Glycaemic Index

What is the glycaemic index of food?
The glycaemic index (GI) of food is a method for ranking foods according to their immediate effect on blood glucose levels. The blood glucose response to an individual carbohydrate-containing food is compared to glucose. GI is therefore expressed as a percentage (ranging from 0-100).

Carbohydrate foods that are digested quickly have the highest GI factors. Those which are digested slowly have the lowest GI factors.

A low GI food is defined as being less than or equal to 55, whereas a high GI is greater or equal to 70.

The effect of carbohydrate on blood glucose and insulin levels depends on both the type and amount of carbohydrate. Glycaemic load (the GI of a food multiplied by its carbohydrate content) describes the overall effect of food consumption on glucose metabolism.

In the past, carbohydrates were classified as simple and complex carbohydrates where simple sugars caused a rapid rise in blood glucose levels whereas complex carbohydrates were digested slowly, causing a small rise in blood glucose.

However, nutrition research has shown that the chemical structure of carbohydrates (i.e. simple and complex carbohydrates) is not useful for predicting blood glucose and insulin levels. For instance, the carbohydrate in some sweetened breakfast cereals is digested and absorbed at the same rate as the carbohydrate in oat bran. There is general agreement that the GI should be used to describe the type of carbohydrate in foods. In 1998, the FAO and WHO recommended that the GI, as well as nutrient composition, be considered when choosing carbohydrate-containing foods1. Selecting one or two low GI foods in a meal will lower the overall GI of the meal. It is not necessary to only eat low GI foods.

What effect does the glycaemic load have on the body?

Blood glucose

Low GI foods
Since the carbohydrates in low GI foods are digested slowly, glucose is released gradually into the bloodstream, preventing fluctuations in blood glucose levels.
The ability to control blood glucose and consequently, insulin levels is central to the management of diabetes mellitus.

Low GI foods also help to sustain endurance exercise longer (e.g. marathon runners).

High GI foods
The carbohydrates in high GI foods are quickly digested resulting in a fast and high blood glucose response.

This is useful for athletes who need to re-fuel carbohydrate stores rapidly after exercising. High GI diets have been found to restore muscle glycogen levels faster than low GI diets.

Insulin levels

High GI foods
High GI diets increase insulin demand.

Low GI foods
Low GI diets reduce the level of insulin required to maintain normal blood glucose levels.

In this way, low GI diets can improve the body's sensitivity to insulin and reduce insulin resistance. Insulin resistance describes a condition where insulin does not function effectively, resulting in high glucose levels and increasing the risk of diabetes mellitus.

Weight control

• Animal models suggest that high GI foods promote high blood insulin levels as well as deposition of visceral fat (fat around the organs in the body) which is known to increase health risks.
• Several studies have indicated that low GI foods are associated with greater feelings of fullness and in some cases, result in lower energy intakes at subsequent meals.
• GI may therefore be a useful tool for controlling food intake in weight management.

Fat metabolism
Some studies suggest that high GI foods reduce HDL-cholesterol levels and increase triglycerides (both of which are risk factors leading to high cholesterol levels). However, results from studies investigating the effect of the amount of dietary carbohydrate on fat metabolism have been inconsistent. Further research is therefore required to confirm the effect of the glycaemic load on fat metabolism.

Preliminary research indicates that a high glucose load after a meal may have adverse effects in the arteries of the body and in so doing, may contribute to increasing the risk of heart disease.

Factors that affect the glycaemic index
Although individuals vary in their glycaemic response to foods, in general, the following factors will influence the GI of foods:

