What Is the Glycemic Index? Purpose and Limitations

Medically Reviewed By Jared Meacham, Ph.D., RD, PMP, CSCS
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The glycemic index (GI) refers to the total increase of blood glucose after consuming a specific carbohydrate-containing food relative to a reference food. Typically, the index uses glucose or white bread as a reference. You may have heard or read about high GI foods and low GI foods, but what do these terms mean?

This article will explain the GI, the potential effects of high GI foods, and the purpose of a low GI diet. It will also provide a list of low and high GI foods and discuss meal planning with the GI.

The glycemic index

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Physicians developed the GI in 1981 for people with diabetes. It is a guide to food selection, people how to consume lower GI foods. Consuming low GI foods helps prevent large increases in blood glucose (blood sugar), which is important for managing diabetes.

Learn more about diabetes here.

These physicians assigned each food a numerical value on a scale of 0–100. The body more rapidly digests and absorbs foods with a higher GI, which translates to a greater rise in blood sugar per gram (g) of carbohydrate. The classifications are as follows:

  • A low GI is 55 or less.
  • A medium GI is 56–69.
  • A high GI is 70 or more.

High blood sugar and its clinical implications

Glycemic variability measures the peaks and valleys in blood sugar levels over time. This and hyperglycemic excursions (blood sugar spikes) are known risk factors for developing many clinical conditions, including diabetes and cardiovascular disease. Why is this?

When you consume carbohydrates, your body breaks them down into glucose, a type of sugar. As your body produces glucose, your blood sugar rises. This stimulates the pancreas to make insulin, a hormone that processes glucose and lowers blood sugar.

If you consume many high GI foods, blood sugar may rise more frequently and for longer periods. In response, the pancreas produces more insulin to keep blood sugar in a healthy range.

The body may become resistant to insulin after sustained periods of high blood sugar. Insulin resistance means your body requires more insulin than usual to lower your blood sugar. Eventually, your pancreas wears out. If your pancreas cannot make enough insulin, your blood sugar remains high. Ultimately, insulin resistance can lead to diabetes.

Additionally, chronically high blood sugar can damage blood vessels and nerves. Insulin resistance and hyperglycemia associated with diabetes are underlying drivers of many different diseases.

Learn more about glucose here.

The purpose of a low GI diet

The purpose of a low GI diet is to keep your blood sugar more stable throughout the day. Certain carbohydrate-containing foods are less likely to cause major blood sugar spikes and dips. This stability may reduce your chance of developing diabetes and other conditions.

Additionally, a rapid rise in blood sugar tends to drop below standard blood sugar levels later. This sharp drop can result in postprandial or reactive hypoglycemia. This is different from hypoglycemia from fasting, which can be life threatening.

Chronically high blood sugar and insulin resistance also leads to more body fat, explains the Centers for Disease Control and Prevention (CDC).

Benefits

Potential benefits to selecting more low glycemic foods include:

  • reduced chance of insulin resistance and diabetes
  • fewer fluctuations in appetite
  • greater ability to maintain a moderate weight
  • fewer blood sugar spikes and valleys

Limitations and the glycemic load vs. glycemic index

The GI has some limitations. First, the composition of a meal alters the GI. Different types of fiber, protein, and fat may reduce any specific food’s impact on your blood sugar. Other factors that influence food’s impact on blood sugar include:

  • how you prepare, cook, or process it
  • how ripe it is
  • how refined the carbohydrates are

Second, the GI does not reflect real-world portion sizes. The GI is based on 50 grams (g) of digestible carbohydrates in the test food. The glycemic load (GL) refers to the quantity in grams of carbohydrate content you eat. To calculate GL, multiply the grams of carbohydrate in the serving size by the food’s GI. Then, divide that number by 100.

The GL scale is:

  • A number equal to or less than 10 is a low GL.
  • A number from 11–19 indicates an intermediate GL.
  • A number equal to or more than 20 is a high GL.

Lowering the GL of your diet may help you manage blood glucose levels if you have type 2 diabetes.

Thirdly, the GI does not account for the amount of fructose in foods because the body metabolizes fructose differently. Therefore, fructose does not spike blood sugar. However, large quantities of fructose can be toxic to the liver and have harmful effects, including:

Finally, everyone’s bodies are unique and respond differently to foods with a particular GI value. Two people could eat the same food in the same quantity and experience a different glycemic response (rise in blood sugar). An older study from 2010 supports this limitation of the GI.

Meal planning with the glycemic index

A diet plan incorporating the GI will typically prescribe foods with GI values less than 55. These foods are “slower” carbohydrates, meaning they take longer to digest. The diet may include consuming foods in a specific order, such as eating protein and vegetables before the carbohydrate.

The following table displays the GI, serving size, carbohydrate per serving, and GL of some common foods.

FoodGI (pure glucose=100)Serving sizeCarbohydrate per serving (g)GL
russet potato, baked 1111 medium3033
cornflakes cereal791 c2620
Kellogg’s All-Bran cereal451 c2110
doughnut761 medium2317
watermelon761 c118
bread, white-wheat flour711 large slice1410
pancake676-in diameter5839
rice, white and boiled661 c5335
rice, brown and boiled501 c4220
spaghetti, white flour581 c4425
spaghetti, whole wheat321 c3714
honey (pure)581 tsp1710
maple syrup541 tsp147
banana551 c2413
orange421 medium115
apple391 medium156
skim milk338 fl oz134
carrots, boiled331/2 c41
kidney beans281 c298
peanuts181 oz61

Read 7 Foods with a High Glycemic Index here.

Frequently asked questions

Below are some questions people have asked about the GI. They have been reviewed by Jaren Meacham, Ph.D., RD, PMP, MBA, CSCS.

What should my daily glycemic load be?

2021 study shows that the optimal range for GL is 85–100 g per 1000 kilocalories (kcal). This range can help reduce the risk of diabetes and obesity. However, the plate method is a more practical and evidence-based approach to meal planning.

With this method, you fill half your plate with nonstarchy vegetables, such as carrots. You then fill one-quarter of the plate with lean proteins and one-quarter with carbohydrates, such as whole grains, starchy vegetables, or black beans.

What is a low GI?

Foods that are assigned a value below 55 are considered low GI.

Which sugar has the highest GI?

Glucose has a GI of 86–97, sucrose has a GI of 59–64, and fructose has a GI of 23. Foods containing higher levels of fructose tend to have a lower GI score.

Summary

The GI can be a useful tool for selecting foods that potentially have less of an impact on your blood sugar. Repeated blood sugar spikes and chronically high blood sugar can lead to various clinical conditions. Therefore, it is important to maintain stable blood sugar levels throughout the day.

Low GI foods may help you maintain a moderate weight and appetite, especially if you pair them with other nutrients. Different proteins, fats, and fibers can alter an individual food’s impact on blood sugar.

However, the GI is not a perfect science. Consider other factors alongside GI when determining what types and quantities of food are best for you. Focusing on overall dietary quality is more important than just the GI. Consult your primary care physician or registered dietitian for more personalized advice on a diet plan best suited for you.

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Medical Reviewer: Jared Meacham, Ph.D., RD, PMP, CSCS
Last Review Date: 2022 Aug 3
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THIS TOOL DOES NOT PROVIDE MEDICAL ADVICE. It is intended for informational purposes only. It is not a substitute for professional medical advice, diagnosis or treatment. Never ignore professional medical advice in seeking treatment because of something you have read on the site. If you think you may have a medical emergency, immediately call your doctor or dial 911.
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