How is GI calculated? A Guide to the Glycemic Index

November 30, 2025 •

4 min read

The concept of the glycemic index (GI) was first introduced in 1981 by Dr. David J. Jenkins and his colleagues, providing a new way to rank carbohydrate foods based on their effect on blood glucose levels. Understanding exactly how is GI calculated is crucial for anyone looking to use this metric to manage blood sugar or make informed dietary choices.

Quick Summary

The glycemic index is calculated by comparing a food's blood glucose impact to a reference food, like pure glucose. The process involves testing volunteers and measuring the incremental area under their blood glucose curve, and various factors can influence a food's final GI rating.

Key Points

Scientific Basis: GI is calculated in controlled laboratory studies involving human volunteers who consume specific amounts of a test food and a reference food (glucose).
Incremental Area Under the Curve (iAUC): The core of the GI calculation involves measuring the area under the blood glucose curve after consuming the food, known as the iAUC.
Influencing Factors: Many variables can alter a food's GI, including cooking time, processing, ripeness, and the other nutrients present in a meal.
GI vs. GL: While GI measures the quality of carbohydrates, Glycemic Load (GL) provides a more complete picture by factoring in the typical quantity consumed per serving.
Practical Application: For real-world dietary planning, considering both a food's GI and its GL provides a more accurate understanding of its impact on blood sugar.
Meal Balancing: To manage glycemic response, combine carbohydrates with protein and fats, as these macronutrients slow down glucose absorption.

The Scientific Methodology: How is GI Calculated?

The glycemic index (GI) is not a theoretical value but is determined through a standardized, scientific testing process involving human volunteers. This method relies on comparing the blood glucose response of a test food to a reference food, typically pure glucose. The core of the calculation is the 'incremental area under the blood glucose response curve' (iAUC).

The In Vivo Testing Protocol

To determine a food's GI, the following steps are taken in a controlled laboratory setting:

Assemble a Volunteer Group: At least 10 healthy, fasting volunteers are recruited for the study.
Test the Reference Food: On one day, each volunteer consumes a portion of pure glucose containing exactly 50 grams of available carbohydrates. Capillary blood samples are then collected at specific intervals over the next two hours to measure blood glucose levels.
Calculate the Reference iAUC: The blood glucose measurements are plotted on a graph over time. The incremental area under this curve (iAUC) is then calculated, ignoring any area below the fasting level. This is often done using the trapezoidal rule.
Test the Target Food: On a separate day, after fasting again, each volunteer consumes a portion of the test food that also contains 50 grams of available carbohydrates. The same blood sampling procedure is repeated.
Calculate the Test Food iAUC: The iAUC for the test food is calculated in the same manner as the reference food.
Compute Individual GI: For each volunteer, the GI is calculated using the following formula: GI = (Test Food iAUC / Reference Food iAUC) x 100.
Determine the Final GI Value: The final GI for the food is the average of the individual GI values from all volunteers.

This robust, repeated testing protocol in the same individuals is necessary to account for day-to-day and individual variations in glucose responses.

Factors Influencing GI Calculation and Values

The GI of a food is not static and can be significantly altered by a variety of factors. This is why a standardized protocol is essential, but also why published GI values can serve as a guide rather than an absolute truth.

Food Processing: Highly processed foods often have a higher GI. For example, milling grains into flour removes fiber and makes the starches more accessible to digestive enzymes, speeding up glucose absorption.
Cooking Method: The way a food is prepared changes its physical structure. Cooking and softening food, like mashing potatoes versus eating them whole, can increase its GI. Prolonged cooking time, as with pasta, also tends to raise the GI.
Ripeness: The ripeness of fruits and vegetables affects their GI. An unripe banana, for example, contains resistant starch and has a lower GI than a ripe, soft banana.
Presence of Other Nutrients: Including protein, fat, and dietary fiber in a meal can slow the absorption of carbohydrates. This means eating a carbohydrate food with other macros will result in a lower glycemic response than eating it alone. Soluble fiber, in particular, slows down glucose absorption.
Varietal Differences: The specific variety of a food can impact its GI. Different types of rice, for instance, have varying GI values.

