley and bran, use mutigrain breads instead of white ones).
References 1. WHO/FAO (1998) Carbohydrates in Human Nutrition. Food and nutrition paper no.66 Rome Italy
2. Opperman AM, Venter CS, Oosthuizen W et al (2004) Meta-analysis of the health effects of using the glycaemic index in meal planning. British Journal of Nutrition - 92:367-381
3. Brand-Miller J, Foster Powell K & McMillan P (2005) The Low GI Diet revolution ISBN 1-56924-413-8 New York
4. Flint A, Moller BK, Raben A, et al (2004) The use of glycaemic index tables to predict the glycaemic index of composite breakfast meals. British Journal of Nutrition 2004 91:979-89