06 October 2008

Glycaemic index: the latest

Several factors determine whether a food has a low or high glycaemic index. Here are some of the latest findings, as discussed at the 2008 Nutrition Congress in Pretoria.

Interesting new findings were presented at the recent 22nd Biennial Nutrition Congress at the University of Pretoria, and I was privileged to attend a Master Class on the glycaemic index (GI).

No absolute values
One of the most important facts about both the GI and the GL that everyone (health workers, dieticians and you, the consumer) must remember is that these values aren't absolute values.

GI and GL values are not like temperature values that are scientifically exact. Instead, they should be viewed as indications of how a food will influence your blood-sugar and insulin levels. A high-GI food will cause your blood glucose and insulin values to shoot up rapidly, while a low-GI food will cause a gradual, balanced increase of your blood and insulin values over a much longer period. Intermediate-GI foods have an effect that falls in between these two extremes.

Different factors at play
The following factors affect the GI of a food:

  • How easily and rapidly the food is digested and absorbed by the body (foods that are easy to digest, such as finely milled cake flour, will generally have a high GI).
  • Which type of starch the food contains. Starch is classified into two different types, namely amylose and amylopectin:
    • Amylopectin starch has a loose structure that can easily be broken down by digestive enzymes. This means that the food is rapidly digested and easily and rapidly absorbed and will have a high GI. An example is Arborio rice.
    • Amylose starch has a compact structure that is difficult to digest, and the digestive enzymes will have to work harder and longer to digest this type of starch. Food which contains a lot of amylose starch will take longer to digest and be absorbed and have a low-GI. An example is Tastic rice.

    This is why two different types of the same kind of food can have different GI values.
  • Heating and cooling. When starches like maize meal or oats are cooked and then cooled down, the cooled porridge will have a low GI, thanks to a process called "retrogradation". Hot, cooked maize meal has a GI of 74 (high) and cooked, cooled maize meal porridge has a GI of 50 (low). Another example of this effect is hot potato salad, which has a much higher GI than cold potato salad.
  • Gelatinisation of starch (conversion of starch in the presence of water into a more readily absorbable form). Starches with fine particles like finely milled cake flour will have a high GI because they're easily gelatinised, while intact grains like crushed wheat will have a low GI, because the gelatinisation of the starch in the crushed wheat is more difficult.
  • Sugar content. Sugar will bind water in foods and make the water less available for the process of gelatinisation, thus lowering the GI of the food. For example, corn flakes have a GI of 189 (high) and will take approximately 1 hour to digest, but Frosties, with their added sugar, have a GI of 67 (intermediate GI) and will take two hours to digest.

Fats and protein
All fats and proteins have a low GI, and adding proteins and fats to carbohydrates will lower the GI of the carbs.

  • should eat only fats and proteins and cut out all carbohydrates;
  • should add masses of fat to carbohydrates to lower the GI.

Fibre and the GI
There are two types of fibre:

  • Soluble fibre (which is found in foods like oats, legumes and fruit, especially apples) generally forms a sticky gel and slows down digestion, which will lower the GI of the food.
  • Insoluble fibre (which is found in wholewheat and bran) will only lower the GI when it's present in very large amounts. This is why brown and wholewheat bread have a high GI, but All Bran, with its very high insoluble fibre content, has a low GI.

Good sources of dietary fibre include:
Oat bran, baked beans, lentils, dense and heavy breads (e.g. Pumpernickel), crushed wheat (‘stampkoring’), pasta made from durum wheat (check the label), Tastic and brown rice, sweet potatoes, high-fibre bran, deciduous fruits (apples, pears, plums), citrus fruits (oranges, grapefruit, naartjies), and vegetables.

Use acid to lower the GI
Acids such as vinegar and fermentation processes that produce acid will lower the GI of food combinations. For example, sour sorghum porridge has a lower GI than sorghum porridge that hasn't been fermented. In a similar fashion, adding vinegar and sugar to beetroot will lower the GI of your beetroot salad.

(Steenkamp, G (2008) Master Class on the Glycaemic Index. 22nd Biennial Nutrition Congress on Evidence Based Nutrition Leading the Way in Innovation, University of Pretoria)


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