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How Is the Glycemic Index (GI) Calculated?

A Complete Guide to Understanding GI and Its Practical Use

I. Why Should We Understand the Glycemic Index (GI)?

The Impacts of Blood Glucose Fluctuations on the Body

Blood glucose levels that spike and crash rapidly (rather than rising and falling gradually) can have significant negative effects on overall health, including:

  • Increased insulin burden: The pancreas must secrete more insulin quickly to lower sudden blood glucose spikes, which can wear down pancreatic function over time.
  • Higher risk of type 2 diabetes and insulin resistance: Chronic high insulin levels can lead to cells becoming less responsive to insulin, a precursor to type 2 diabetes.
  • Challenges with weight management and persistent hunger: Rapid blood glucose drops often trigger intense cravings for sugary, high-calorie foods, leading to overeating and weight gain.

Who Benefits Most from Understanding GI?

While GI is a useful tool for everyone, it is particularly critical for specific groups of people who have unique metabolic needs:

  • People with diabetes or prediabetes: Managing blood glucose levels is essential for preventing complications, and GI helps them choose foods that avoid dangerous spikes.
  • Individuals focused on fat loss: Low GI foods promote satiety and stable energy, reducing unnecessary snacking and supporting calorie control.
  • Sedentary workers and mental laborers: This group often has lower energy expenditure and higher risk of insulin resistance, making GI a valuable tool for maintaining metabolic health.

Important Precaution: A food's GI value does not equal its overall healthiness. This is a common misconception we will debunk later in this guide.

II. What Is the Glycemic Index (GI)?

Plain-Language Definition

The Glycemic Index (GI) is a standardized metric that measures the speed at which carbohydrate-containing foods raise blood glucose levels after consumption. In other words, it tells us how quickly the carbs in a food are broken down into glucose and absorbed into the bloodstream.

GI Value Range

GI values range from 0 to 100, with a reference food serving as the "benchmark" for comparison. The two most common reference foods are:

  • Glucose: The most widely used reference (assigned a GI of 100, as it is absorbed and raises blood glucose the fastest).
  • White bread: Used as an alternative reference in some countries and regional studies (also assigned a GI of 100).

GI Classification Standards

  • Low GI Foods: ≀ 55 (These foods cause a slow, gradual rise in blood glucose)
  • Medium GI Foods: 56 – 69 (These foods cause a moderate rise in blood glucose)
  • High GI Foods: β‰₯ 70 (These foods cause a rapid, sharp rise in blood glucose)

III. How Is the Glycemic Index (GI) Calculated? (Core Focus)

1. Who Are the Experimental Subjects?

GI testing relies on human subjects to measure real-world blood glucose responses, with strict criteria for participant selection and testing conditions:

  • Typically, 10 to 12 healthy adult volunteers (free of diabetes, insulin resistance, or other metabolic disorders).
  • All tests are conducted in a fasted state (usually 8–12 hours without food) to ensure a consistent baseline blood glucose level.
  • Multiple cross-over experiments are performed (each subject tests all foods, with rest periods in between) to account for individual variation, and the final GI value is the average of all results.

2. What Is the Standard for Test Foods?

Unlike many food tests that use a fixed weight (e.g., 100 grams), GI testing uses a fixed amount of digestible carbohydrates β€” this is a critical distinction:

Key Standard: The test food must contain 50 grams of digestible carbohydrates (excluding fiber, which is not digested into glucose).

Example: 50 grams of digestible carbs is not the same as 50 grams of food. To get 50 grams of carbs from watermelon (which is ~92% water), you would need to eat over 700 grams of watermelon. In contrast, you only need ~150 grams of cooked white rice to get 50 grams of digestible carbs.

3. Simplified Experimental Process

  1. Measure the subject's baseline blood glucose level after an overnight fast (this is the starting point for comparison).
  2. The subject consumes the test food (containing exactly 50 grams of digestible carbohydrates) within a short time frame (usually 15–30 minutes).
  3. Blood glucose levels are measured at specific time intervals: 0, 15, 30, 45, 60, 90, and 120 minutes after eating the test food.
  4. Record all blood glucose values and plot them on a graph to create a blood glucose response curve (showing how glucose levels change over 2 hours).

