GI - The Glycaemic Index Explained

GI - What does it Mean?

The Glycaemic Index (or GI) of a food refers to the speed at which carbohydrate* from that food is converted into blood glucose* by the body.

Generally, the speed at which the carbohydrate in a food is broken down during digestion, relates to the GI level in that food. The faster the carbohydrate is digested and turned into blood glucose, the higher the GI value.

The standard by which all foods are compared to get their GI is pure glucose - as this is digested by the body and converted into blood glucose very quickly. The GI value for blood glucose is 100, and all other carbohydrate containing foods are given a GI value relative to this.

Carbohydrate foods are divided into low GI (GI < 55), medium GI (GI = 56-69) and high GI (GI > 70).

Fibre, starch and sugars are all types of carbohydrate. Fibre takes longer for the body to digest, so carbohydrate foods with fibre tend to have a lower GI value.

Examples of some common low, medium and high GI foods are shown in the table below.

Common foods and their GI values**

Source: http://www.ajcn.org/cgi/content/full/76/1/5

**note that while one GI value is given to each food in this table, this is an average figure. The GI of any food can vary within and between individuals. Often the degree of variation is very large - especially for foods in the medium and high categories.

How do fat and protein affect GI?

When a single food or a meal contains fat or protein, the carbohydrate component of that food is digested more slowly than it would be on its own. That is because the presence of fat or protein delays stomach emptying. Therefore, a mixed meal will have a lower GI value. For example, cornflakes on their own have a high GI value, but when they are eaten with standard milk, it brings down the overall GI of that meal.

The addition of a low GI food to a meal (eg drinking milk with a meal) helps increase the time taken for the carbohydrate in the meal to be digested and converted into blood glucose - so it will lower the overall GI of the meal. So it's good to include at least one low GI food in each meal.

High fat foods also tend to have lower GI values, but it is important to remember that they should only be consumed in moderation, due to their high energy content. The GI of foods does not in any way reflect their energy content. Although high fat foods may lower the GI of a meal, it does not mean that they are any better for you, especially if you are trying to control your weight.

The acid content of foods also tends to lower their GI value - so foods such as yoghurt which contain protein and lactic acid have a lower GI value than unfermented dairy products.

Why is low GI a good thing?

While it is normal to have higher blood glucose levels after a meal, continuously high levels puts extra strain on the body and may increase the risk of developing Type 2 diabetes and other health problems in the future.

Blood glucose levels which rise slowly following meals, and stay at a moderate level for longer before falling again, are good for several reasons:

• This gives a feeling of "fullness" for longer - which means that you are less likely to get hungry between meals and snack unnecessarily.
• For example, a high GI breakfast of white toast and orange juice is likely to leave you feeling hungry not long afterwards.? A low-medium GI breakfast of wholegrain toast and a glass of milk, while containing a similar level of energy, is more likely to keep you feeling full until lunch-time due to the slower release of glucose in the bloodstream.
• It provides a sustained source of energy, which can be very important for sportspeople.
• Over a long period of time, eating meals with a low GI may reduce your risk of developing obesity, heart disease and Type 2 diabetes.
• For people with diabetes, this is a good way of maintaining control of blood sugar levels, without having to use medication.

Who should use the GI concept?

Traditionally the GI concept of eating has been used by people with diabetes who need to control their blood sugar levels closely. More recently however, this way of eating has become more popular with other groups such as the weight conscious and athletes.

Glossary

Blood glucose (or blood sugar, or glycaemic) is formed by the breaking down of carbohydrates in food. The blood transports glucose molecules to individual cells to use or store for fuel. The normal range of blood glucose levels is between 4-7 mmol/L. People with diabetes and pre-diabetes may often experience blood sugar levels outside of this range. Your blood sugar should not be too high or too low, so it is important to maintain blood glucose at a healthy level. Low blood glucose levels results in lack of energy, disorientation and even unconsciousness, however this usually only occurs in people with diabetes.

Carbohydrate is a type of nutrient in food that our body uses as fuel. The other main nutrients of this type from foods are protein and fat. Carbohydrate from food is broken down in the intestine and converted into glucose (small carbohydrate molecules).