Limitations of the Glycemic Index (GI)
At best, the GI measures something very specific - how quickly carbohydrate foods are digested and converted into glucose that enters the bloodstream. But there are many flaws in this measurement.
- Low GI foods are not always healthier than high GI foods. Generally they are, especially for those who are overweight. But as we saw above, there are some inconsistencies and some puzzling Gl values.
- GI does not consider the quality of food. As long as a food has a low GI value, it is deemed to be "good", regardless of whether the food is natural, or artificial and highly processed. Therefore GI is best applied to natural food.
- People generally do not eat single foods. More often, we eat meals comprising different foods, like rice with vegetables and meat or fish, cooked in oil. The GI of the meal would be much lower than that of rice alone, because protein, oil and fibre slow down digestion and absorption. For this reason, the GI chart does include meals like hamburgers or pasta dishes. However, there are endless combinations of meals. Even the same meal is prepared differently by different people, such that the GI values can vary greatly, making any official values quite meaningless.
- Many factors influence GI. For example, oil and acid will reduce the Gl. So if you eat brown rice as part of a meal that contains oil, your blood glucose will not rise as quickly as if you eat plain brown rice. Protein and fibre, too, will delay digestion. Protein makes your food stay longer in the stomach while fibre impedes the interaction between digestive enzymes and your food. Both factors will slow down the rate at which carbohydrates are digested and glucose absorbed into the blood. In other words, they reduce the Gl.
Since there are many factors that can greatly affect the Gl, it is actually quite meaningless to be make healthy food choices based solely on Gl values. At best, the GI should serve only as a rough guideline.
Glycemic load (GL)
Yet another problem with the GI is that while it indicates how quickly glucose enters the bloodstream, it does not consider the total amount that enters the bloodstream. As such, the index does not fully reflect the impact on blood glucose levels.
The most often cited example is watermelon. It has a high Gl of 74. But watermelon contains mostly water and very little sugar. So even though watermelon is quickly digested and absorbed, eating it will not significantly raise your blood glucose level.
Introduction of the concept of Glycemic Loading (GL)
To get around this problem, scientists have developed another measurement called the glycemic load (GL). This formula multiplies the GI by the number of grams of carbohydrates in a typical serving and then divides the result by 100.
Explanation of GL categories: high, medium, and low.
For watermelon, the Gl is 74. But a typical serving of watermelon contains only 6g of sugar. So the glycemic load of watermelon is 74 x 6 ÷ 100 = 4.4, which is very low.
For white rice, the GI is 64. But a typical serving of white rice has 40g of starch. So the glycemic load is 64 x 40 ÷ 100 = 25.6, which is rather high.
Even though white rice has a lower GI than watermelon, it has a very much higher GL. In general:
- GL higher than 20 is considered high;
- GL between 11 and 19 is medium;
- GL below 10 is low.
The GL is a better indicator of the outcome of what you are eating. However it is easier to find data on the GI than on the GL. Note, too, that since the GL is based on the Gl, it is also subject to the same limitations and flaws discussed previously. So again, the GL is, at best, a rough guideline.
This content is adapted, with permission, from Book 1 of 2 : The Wonders of Nutrition by Dr Ang Poon Liat. MBBS, M.MED (PAED), MRCP (UK PAED), FAMS, MD.