The glycemic index (GI) is a number, typically between 50 and 100 indicating a particular food's effect on a person's blood glucose (sugar) level. A value of 100 represents an equivalent amount of pure glucose and represents the standard measurement.
Foods that break down and are absorbed quickly in the guts, rapidly releasing glucose into the blood tend to have a high GI. Those that breakdown and are absorbed more slowly and hence have a more gradual release of glucose into the blood tend to have a lower GI. A rapid release of glucose into the blood generates a spike in the hormone Insulin in order to bring blood sugar levels back to normal. Insulin is a stress hormone, and repeated spikes are not good for the body. There is some limited evidence that lower spikes in insulin lead to better long term blood sugar control.
GI is calculated by measuring blood sugar levels repeatedly over a 2-hour period after consuming 50g of the food (following a 12-hour fast) and then calculating the area under the curve over time. Comparison is then made to glucose and the result multiplied by 100. When compared to glucose as the standard (some tables use white bread) there will be a maximum GI of 100 and a minimum of 0. GI is then categorised into low, medium and high as below:
Put simply, a low GI food will release glucose more slowly.
There are several problems with GI measurement and using it for dietary manipulation. Firstly the number of grams of carbohydrate consumed can have a bigger impact on blood sugar levels than the GI value of the food itself. GI also does not take into account the insulin response of the person to the food consumed. GI only tells us how quickly blood sugar rises and not how much of the carbohydrate consumed is turned to glucose.
An alternative and slightly better method is Glycaemic Load:
The Glycaemic load (GL) will take into account not only the GI of the food, but also the amount consumed:
A good working example is to consider watermelon with a GI of 72. a 50g serving has 2.5g of carbohydrate and therefore has a GL of 1.8. A double portion still has a GI of 72 but a GL of 3.6. This clearly shows how GL is easier to interpret than GI. In general GL figures can be categorised as:
The insulin index (II) quantifies the typical insulin response to certain foodstuffs and is based on eating 1000Kj (approx 240 calories) of the food. The II and GI are well correlated, but foods high in protein, fat or refined carbohydrates elicit a disproportionately higher response than the glycaemic response.