05 February 2010

Glycaemic index: unresolved issues

The glyceamic index has become a popular tool. However, it's still a relatively new dietetic tool and there are a number of issues that need to be resolved.


The glyceamic index (GI) has become a popular tool. However, it's still a relatively new dietetic tool and there are a number of issues that need to be resolved.

1) A unique concept

The GI is a unique concept because it's an indicator of a physiological reaction in the human body. Most other measures of nutritive content, such as the carbohydrate, protein, fat, moisture, dietary fibre, vitamin and mineral contents, can be determined by doing a chemical analysis of the given food.

If Laboratory A analyses the carbohydrate content of a sample of bread and Laboratory B performs the same analysis on the same sample of bread, then the chances are excellent that both laboratories will obtain the same result.

In contrast, the GI measures the reaction of the human body to the ingestion of a portion of food that contains carbohydrate. The analysis is carried out on blood samples obtained from test subjects who ate specified portions of the food. The test subjects also have to eat a specified amount of the standard substance, which is usually glucose or white bread.

2) A difficult analysis

Understandably, it's difficult and expensive to carry out such tests with human subjects. Test-tube analyses are much cheaper. Other problems arise because of the variability of the human response: Subject A may give a much higher result (increase in blood-glucose level) than Subject B today, when ingesting the test portion, while tomorrow the reverse may apply. To obtain a relatively accurate result, the tests have to be repeated a number of times, which increases costs.

This is one of the reasons why we don't at present have GI values for each and every food on the market. There are literally thousands of foods out there and it will be a very involved and expensive exercise to test all these foods for their GI values. Some smaller manufacturers also can't afford to have their products tested.

3) No precise results

Due to the variability of the human response to GI testing, it's impossible at this stage to obtain GI values that are 100% accurate. GI values represent a broad indication of how the blood-glucose levels of individuals will respond to the intake of foods.

So, don’t waste time agonising about the fact that one brand of breakfast cereal has a GI of 55, while another has a GI of 50. What we can say is that both these breakfast cereals have GI values that are low and that they can be used by people who need to eat low-GI foods.

4) Control of diabetes mellitus and other conditions involving insulin

The speakers at a recent symposium on GI foods concurred that there's sufficient scientific evidence available to conclude that:

"The habitual intake of foods with low-GI values improves the prevention and control of diabetes mellitus through their positive effect on blood-glucose values and insulin levels."

If you suffer from diabetes (both type I and type II), insulin resistance and hypoglycaemia, it's a good idea to eat foods with a low-GI value.

5) Not the only criterion

The speakers at the symposium underlined the fact that:

"The GI values of foods are NOT the only criteria that should be used to judge the worth of a food or a diet."

This means that a food like oats porridge, which has a high GI value, can still be included in a diabetic diet because it has other positive effects on human metabolism, such as lowering blood-cholesterol levels.

6) Combining foods

It's also important to keep in mind that combining foods, as is normally the case when we eat mixed diets, will change the GI values of the given foods. Oats are again a good example. Eaten on its own, oats has a high GI, but if oats porridge is combined with low-fat milk, the GI of the combination is reduced dramatically.

Don’t cut out valuable foods just because they have a high GI. Combine them with other foods that have a low GI to obtain an overall lower GI effect.

7) Sources of GI information

The following books by Gabi Steenkamp and Liesbet Delport are available both in English and Afrikaans and can help answer your GI questions:

  • “The SA Glycaemic Index Guide”
  • “Eating for Sustained Energy”
  • “Eat Smart & Stay Slim”

Order these and other books by the same authors via

– (Dr I.V. van Heerden, DietDoc,updated October 2008)

Any questions? Ask DietDoc


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