• Particle size - the larger the particle size, the lower the GI. For instance, breads made from coarsely ground wheat, rye and barley have lower GI than white bread. A study comparing boiled rolled oats, raw rolled oats and intact boiled oat kernels found that the latter had the lowest GI.
• Cooking - heat and water make starch granules swell. This destroys the crystalline structure of the starch and makes it easy to digest (e.g. thickening of flour and water when making a sauce illustrates this process which is called gelatinisation). Cooked foods, depending on the type of starch, generally have a higher GI than raw foods. However, the type of starch and the particle size influences the degree of gelatinisation and hence the digestibility of starch. Large particle size and high amylose content are less likely to gelatinise.
• Grain variety - the higher the proportion of amylose in the starch of the grain, the lower the GI. Grains naturally high in amylose include Doongara rice and Hi-maize(r). High amylose starch remains mostly ungelatinised with cooking and therefore tends to have a lower GI.
• Dietary fibre - although there is no correlation between GI and total dietary fibre, GI can be affected by soluble dietary fibre which slows the rate of digestion of carbohydrate.
• Antinutrients in grains - Phytic acid, lectins, tannins and amylase inhibitors have been shown to lower glycaemic response. In one study, the removal of phytic acid from navy beans caused an increase in blood glucose and its addition, a decrease. When added to wheat flour in unleavened bread preparations, phytic acid also reduced starch digestion rate and glycaemic response.
• Dietary fat increases insulin and decreases glucose levels (and hence GI).
• Acidity of the food - adding vinegar reduces the blood glucose responseby 30%.
• Adding water to a meal (either drinking or adding water to the recipe) increases the overall glycaemic response of the meal.

Low GI meals
The GI of a whole meal can be calculated by working out the total carbohydrate content of the meal and the percentage contributed by each individual food. The meal's GI is the sum of the individual GIs multiplied by that food's percentage contribution to total carbohydrate.

To decrease the GI of the whole meal, it is sufficient to include a few low GI foods in the meal. All foods do not need to be low in GI.

Low GI grains and pulses
Grains are generally low GI foods. However, the manufacture of grain-based foods can alter the GI of grains.

Breads

• Grain breads have a lower GI than white, brown or wholemeal breads.
• Stone-ground wholemeal breads (due to a larger particle size) and sourdough breads (due to acidity) have a low glycaemic index.
• The addition of bran or kibbled grains to bread can lower the GI.
• The starch in breads which have a long fermentation time in the preparation of the dough seems to be more slowly digested and hence has a lower GI.
• Breads with a denser food matrix, such as pita bread, have a lower GI.
• Breads containing barley or oat bran have a higher soluble dietary fibre content and hence a lower GI.

Breakfast cereals

• Wholegrain cereals and coarsely milled grains, which have a larger particle size, such as rolled oats and muesli, have a lower GI. (Quick cooking oats which have a smaller particle size have a high GI).
• High fibre cereals which have added bran (oat or wheat) or psyllium have a lower GI.
• The manufacture of breakfast cereals may affect the extent to which starch is gelatinised and hence determine the GI. It is difficult to predict the GI of breakfast cereals.
• Although breakfast cereals with added sugar generally have a higher GI, it is not always the case.

Rice

Most rice contains about 20% amylose. Milled rice, including long grain and glutinous rice, generally has a high GI.

Rice varieties, such as Australian-grown Doongara rice, has about 28% amylose. Its compact structure and higher amylose content makes it more slowly digested, giving it a lower GI than other rice varieties.

Pasta

Pasta is made from semolina (large particles of wheat). During its manufacture, the starch becomes retrograded. These characteristics make pasta a low GI food.

Gluten-free pasta made with pulse flour also has a low GI.

Noodles

The high amylose content of mung bean noodles (also called cellophane noodles) and their shape (dense texture) gives them a low GI.

Noodles generally have a low GI due to manufacturing process which is similar to that of pasta.

Pulses

The higher amylose content, the seed coat, and the presence of antinutrients make pulses one of the lowest GI foods. Combining pulses with rice, lowers the GI of the rice meal.

References
1. The FAO/WHO (Food and Agriculture Organisation/World Health Organisation) position on carbohydrates in human nutrition can be found in its "Report of a joint FAO/WHO report. Rome 14-18 April 1997. Paper 66, 1998. FAO Food and Nutrition". Visit www.fao.org for more information.
2. Brand Miller J et al. The GI Factor. 2nd edition. Sydney: Hodder; 1998.

For Further Research