Comparison: Glycemic Index vs. Glycemic Load

While GI is a useful measure of carbohydrate quality, it doesn't account for the quantity of carbohydrates consumed in a typical serving. The concept of Glycemic Load (GL) addresses this limitation by combining a food's GI with its carbohydrate content per serving.


Feature	Glycemic Index (GI)	Glycemic Load (GL)
Measurement	A ranking of how quickly carbs raise blood glucose, independent of portion size.	A number estimating a food's total impact on blood glucose, considering portion size.
Calculation	(iAUC of Test Food / iAUC of Reference Food) x 100.	(GI x available carbohydrate (g) per serving) / 100.
Classification	Low (≤ 55), Medium (56–69), High (≥ 70).	Low (≤ 10), Medium (11–19), High (≥ 20).
Best For	Comparing the carbohydrate quality of different foods.	Assessing the realistic impact of a mixed meal or typical food portion on blood glucose.
Example	Watermelon has a high GI (76).	A typical serving of watermelon has a low GL (8) because of its low carbohydrate density.

Why Glycemic Load Provides a More Comprehensive Picture

GL is often considered a more practical tool for everyday diet planning than GI alone. Because it accounts for portion size, it provides a better estimate of a food's overall impact on your blood sugar. A high-GI food with a small serving size and low carbohydrate count, like carrots, may have a low GL and a modest effect on blood sugar. Conversely, a large portion of a low-GI food, like pasta, could have a high GL and still cause a significant blood sugar increase.

For most people, paying attention to both GI and GL offers a more complete understanding of how food affects their glucose response. The ultimate goal is to balance meals, combining carbohydrates with protein and healthy fats to moderate the glycemic response. Authoritative resources, such as the Linus Pauling Institute's guide on Glycemic Index and Glycemic Load, offer extensive information on the topic.

Conclusion: Navigating GI for Better Health

In conclusion, the glycemic index is a scientifically derived measurement based on human trials comparing a food's effect on blood sugar against a pure glucose reference. The calculation is complex, relying on the incremental area under the blood glucose curve. However, GI is not the full story. Factors like cooking, processing, and the presence of other nutrients all influence the actual glycemic response. This is where the glycemic load, a more practical measure incorporating portion size, becomes a valuable tool. By understanding both how is GI calculated and the distinction between GI and GL, individuals can make more informed and beneficial food choices for managing blood sugar and promoting long-term health.

Frequently Asked Questions

The glycemic index (GI) measures how quickly a carbohydrate food raises blood sugar, independent of portion size. Glycemic load (GL) provides a more practical measure by also factoring in the amount of carbohydrates in a typical serving of that food.

Not necessarily. A food can have a low GI but a medium or high GL if a typical serving contains a large amount of carbohydrates. Conversely, a high GI food like watermelon can have a low GL due to its low carbohydrate density per serving.

GI values are based on the average glycemic response of a group of healthy individuals to create a standard ranking system. While individual responses can vary, testing in healthy populations establishes a consistent baseline.

When consumed with carbohydrates, fats and protein slow down digestion and the rate of glucose absorption into the bloodstream. This typically lowers the overall glycemic response of the meal compared to eating the carbohydrate alone.

The iAUC is a metric used in the scientific calculation of GI. It quantifies the total increase in blood glucose over a two-hour period after eating, specifically the area above the fasting blood glucose level.

Yes, factors like cooking method and processing have a significant impact. For example, cooking can break down starch molecules, making them easier to digest and raising the food's GI. Highly processed grains often have a higher GI than their whole-grain counterparts.

While GI is primarily tested on individual foods, a meal's GI can be estimated using a formula based on the GI values of its components. However, this estimation can be inaccurate because protein and fat can significantly alter the glycemic response in mixed meals.