4. Key: Area Under the Blood Glucose Curve (AUC)

To quantify the total blood glucose response to the test food, researchers calculate the Area Under the Curve (AUC) β€” but with an important caveat:

  • Only the area above the fasting baseline blood glucose level is included in the calculation.
  • Any portion of the curve where blood glucose drops below the baseline (negative values) is ignored, as it does not contribute to the food's glucose-raising effect.
  • The resulting value is the blood glucose response AUC for the test food, which represents the total magnitude of glucose elevation over 2 hours.

5. Mathematical Formula for GI

The GI value is calculated as a percentage of the test food's AUC compared to the reference food's AUC:

GI = (AUC of Test Food Γ· AUC of Reference Food) Γ— 100

For example, if a bowl of oats has an AUC of 250, and glucose (the reference) has an AUC of 500, the GI of oats would be (250 Γ· 500) Γ— 100 = 50 (a low GI food).

6. Plain-Language Analogy

To better understand GI's focus on "speed" rather than "height", consider this analogy:

  • Glucose (GI=100): Imagine blood glucose levels shooting straight to the roof like an elevator β€” fast, steep, and rapid.
  • Low GI foods (e.g., oats, broccoli): Imagine blood glucose levels climbing slowly uphill like a hiker β€” gradual, steady, and without a sharp spike.
  • Core Truth: GI measures how quickly blood glucose rises, not how high it rises overall. The total glucose elevation (height) is primarily determined by the total amount of carbs consumed, not the GI value.

IV. Why Does the GI of the Same Food Vary?

1. Food Processing Method

Processing breaks down food structure and fiber, making carbs easier to digest and raising GI:

  • Refined grains (white rice, white bread) have higher GI than whole grains (brown rice, whole wheat bread).
  • GI ranking: Porridge ＞ Cooked rice ＞ Cold cooked rice (cold rice forms resistant starch, which lowers GI).
  • Juiced fruit has higher GI than whole fruit (juicing removes fiber and breaks down cell walls).

2. Cooking Method

Cooking time and temperature affect starch gelatinization, which impacts GI:

  • The longer and more thoroughly food is cooked (softer texture), the higher its GI.
  • Higher gelatinization degree (starch absorbs water and swells) β†’ Easier digestion β†’ Higher GI.
  • Example: Overcooked pasta has a higher GI than al dente pasta.

3. Food Structure

Components that slow carb digestion will lower a food's GI:

  • High fiber content (soluble and insoluble fiber) slows gastric emptying and carb absorption.
  • Concurrent intake of fat and protein (e.g., rice with chicken and avocado) slows down digestion and lowers the overall GI of the meal.
  • Resistant starch (found in cold rice, legumes) is not digested in the small intestine, so it has a lower GI impact.

4. Individual Differences

Each person's body processes food differently, leading to variable GI responses:

  • Intestinal microbiota composition (different bacteria affect carb fermentation and absorption).
  • Insulin sensitivity (people with insulin resistance may have slower glucose clearance, altering their response).
  • Meal consumption order (eating veggies/protein first slows carb absorption compared to eating carbs first).

V. Common Misconceptions About GI

❌ Misconception 1: Low GI foods can be eaten in unlimited quantities

Many low GI foods are high in calories, fat, or non-digestible carbs. For example, dark chocolate (GI ~25) and nuts (GI ~15) are low GI but calorie-dense β€” overeating them will still lead to weight gain and metabolic stress, even if they don't spike blood glucose.

❌ Misconception 2: High GI foods are always unhealthy

High GI does not equal poor nutritional value. Watermelon (GI ~72) is high in vitamins A and C and hydration; potatoes (GI ~80) are rich in potassium and vitamin B6. These foods can be part of a healthy diet when consumed in appropriate portions and combined with other foods.

