A Closer Look at the Glycemic Index

Blog Post

Stephen W. Farrell, PhD, FACSM
The Cooper Team
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March 14, 2022

Many popular diets focus on limiting foods and beverages that are high on the glycemic index. Is this really the best approach for healthful and long-term weight control? Read on to learn more!

The glycemic index (GI) for carbohydrate-containing foods and beverages was developed in the early 1980’s with the noble goal of improving blood glucose control in diabetic patients. The GI of a food or beverage is determined by the rise in blood glucose (sugar) levels during the two-hour period following its ingestion versus the blood glucose response to an equivalent amount of carbohydrate in a standard food such as white bread. As a reference point, white bread has a GI of 100. Foods and beverages with a GI higher than 100 (e.g. instant rice) cause a greater rise in blood glucose than white bread, while foods and beverages with a lower GI than 100 cause less of a rise in blood glucose than white bread. For example, if a food has a GI of 33, it would cause only about one-third of the increase in blood glucose as compared to eating an equivalent amount of white bread. Examples of high and low GI foods are shown in Table 1 below. Please note that this Table should not be interpreted as to what to eat or what not to eat!

Table 1. Examples of high and low GI foods and beverages

Table 1
High GI foods Low GI foods
White bread Beans
Instant rice Apples
Baked potato Whole grains
Corn flakes Chocolate
Raisins Milk
Watermelon Bran cereal
Non-diet soft drinks  French fries
Pretzels Full-fat ice cream

Carrots are considered a High Glycemic Index food
Carrots are classified as a high GI food

Some hip and trendy diets, e.g. South Beach, Atkins, and The Zone strongly recommend the use of the GI to determine what foods should or should not be consumed in order to lose weight and maximize health. Virtually all legitimate nutrition experts (e.g., Registered Dietitians) agree that there are many limitations and flaws to this very narrow type of approach:

  • The GI of any individual food is based on consuming an amount of that food which contains 50 grams of carbohydrate. For example, although carrots have a high GI, this is based on consuming 3.5 cups of carrots at one time. That’s a whole lot of carrots!
  • The GI is based on consuming a food by itself on an empty stomach! Very few of us would eat just a plain baked potato on an empty stomach. Most of the time we consume carbohydrate-containing foods in combination with other foods that contain various amounts of fat and/or protein. The GI pretty much goes ‘out the window’ when this occurs.  
  • Individuals have different blood glucose and insulin responses to the same food! More specifically, physically active individuals have a much smaller rise in blood glucose and insulin levels than inactive individuals when given the same amount of carbohydrate-containing food.
  • Weight that is lost on a low GI type of diet plan occurs simply because so many foods are restricted, or even eliminated. When foods are restricted, people tend to consume significantly fewer calories than they did previously. Therefore, the weight loss occurs simply because of negative caloric balance (calories burned > calories consumed), not because people are consuming low GI foods.
  • The ripeness of a food can affect its GI. For example, the GI of bananas can range from 30 (greenish colored-banana) to 70 (yellow and brownish-colored banana).

Perhaps the greatest limitation of using the GI to determine whether or how often a carbohydrate-containing food should be consumed is that it completely ignores the nutrient and fiber content of the food!

For example, a bowl of full-fat ice cream has a low GI, but is loaded with simple sugar, saturated fat and calories, and completely lacking in fiber. On the other end of the spectrum corn, carrots, and raisins have a high GI, but are a great source of fiber, vitamins, minerals, and phytochemicals (non-nutrients found in plant-based foods that have been shown to help prevent disease). For athletes and others who are training for long periods of time on a daily basis, it is extremely important to replace muscle and liver carbohydrate stores (i.e., glycogen) that are depleted each day. Many peer-reviewed studies have shown that consuming high GI foods and beverages during the 30-60 minute period post-exercise actually speeds up muscle and liver carbohydrate replacement, thus speeding up recovery from exercise! So, for highly active individuals, the low GI approach is actually very counterproductive.

Chocolate is considered a Low Glycemic Index food
Chocolate is classified as a low GI food

Since individuals with diabetes struggle to control their blood glucose level, it might be of interest for you to know what the American Diabetes Association (ADA) has to say about the GI. In their recommendations, they state that individuals with diabetes do not have to limit their food choices to only those that have a low GI. Instead, people with diabetes should focus on the nutrient and fiber content of carbohydrate-containing foods, as well as the amount of carbohydrate consumed. The GI of a food, when considered together with the amount of carbohydrate in the food consumed is known as glycemic load. When it comes to weight loss and good health, individuals are advised to be more concerned with glycemic load as well as nutrient and fiber content, and less concerned with GI.

It is interesting to note that none of the fad diets which strongly promote use of the GI have been shown to be effective for long-term weight control or long-term significant improvements in health, despite claims to the contrary made by their vocal proponents.

As was eloquently stated several years ago by Dr. Xavier Pi-Sunyer, a prominent expert in diabetes, “People and the experts advising them on nutrition can find more worthwhile issues to concentrate on than the glycemic index.” That sums it up quite nicely I think!

Foster-Powell, K., Miller, J.B. (1995). International tables of glycemic index. Am J Clin Nutr. 62:871S-893S.


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