07 May 2012
What is the glycemic index? -Steve
Steve, I'm glad you asked this, since I think there are many misconceptions about this tool out there. The glycemic index was developed in the 1980's, and is a measure of the effect that ingesting a particular type of carbohydrate has on blood sugar levels. If you recall, carbohydrates are essentially long chains of glucose molecules, which are broken down in the GI tract and released into the bloodstream.
A carbohydrate that is HIGH on the glycemic index is broken down very quickly and appears in the bloodstream as glucose after a very short time. Alternatively, a carbohydrate that is LOW on the glycemic index takes a long time to break down, and is similar to a time-release pill in that it appears in the bloodstream little by little.
Glucose is usually given the arbitrary value of 100, which is the highest value on the glycemic index (remember, the glycemic index is measuring how fast a carbohydrate breaks down into glucose, so glucose does not need to break down at all to become glucose, therefore, it is given the highest value), meaning that ingesting pure glucose would produce a very quick rise in sugar levels in the bloodstream.
The speed with which glucose is dumped into the bloodstream has an impact on insulin secretion. Glucose is the main fuel for our cells, but it has to get inside the cells before it can be used. Insulin, which is produced and released by the pancreas, functions as a metabolic key which allows glucose to enter inside the cell; the more glucose that is present, the more insulin is released to allow it to enter into cells.
For several different reasons, which are beyond the scope of this article, it is better to have a gradual appearance of glucose in the blood stream, along with a gradual insulin release, rather than several spikes of glucose, with massive amounts of insulin released from the pancreas. Eating lower glycemic index foods provide this gradual release of glucose+insulin, as opposed to the spike which is produced with high glycemic index foods.
However, this does not mean that you should avoid all high glycemic index foods. The glycemic index was constructed under controlled conditions which do not always apply in reality. Firstly, the glycemic index values were calculated after an overnight fast and eating a 50g serving of the particular carbohydrate first thing in the morning. Some examples of 50g of carbohydrate include 1 cup of uncooked oatmeal, an entire bagel, or ½ cup of raisins. This might be much more, or much less than what is consumed in reality.
Furthermore, most people don't just eat a bagel for breakfast. Usually, a bagel, eggs, cream cheese, lox, etc… are all consumed together. A mixed macronutrient meal cannot be accurately measured using the glycemic index, since different macronutrients affect digestion and absorption differently.
Due to these faults in the glycemic index, the concept of glycemic load was introduced. The glycemic load equalizes the glycemic index value based on the amount of carbohydrates in one serving of the food. A perfect example of using the glycemic load value is with watermelon, which has a glycemic index value of ~75, which is very high (remember, 100 is pure glucose). Compare this to Coca Cola, which has a glycemic index score of ~60. If you just look at the glycemic index numbers, you may think that soda is a healthier alternative to watermelon.
However, remember that the glycemic index is based on a 50g sample of the carbohydrate. To get 50g of carbohydrates from Coca-Cola, one would have to drink ~16 fl oz., which is a plastic bottle. To get 50g of carbohydrates from watermelon, one would need to eat ~903g, which is close to 2 lbs. of watermelon.