The Glycemic Index is an indication of how quickly a specified amount of food will cause a rise in blood sugar level. The amount of food is the portion that contains 50 grams of carbohydrate (200 calories from carbs). So it is really an indication of how one carb compares to another.
Certain foods cause a spike, or rapid rise, in blood sugar level. This spike causes an insulin response and may over time lead to health problems such as diabetes, insulin resistance, metabolic syndrome and other issues. Maintaining a more even blood sugar level appears to be beneficial in many ways.
Only foods that contain carbs cause this spike, proteins and fats do not. They provide calories but do not cause an immediate rise in blood sugar levels. Meat and eggs contain no carbs at all, they can be considered to have a zero glycemic index. However this is not strictly true as the test cannot be carried out on them: no amount of eggs will give the required 50 gm of carbs.
The test is carried out on volunteers who have been fasting for a period of time. They are fed a portion of food containing 50 gm of carbohydrate and their blood sugar level monitored over a 2 hour period. This data is drawn on a graph and the area under the curve measured, the larger the area, the higher the glycemic index (GI). Glucose is used as the standard with a value of 100 and all other foods are compared to this.
Many factors influence the GI for foods. The index of the carbs available, the amount of non carb food in the serving, and the amount of fiber and ash (yes ash!) present. Protein and fat in the food will tend to lower the GI because they reduce the body's ability to digest the carb quickly. Soluble fibre (inulin) has the same effect, insoluble fibre (bran) does so to a lesser extent.
The irony here is that a serving with more calories can have a lower GI, but exactly the same type and amount of carbs. This also shows the danger of consuming significant amounts of refined carbs (even fruit juices) on their own as they can produce a rapid blood sugar spike. Again balance is best in all things.
One might assume because of this that the answer is to avoid all carbs, and certain diets (Atkins for example) do go in this direction. However the body needs a good balance of protein, fat and carbs for health. So the answer appears to be the correct choice and amount of carbs. The Glycemic index and the Glycemic Load are useful tools in the achievement of this end.
And that brings me on nicely on to the next subject - Glycemic Load - what is it?
The glycemic load (GL) is the glycemic index multiplied by the amount of carbs in the serving. So in a way it represents the actual effect the serving of food will have on blood sugar level.
The Glycemic Index on it's own can be a little misleading because portion sizes are not taken into account. Foods that contain low quantities of carbs can still score highly because large amounts are required for the test.
To produce the required 50 grams of carbs about 12 carrots are necessary but only 3 slices of bread. So the GI for carrots is almost 50, for bread it is 70, not a huge difference. In reality nobody eats 12 carrots at a sitting.
This is where the glycemic load comes in. The GI for carrots is 50, the amount of carbs per serving is 4. So the GL is 2 (50 by 4 divided by 100). The glycemic load for a serving of bread is 10. This is a much more realistic indication of the effect carrots and bread have on blood sugar levels.
Rating System for Glycemic Index:
Below 55 - low GI.
56 to 69 - medium GI.
Above 70 -high GI.
Rating System for Glycemic Load:
Below 11 - low GL.
11 to 19 - medium GL.
Above 19 -high GL.
For sweeteners the glycemic index is useful as in most cases they consist of pure carbohydrate. Thus the GI offers a fair comparison between them. Natural sweeteners can contain soluble fiber and other substances that tend to slow metabolism and reduce the effect of blood sugar.
For information on GI for sweeteners click here.