Definition of the Glycaemic Index
What you need to remember: The glycaemic index (GI) is a measurement describing the influence of sugar (from food) on glycaemia. It is used to describe the type of carbohydrate content (in particular sugars) in a food and likely to influence an increase in a person’s level of blood glucose.
(Source: Index glycémique – Mes P’tits Marabouts – Pr. J. Brand-Miller, K. Foster-Powel and S. Colagiuri)
Glycaemia is the level of glucose in the blood. On an empty stomach, your glycaemia is roughly 1 g of glucose per litre of blood. This repercussion on the blood glucose level is measured in relation to a reference carbohydrate: glucose, which is given a value of 100. A food with a high GI has a considerable impact on glycaemia, while a low GI food contains carbohydrates with little effect on glycaemia. For more information about the glycaemic index, we recommend you visit the following websites:
How do you calculate your Glycaemic Index?
The chart below represents 2 glycaemia lines dependent on time for X g of glucose and X g of carbohydrates for a tested food. The glycaemic index value of a food is determined by the following formula:
Here two different glycaemia lines represent the effect of the glycaemic index of two foods on a person’s blood sugar level: one has a high GI, the other a low GI.
How do you regulate your glycaemia?
During digestion, your glycaemia rises. However, this level does not remain high forever. It is regulated by the action of a hypoglycaemic hormone (reducing the level of glucose): insulin. This is produced by the pancreas.
The more you eat high GI foods, the higher the level of glucose in the blood and the more the pancreas produces insulin. Eating too much food with a high GI wears out the pancreas.
Why should we check our glycaemia?
Checking your glycaemia using the glycaemic index of food eaten daily allows you to:
- Improve your general health,
- Eat more healthily,
- Favour the normal working of the pancreas,
However, you should not focus your diet solely on the notion of the glycaemic index for all products, since there is a risk that your diet will be too high in fats and lipids. For example, products such as crisps have a low GI, but are known to be high in fat content.
This sort of diet does not exempt you from regular physical activity for your health (i.e. 30 minutes of quick walking daily).
It is important to have a healthy lifestyle and a balanced diet. Do not forget to eat at least 5 fresh fruit and vegetables per day!
Is the glycaemic index of a food always the same?
The answer is no. The glycaemic index of a food varies according to several parameters that influence the structure of carbohydrates and the speed at which they are digested and then pass into the blood.
These parameters are as follows:
- The fruit and vegetable’s variety
- Thermal treatment (cooking, refrigeration)
- Processing (crushing, mashing, etc.); e.g. a potato cooked 20 minutes in water will have a lower GI than a potato cooked in water and mashed.
- Other foods with which the food in question is digested (a food rich in fibre will lower the GI of the dish in question).
The notion of glycaemic load:
The glycaemic index of a food is an important item of data we must also consider the quantity of food consumed. This differs from one food to another. For example, we do not eat as much jam as we do toast for breakfast.
To determine the glycaemic load (GL) of a food the following calculation needs to be made:
The quantity of carbohydrate contained in a specific portion of food is indicated in glycaemic index tables.
Source: Index glycémique – Mes P’tits Marabouts – Pr. J. Brand-Miller, K. Foster-Powel and S. Colagiuri)
Scientific studies on our glycaemic index
The results of a large number of scientific studies confirm the significance of using a food glycaemic index:
- Crapo, a researcher in the study of diabetes, published studies, in 1975, 1977 and 1981, showing that the increase in blood sugar levels following a meal was very different depending on the carbohydrates found in the food. (Crapo, P.A., Reaven, G. & Olefsky, J (1977) Postprandrial plasma glucose and insulin responses to different complex carbohdrates. Diabetes 26 : 1178-1183). Crapo suggested that a diet composed exclusively of carbohydrates resulting in low blood sugar levels could have a therapeutic value in the monitoring of diabetic blood sugar levels.
- Jenkins, a Canadian researcher, undertook to prioritise the impact of each carbohydrate on blood sugar levels in relation to a standard value arbitrarily attributed to glucose 100. A glycaemic index is therefore attributed to each carbohydrate. (Jenkins, D.J.A., Wolever, T.M.S. & Taylor, H.T. (1981)
- The Australian researcher, Jennie Brand Miller, now the reference in matters related to the glycaemic index, is Professor of Human Nutrition at the University of Sydney and holds the Chair of the Australian Academy of Nutrition. Her work provides additional scientific confirmation of the link between weight gain and the eating of high glycaemic index foods (Brand-Miller JC, Petocz P, Colagiuri S. (2003) Meta-analysis of low glycemic index diets in the management of diabetes : response to Franz. Diabetes Care 26 :3363-4).
Did you know? The University of Sydney’s SUGiRs (Sydney University’s Glycaemic Index Research), where studies on the Glycaemic Index were conducted by Prof Jennie Brand Miller, is internationally famous for its outstanding research.
In the same spirit, Prof Jeya Henry, working at Oxford University suggests we choose foods according to their rating on the GI scale in tackling obesity and type II diabetes. The role of a low-glycemic-index diet in the management of obesity. Am. J. Clin. Nutr. 81, 940-941).
David Ludwig, (Doctor at the Boston Children’s Hospital) carried out a study on the reaction of 12 overweight boys to 3 types of meals.
All the meals had the same number of calories but different glycaemic indexes. Dr Ludwig observed that the meals with a high GI resulted in an increase in appetite: the boys that had a high GI meal consumed a greater number of calories than the others for the rest of the day. For Dr Ludwig, high GI foods are directly responsible for eating between meals including among people who are not obese: “roughly 2 hours later”, he said “the insulin peak results in a fall in blood sugar level. This is when people feel the need to take a snack before the next meal.” (Ludwig DS : High glycemic index foods, overeating, and obesity. Pediatrics 1999, 103(3):E26).
Very recently, at Congress on nutrition and diabetes, Dr Frank Hu, from the Department of Nutrition of the Harvard School of Public Health, demonstrated that the consequences of carbohydrate intake on diabetes depend chiefly on the nature of the carbohydrates. He emphasised that several studies show the involvement of a high glycaemic index and load (GI quantity of carbohydrates present in the portion) in the increase in the risk of type II diabetes. (Schulze MB, Manson JE, Ludwig DS, Colditz GA, Stampfer MJ, Willett WC, Hu FB. Sugar-sweetened beverages, weight gain, and incidence of type 2 diabetes in young and middle-aged women. JAMA 2004 ; 292:927-34.)