The Glycemic Index Explained

Counting carbs is not just a matter of adding two and two together. Other factors also have to be taken into consideration, such as how fast the carbohydrate is digested, and how much it causes your blood sugar levels to rise.

The glycemic index is a system that takes these factors into account when describing or classifying a carbohydrate food. A food with a low-GI rating will cause a small, slow rise in blood sugar levels, while a high GI food will cause a fast and dramatic spike. The GI rating of a food is based on glucose – the fastest releasing carbohydrate – having a rating of 100 . A food that releases glucose at half the rate of pure glucose has a GI of 50; a food with a quarter the rate of glucose release has a GI of 25, and so on.

High GI foods, such as white bread, potatoes, white rice, and honey have a GI of 70 or more. Intermediate GI foods, such as ice cream, sugar, and orange juice have a GI between 56 and 69. Low GI foods, including mixed-grain breads, legumes, milk and yogurt, and most fruits have a GI of 55 or less.

The glycemic load (GL) goes a step further than the GI by taking into account the amount of carbohydrate in a food. A weak point of the GI is that it fails to do this.

For example, carrots have a high GI of 47, but you have to eat a pound and a half of them for there to be a steep rise in blood sugar. Even a rabbit won’t eat that many carrots in one sitting! Because carrots have a high GI number, it seems like they are a food to avoid, whereas in fact they are full of excellent nutrients and, when eaten in normal proportions, are unlikely to cause a dramatic influx of blood sugar levels. The GL provides a more practical way of evaluating the effect of carbohydrates on blood sugar by combining both quantity and quality of carbohydrate into one number. According to the GL system, therefore, carrots are given a relatively low rating of 3.

A GL of 20 or more is high, a GL of 11-19 is medium, and a GL of 10 or less is low. Almost all foods with a low GI will also have a low GL, but foods with an intermediate or high GI often have a low GL.

GL is calculated by dividing the GI of a food by 100 and then multiplying by the food’s available carbohydrate (i.e. not including fiber) in grams. For example, the GI of an apple is 38 and its carbohydrate content is 16. Therefore: 0.38 X 16 = 6.08. So an apple has a GL of around 6.

There are some reasons why a low GI diet can contribute to weight loss. For example, many low GI foods are bulky, high in fiber, and more satiating than high GI foods because they take longer for the body to process; e.g. legumes take longer to process than bread. Consuming these foods therefore helps people to feel fuller for longer and means they are less likely to overindulge. Low GI foods also produce less insulin, and low insulin levels help people to burn fat instead of carbohydrate.

However, following a low GI or GL diet does not automatically mean that you will lose weight. These indexes only measure the carbohydrate content of foods and do not account for calories, fat, and other nutrients. It is vital to consider overall calorie and fat intake, as well as carbohydrate intake, for effective weight management. It is also easy to make unhealthy choices based on GI or GL rating alone. For example, chocolate has a low GI compared to a slice of white bread, but bread has more nutrients and less calories and fat than chocolate, and is therefore a much better option.

According to Jennie Brand-Miller, Professor of Human Nutrition at the University of Sydney and a leading researcher in the development of the GI, the GI/GL is essentially about making smart carbohydrate choices and should be thought about in terms of swaps: "People should not think that chocolate is a good swap for watermelon!" she warns.

Portion control also remains vital for weight loss when using the GI or GL. A low GI or GL rating should never suggest free reign on portion sizes. This is as important to those who are controlling their blood sugar levels as it is to those who are controlling their weight.

It is good to keep in mind that GI and GL are really research tools for analyzing carbohydrates in foods. Calculating the GI or GL of every food before you eat it is neither practical nor sensible. However, being familiar with the GI/GL rating of foods can help you improve the quality of your carbohydrate choices, whether for insulin moderation or weight loss purposes.

People who have diabetes do not produce enough insulin in their bodies to effectively manage glucose and maintain their blood sugar levels. Consequently, it is very important for people with diabetes to manage their glucose intake carefully. Low GI and GL diets are believed to be helpful to people with diabetes because low GI and GL foods release glucose into the bloodstream slowly and more evenly than high GI and GL foods, making glucose levels easier to regulate.

Brand-Miller says that "choosing low GI sources of carbohydrate is a natural way of keeping blood glucose levels on an even keel, which helps people avoid marked fluctuations." However, the GI or GL should clearly not be the only point of reference in managing carbohydrate intake; instead they should be used as tools.

A low GI diet may also help to prevent Type 2 diabetes. A report on GI, GL, and the risk of Type 2 diabetes in the American Journal of Clinical Nutrition (July, 2002) found that diets high in both high glycemic index and high glycemic load foods have been associated with a greater risk of type 2 diabetes in both men and women.

Researchers agree that larger studies and clinical trials are needed to definitively answer the question of whether there is a link between a diet of high GI foods and the development of diabetes. But as it stands, the current research gives a fairly strong indication that a low GI/GL diet is a wise choice for both managing and preventing diabetes.

Low GI, high GL, swapping white rice for sweet potato... All of these figures and rankings can seem a little confusing at first. It's important to remember that GI and GL are simply tools to use for controlling your blood sugar levels and carbohydrate intake.

Try our tips for some simple ways to make the switch to low GI foods:

• Don't worry about exact GI values. Instead consider whether the food falls into the low, medium or high GI category. For example, a food with a GI of 40 is as suitable a choice as a food with a GI of 30 - they're both low GI
• Consider which food will give you the most nutritional value, as well as the GI rating
• Remember portion size - low GI foods can still be high in calories
• It's okay to eat some moderate and high GI foods. Try to combine them with low GI foods to reduce the overall GI rating of the meal
• Use the GI ratings to make food swaps. Swap a high GI food for a low GI food - such as yams instead of potato, brown rice instead of white rice, and so on

Compiled by Joan Bushman, MPH, RD  (in reference to K. Foster-Powell, S. Holt and J. Brand-Miller:  ‘International table of glycemic index and glycemic load values: 2002’. American Journal of Clinical Nutrition 2002; 76:5-56)