Understanding the Glycemic Index

Many of you would have heard about the Glycemic Index (GI).  The GI measures how quickly the carbohydrate portion of a food is digested and absorbed into the bloodstream.

Generally speaking, foods with a high GI have the glucose component digested and absorbed rapidly into the blood stream, resulting in marked fluctuations in blood sugar levels.  For example, white bread has a GI of 70.  Those foods with a low GI, have the glucose digested and absorbed much slower and more gradually, producing gradual rises in blood sugar and insulin levels, providing long-lasting energy.  Chickpeas have a GI of 10.  Choosing a low GI diet has been shown to help in the prevention of obesity, cardiovascular disease and diabetes.

Source: glycemicindex.com










There are certain limitations to the GI system which people should be aware of:

•  If a food is eaten in combination with other foods, the overall glycemic affect changes.

•  A low GI food doesn’t always translate to a healthy food.  Icecream has high sugar content but a low GI.  This is because the saturated fat content of icecream slows down the absorption of sugar.

•  The glycemic response differs between people due to genetics, hormones, activity, etc.  The response may even be different in the same person from day to day, depending on blood glucose levels, insulin resistance, and other factors.

•  Factors such as plant variety, ripeness, cooking methods and processing techniques may affect a food’s GI.  For example, bananas, rice and potatoes are sensitive to these factors (the GI for under-ripe bananas is 43 and for over-ripe bananas it is 74).

•  GI tests are not performed using typical portion sizes.  Using the glycemic index alone, the blood sugar effects of foods containing only a small percentage of the carbohydrates (eg: watermelon) are likely to be overstated. Conversely, foods containing a high percentage of carbohydrates are likely to be understated.

The GI value tells you only how rapidly a particular carbohydrate turns into sugar.  It does not tell you how much of that carbohydrate is in a serving of a particular food.  You need to know both things to understand the effect this will have on your blood sugar.  That is where the Glycemic Load (GL) is more useful.

The GL combines both the quality and quantity of carbohydrate into one number.  It considers the serving size of the food and calculates the number of carbohydrates in that particular serving of food.  You then have a more accurate way of predicting how the carbohydrates of a given food will affect your blood sugar.

For example, the carbohydrate in watermelon has a high GI.  But there isn’t a lot of it, so watermelon’s GL is relatively low.

To calculate the GL of a food, multiply the GI value by the amount of carbohydrate per serving and divide the result by 100.   You can then measure the glycemic load accordingly:

high = 20 or more
medium = 11-19
low = 10 or less