Glycemic Index

What is Glycemic Index?

Our body and our cells get their energy mostly from a simple, six-carbon sugar called glucose. Humans are not able to make glucose, so we need to obtain it from the food we eat. Glucose is a carbohydrate found in starchy foods (rice, bread, potatoes, pasta, etc.) and/or sweet foods (candies, fruits, sodas, etc.). After eating food and digesting it (breaking it into the small molecules such as glucose), these molecules are absorbed into our blood. Therefore, we can expect an increase in the amount of glucose in our blood (also called blood sugar) after eating a meal or a snack. Different foods that contain glucose can increase blood sugar at different levels. Some foods cause a sudden increase in blood sugar, while others lead to a more slow, steady increase. Glycemic index is a number, between 0 and 100, assigned to different types of food that contain carbohydrates, based on how much they elevate blood sugar 2 hours after ingesting. As a reference, pure glucose is given an index of 100, and all other types of food are compared to it. A food with a high glycemic index (GI) will raise the blood sugar to a higher level compared to a food with a lower GI.

What Factors Determine the GI of a Food?

One of the main factors that affect a food’s GI is how fast or slowly it is broken down and absorbed during digestion. Foods that get broken down pretty fast will have a higher GI compared to the ones that are broken down more slowly. There are several other factors that can affect the same type of food to have different values of GI. These include:

  • Cooking time/methods: Slow-cooking or overcooking the same type of food leads to it being more broken down before consumption, leading to a higher GI.
  • Fruit Ripeness: When a fruit ripens, it increases in the amount of its sugar, leading to a higher GI.
  • Refinement/processing: For example, white bread is purely starch, while whole-wheat bread also has a considerable about of dietary fiber and other nutrients, so white bread has a higher GI.

What Standards Are Used to Determine GI?

There are two main standards for measuring GI. One standard gives a GI value of 100 to white bread, and other foods are compared to it, but this leaves glucose, which is the basis of all blood sugar measurements, with a GI value of about 130. To overcome this complication, a different standard gives a value of 100 to pure glucose, and all other foods are compared to glucose. To determine the GI value of each type of food, different types food containing carbohydrates are given to people who have been fasting for 12 hours, at the same amount (typically 50 grams, or 1.7 ounces), to a sample group of 10 people. Then their blood glucose level is measured continually and recorded after 2 hours. It has been shown that foods with lower glycemic index will raise blood sugar less that foods with higher glycemic index (see figure below).

What Are Some Foods with Low, Medium, and High GI?

The following table lists some examples of foods with various glycemic index:

Pros and Cons of Foods with High Glycemic Index:

Pros:

  • They give a quick boost of energy
  • You feel full immediately after a meal
  • If you are feeling hypoglycemic, they will elevate your blood sugar quickly
  • They are a good choice for quick muscle recovery after an intense workout, if your muscle glycogen is depleted

Cons:

  • You will feel hungry shortly after a meal
  • They cause a sudden increase in blood sugar, leading to an insulin spike

Pros and Cons of Foods with Low Glycemic Index:

Pros:

  • They can keep you feeling full for a longer time, so less cravings and less calorie intake
  • They don’t lead to blood sugar spike
  • They may help in weight loss, reduce the risk of obesity and type 2 diabetes

Cons:

  • Since they take longer to be digested and enter the bloodstream, you can’t rely too much on them for a quick boost of energy

Glycemic Load:

While it sounds that consuming foods with lower GI sounds like a healthy plan and should guarantee weight loss or at least obesity prevention, it is important to mention that in order to manage your weight and monitor your health, simply focusing of the GI of your food is not enough, so instead of having only glycemic index in minds, it is better and more effective to consider your food’s Glycemic Load (GL), which considers both the quality AND quantity of your food. When you are preparing a meal, you may end up with a meal that has much lower GI, much greater amount of carbohydrate, meaning it will not be any healthier than a smaller portion of a food with high GI. And because of this matter, GL is a better indicator of your meal plans. The formula to calculate GL is:

GL = GI(%) x gr

Where GL is the glycemic load, GI is the glycemic index converted to percent, and gr is the amount of carbohydrates, in grams.

Glycemic Index Quiz