Carbohydrate

Physiological Classification of Carbohydrates

Glycaemic Index

The glycaemic index (GI) is a way of rating carbohydrate in foods based on their effects on blood glucose (sugar) levels in the body. Basically the blood glucose response to a food is compared to the blood glucose response after ingesting glucose.

Foods are ranked on a scale of 0 to 100. Glucose is assigned a GI of 100. Foods with a GI of over 70 are described as having a high GI. In general a food with a high glycaemic index is one which is digested and absorbed quickly and tends to produce a rapid rise in blood glucose concentrations.  Low GI goods are described as having a glycaemic index less than 55. Foods with a low GI contain carbohydrates that are digested and absorbed more slowly and will produce a more gradual rise in blood glucose levels.

How is the GI of a food measured?

1. A volunteer eats an amount of food which contains 50g of carbohydrate. This is equal to 3 tablespoons of pure glucose powder.
2. Over the next 2-3 hours a blood sample is taken every 15 minutes for the first hour and every 30 minutes thereafter. The blood glucose level of these samples are measured and recorded.
3. The blood glucose levels are plotted on a graph and the area under the curve is calculated using a computer programme.
4. The volunteer's blood glucose response to the test food is compared with their blood glucose response to 50g of pure glucose (reference food).
5. The GI factor of a test food is the average value of a group of 8-12 volunteers.

Practical implications of the Glycaemic Index

There are many practical applications for the glycaemic index. Research has investigated the links between the glycaemic index and such variables as human health, sports performance and weight control, to name a few.

Diabetes and the Glycaemic Index

Research suggests that the GI may be an important clinical tool in the treatment of individuals who have diabetes.  Foods with a low glycaemic index can help control the blood glucose levels in those with diabetes.

There is some evidence to suggest that low GI diets may be useful in the prevention of diabetes and that for those at increased risk of diabetes, high GI foods should be avoided. However more research is required to determine a role for the GI in diabetes risk.

Weight Control and the Glycaemic Index

There is some suggestion that the glyceamic index may be useful for weight control.

Low glycaemic index foods may increase satiety (feeling of fullness) and decrease hunger, which may subsequently decrease energy intake and therefore induce weight loss.

There is also some interesting research suggesting that the consumption of high glycaemic index foods increase insulin levels to an extent which subsequently decreases the use of body fat as an energy source. Research is ongoing in this area.

Sports Performance and the Glycaemic Index

Some research suggests that the GI may be a useful tool for athletes.

Some investigators have suggested that the consumption of a low GI meal prior to exercise may be beneficial for maintaining blood glucose levels.

Conversely, the consumption of high glycaemic index foods may be beneficial during the post-exercise period to rapidly replenish fuel stores and aid muscle recovery.

The consumption of high GI foods may be beneficial during exercise because carbohydrate foods during exercise need to be rapidly available for the exercising muscle.

What affects the glycaemic index of a food?

Many food variables affect the glycaemic index of a food:

• Physical form of carbohydrate molecules - beans and seeds have a fibrous coating that can slow down digestion and therefore lower the GI
• Presence of other nutrients - the presence of fibre and fat will delay digestion and will therefore lower the GI of carbohydrate containing foods.
• Presence of Anti-nutrients - phytates (in whole grains), phenols, etc slow digestion and therefore decrease the glycaemic index of a food.
• Food processing - Processing such as milling and grinding make foods more digestible and will therefore increase the glycaemic index. For example wholemeal bread will have a higher GI than multigrain bread that contains intact grains.
• Type of starch - Amylopectin and amylose are the 2 starches present in foods. Amylose is a straight chain of repeating glucose molecules whereas amylopectin is a branched chain.  Since amylopectin has a branched chain there is a larger surface area for the action of digestive enzymes. Therefore if a food contains a higher proportion of amylopectin will have a higher GI.
• Type and proportion of sugars  -  Fructose is not absorbed as quickly as glucose. Therefore, foods that contain a higher proportion of fructose (e.g. fruit) have a lower GI than foods that contain glucose and sucrose.

Limitations of the Glycaemic Index

When testing the GI of a food, only single foods are consumed. For example, if one tests the GI of a particular cereal, only the cereal is eaten (ie. no milk or sugar is added). This is not how we tend to eat foods and so we have to extrapolate the GI to real eating situations.

Since the fat content of a food decreases the GI, higher fat tend to have a lower GI.  Therefore if we solely look at the GI of foods we could ultimately be consuming a high fat diet.

The GI of a food is calculated using food portions containing the same amount of carbohydrate (ie. 50g carbohydrate portion). It is the blood response to carbohydrate in a food as a percentage of the response to an equal weight of glucose. Since the GI is a percentage, it is not related quantitatively to food intakes. Because the GI is based on equi-carbohydrate comparisons its practical application is limited to comparison of foods providing equal carbohydrate doses. To overcome this restriction, investigators have developed the glycaemic glucose equivalent (GGE). The GGE is the weight of glucose having the same glycaemic impact (impact on blood glucose levels) as a given weight of food. GGE are likely to give a better indication of glycaemic loading because they are based on food composition and quantity as well as on glycaemic potency of carbohydrate in food.

In a nutshell GGEs should provide an accurate indication of blood glucose response to a meal because it combines GI with carbohydrate dose in diets of varying composition and intake.

Keeping the GI in Perspective

It is important to remember that the glycaemic index is only one component of food. One must also keep in mind other important components of food such as the saturated fat, sodium, fibre, and sugar content.

Incorporating the GI in your Eating Plan

It is a good idea to incorporate at least one low GI food into each meal.

Including some high and moderate GI foods into a healthy diet is fine as many of these foods can contain important sources of nutrients.