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Core Knowledge


Understanding the Glycemic Index


The glycemic index went from being an obscure theory 10 years ago to a trendy topic in the early 21st century to a concept somewhere in between those two extremes today.

The basic premise of the glycemic index is that it gives us a way to rank carbohydrates according to their capacity to influence blood sugar levels. This is important because sugar levels impact athletic performance, weight loss or gain, heart health, and diabetes.

Not surprisingly, lists began popping up categorizing carbs as good or bad, depending on the glycemic index value. The truth is that both high and low glycemic index foods play an important role in providing energy for athletic performance. The role of the glycemic index in weight management and other physical problems is less clear and more controversial.

Glycemic Index by the Numbers

55 or less
A low glycemic index level.

56 to 69
A medium glycemic index level.

70 or more
A high glycemic index level.

How the Glycemic Index Works

The glycemic index measures the impact individual foods have on blood sugar levels. It ranks foods against white bread or glucose by potential to raise blood glucose levels.

Low Glycemic Index

Low glycemic index, or so called good carbs, are high in fiber and not overly processed. They're referred to as low glycemic index carbohydrates. The glucose from this type of carbohydrate is released at a slower rate. This means energy is released evenly over time, which keeps the body from experiencing an insulin spike.

Sources of low glycemic index foods:

  • Fruits
  • Vegetables
  • Whole grains
  • Legumes

High Glycemic Index

High glycemic index, or bad, carbs are those found in highly processed foods (white breads, sugary drinks, and candy). The glucose from these carbohydrates is quickly released and causes an insulin spike. They're then removed from the bloodstream and transferred quickly into cells. When the sugar leaves the bloodstream, a person may experience a crash characterized by a low energy level.

Sources of high glycemic index foods: 

  • White bread
  • Pasta
  • Rice
  • Low-fiber cereals
  • Baked products
  • Bagels with honey
  • Crackers
  • Raisins
  • Bananas 

For a more comprehensive list of foods and their glycemic index values, check out this information from Oregon State University.

The Pros and Cons of the Glycemic Index

There are three major issues with using the glycemic index as an absolute value of carbohydrates:

  1. Most foods aren't consumed in the volume needed to elicit the measured response.
  2. Most foods are eaten with other foods.
  3. Eating combinations of foods can change the glycemic index value for each food in a meal.

There is mixed research to show that the glycemic index of food has an impact on weight management. A few other pros and cons of the glycemic index are below: 


  • It provides a standardized measure of carbohydrates based on their potential as an energy source.
  • It can help control of blood glucose levels.
  • It can help suppress hunger.
  • It can promote improved awareness of the benefits and risks of some foods.
  • It can help an athlete recognize that changes in glucose levels may affect body composition and physical performance.


  • The manner in which foods are prepared impacts the glycemic index value.
  • The glycemic index value can change when fat or protein is eaten during the same meal.
  • The glycemic index doesn't take into account individual responses to glucose and the production of insulin by the body.
  • There are too many variables to support the use of glycemic index values alone to structure a diet.
  • The glycemic index load (amount of that particular food) should also be taken into account.

Glycemic Index and Athletic Performance

From the perspective of a serious exerciser or athlete, both high and low glycemic index carbohydrates are needed for optimal performance. The key is  timing. It's best to include lower glycemic index carbohydrates in meals throughout the day and before training or activity. During intense and prolonged exercise (an hour or more), it may be better to have a high glycemic index sports drink to maintain blood glucose levels.

The optimal time to have higher glycemic index foods is right after a workout. After an activity, there is about a two-hour window of optimal recovery. During these two hours, cells are most receptive to foods and beverages that will replenish depleted glycogen stores. This improves the quality of recovery and ensures a full energy supply for the next game, event, or training session.

Glycemic Index and Weight Loss

The premise of using the glycemic index in weight control is that diets that are low on the glycemic index foods will help people lose weight and reduce their risk for heart disease and diabetes. The original studies evaluating the glycemic index of foods were conducted in a controlled environment on subjects who fasted overnight. They ate a single carbohydrate in a prescribed amount and had their blood glucose measured two hours later.

In a controlled environment, a low glycemic index carbohydrate is broken down more slowly, which produces a more consistent glucose level. A high glycemic index carbohydrate does the opposite. It's broken down quickly and causes a spike in blood glucose, followed by a crash. A moderate glycemic index carbohydrate falls somewhere in the middle.

This scientific research doesn't really apply to real life because breakfast is the only time we truly eat after a fast. Additional factors such as the length of time the food is cooked, the body’s hormones, and any other food (protein or fat) that is eaten in combination with that carbohydrate can alter how the body uses glucose.

Diabetes and Other Conditions

The research is mixed. Some study show that low glycemic index diets positively influence diabetes and other conditions, while others don't. A study conducted at the University of Southern California found that people who followed a diet of low glycemic index carbohydrates didn't have significantly lower blood glucose levels than people who followed a diet of relatively high glycemic index carbohydrates.

Does this mean to run out and get that Wonder Bread that you've been craving? No. Lots of studies link dietary fiber to a decreased risk of heart disease, diabetes, and obesity. This questions whether it 's really the glycemic index value that makes a food healthy or the components of the food that make it fall low on the glycemic index. Foods that are low on the glycemic index are typically higher in fiber. Therefore, many of the studies linking glycemic index to good health may have, in fact, reflected the dietary fiber found in those foods. 

The Big Picture

For those of you who've jumped on the glycemic index bandwagon, don’t get off just yet. The glycemic index has taught us a lot about carbohydrates by helping us realize that all carbohydrates aren't created equal. However, the glycemic index isn't the complete answer to controlling weight, preventing heart disease, or managing diabetes. It's simply one piece of the puzzle.  

Foods that are found lower on the glycemic index tend to be more whole foods with more nutrients and  fiber. People should know about the glycemic index, but at the end of the day they should choose the least processed form of food possible. Don't make it complicated. When choosing carbohydrates, reach for fruits, veggies, beans and whole grains. When you're shopping for breads and cereals, look for a high fiber content. If the cereal or bread has 3 g of fiber or more per serving, go for it. If it doesn't, look for something else.

Tags: Nutrients, Health, Weight Loss, Food


  1. Amanda Carlson-Phillips, Athletes’ Performance/Core Performance
  2. American Diabetes Association
  3. Linus Pauling Institute, Oregon State University
  4. University of Sydney (Australia)
  5. WebMD