❌ Misconception 3: GI determines everything about blood glucose

GI only measures the speed of carb digestion, not the total carb content of a serving. This is where Glycemic Load (GL) comes in β€” GL combines GI with portion size to provide a more accurate picture of how a food affects blood glucose in real-life consumption.

VI. Difference Between GI and GL (Advanced but Important)

Core Differences

Glycemic Index (GI)

Measures the speed of blood glucose rise from a food with 50g of digestible carbs (portion size independent).

Glycemic Load (GL)

Measures the total impact of a specific serving of food on blood glucose (speed Γ— portion size).

GL Calculation Formula

GL = (GI Γ— Grams of Digestible Carbs in a Serving) Γ· 100

Why GL Is More Practical in Real Diets

GI is a laboratory metric that uses a fixed 50g carb portion, which is often larger than a typical serving size. GL, on the other hand, accounts for how much of the food you actually eat. For example:

Watermelon has a high GI (~72), but a 100g serving only contains ~7g of digestible carbs. Its GL is (72 Γ— 7) Γ· 100 β‰ˆ 5 (a low GL), meaning it has a minimal impact on blood glucose when eaten in a normal portion. This makes GL far more useful for planning daily meals and managing blood glucose in real life.

VII. How to "Correctly Use GI" in Daily Diet

  • Refer to GI, but don't worship it: Use GI as a guide, not a strict rule. Prioritize whole, unprocessed foods over relying solely on GI values.
  • Control total carbs + optimize food combinations: Even low GI carbs will raise blood glucose if eaten in excess. Pair carbs with fiber, fat, and protein to slow digestion (e.g., quinoa with roasted veggies and grilled chicken).
  • Understand "low GI diet" correctly: A low GI diet is not about eliminating all high GI foods β€” it’s about prioritizing low to medium GI foods and balancing high GI foods with other nutrients and appropriate portions.

Practical Examples for Daily Meals

  • Meal combination: Cooked brown rice (medium GI) + steamed broccoli (low GI) + baked salmon (protein/fat) β€” this combination slows carb absorption and stabilizes blood glucose.
  • Meal order: Eat vegetables first β†’ then protein/fat β†’ finally staple carbs (rice, pasta, bread). This order delays gastric emptying and reduces post-meal blood glucose spikes by up to 30% in some studies.

VIII. A Final Summary for Ordinary People

The Glycemic Index (GI) is a valuable laboratory-derived indicator, but it is not a one-size-fits-all solution for managing blood glucose or diet. The actual blood glucose response to any food is the product of four key factors:

Food Γ— Portion Size Γ— Cooking Method Γ— Individual Differences

Ultimately, managing blood glucose and maintaining metabolic health is not about "picking the right single foods" based on GI alone. It is about designing a balanced, sustainable dietary structure that aligns with your body's needs, preferences, and lifestyle.

References & Data Sources

  • Jenkins, D. J., Wolever, T. M., Taylor, R. H., et al. (1981). Glycemic index of foods: a physiological basis for carbohydrate exchange. American Journal of Clinical Nutrition, 34(3), 362-366. View on PubMed
  • World Health Organization (WHO). (1999). Diet, Nutrition and the Prevention of Chronic Diseases. Geneva: WHO Press. View on WHO Website
  • American Diabetes Association. (2024). Glycemic Index and Glycemic Load for Diabetes Management. Diabetes Care, 47(Supplement 1), S151-S160. View on Diabetes Care
  • University of Sydney Glycemic Index Research Service (GIS). (2023). GI Database of Common Foods (6th Edition). Sydney: University of Sydney Press. Access GI Database
  • Monash University. (2024). Evidence-Based Guide to GI and GL in Daily Clinical Practice. Melbourne: Monash University Publishing. View Monash Nutrition Research
  • Brand-Miller, J., & Foster-Powell, K. (2022). The New Glucose Revolution: The Authoritative Guide to the Glycemic Index. New York: Penguin Random House. View on Amazon