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The Glycemic Index Explained

Counting carbs is out; the glycemic index is in. But is the glycemic index just another dose of diet hype, or can it really help?

In fact, if used properly, the glycemic index (and even better, the glycemic load) may actually help you to improve your diet and make better food choices. It can also be a great tool for people with diabetes. Just don't count on it for weight control, though.

Read on to learn more about how to use the glycemic index and the glycemic load, and check out your favourite foods on our glycemic ratings table.

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What is the glycemic index?

Pears have a low GI

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 (GI) takes these factors into account when rating 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, white rice, and jelly beans, have a GI of 70 or more. Medium GI foods, such as bananas, cherries and ice cream have a GI between 56 and 69. Low-GI foods, including mixed-grain breads, legumes, milk and yoghurt, and most fruits, have a GI of 55 or less.

What is the glycemic load?

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, pumpkin has a high GI of 75, but you would have to eat a lot of pumpkin for there to be a steep rise in blood sugar. Because pumpkin has a high GI number, it seems like it is a food to avoid, whereas in fact it is full of excellent nutrients and, when eaten in normal proportions, is 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. Foods low in carbohydrates, such as pumpkin which has only 8g carbs per 100g, do not have much 'power' to raise your blood sugar levels. According to the GL system, therefore, pumpkin is given a relatively low rating of 4.

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 fibre) 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.

Food GI Rating GL Rating
Watermelon 72 4
Coca Cola 53 14
Corn on the cob 48 5

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