What Is Glycemic Index (GI)?

I. Introduction: Why Is the Glycemic Index (GI) Worth Paying Attention To?

The Problem of Blood Glucose Fluctuations in Modern Diets

In today's world, processed foods, refined carbohydrates, and sugary beverages have become staples of many people's diets. These foods often cause rapid spikes and subsequent crashes in blood glucose levels — a pattern that is increasingly linked to a range of health issues, even in individuals without diabetes.

Long-Term Impacts of Blood Glucose Fluctuations on Health

• Energy levels: Rapid glucose crashes lead to fatigue, brain fog, and sudden drops in energy, affecting productivity and daily functioning.
• Weight management: Blood glucose spikes trigger increased insulin secretion, which promotes fat storage (especially abdominal fat) and intense cravings for high-calorie, sugary foods.
• Metabolic health: Chronic glucose fluctuations can lead to insulin resistance, a precursor to type 2 diabetes, high blood pressure, and dyslipidemia (abnormal cholesterol levels).

The Status of GI in Global Nutrition

The Glycemic Index is not a niche concept — it is widely recognized and used by leading health organizations (including the World Health Organization, WHO) and nutrition experts worldwide. It serves as a critical tool for guiding evidence-based dietary choices to support metabolic health and prevent chronic diseases.

II. What Is the Glycemic Index (GI)?

Official Definition (Plain Language)

The Glycemic Index (GI) is a metric that measures the speed and magnitude at which carbohydrate-containing foods raise blood glucose levels after consumption.

GI Value Range

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

• Glucose (assigned a GI of 100, the fastest-acting carbohydrate)
• White bread (also assigned a GI of 100, used in some regional and clinical studies)

GI Classification

III. How Is the Glycemic Index Measured? (Simplified Version)

Test Subjects

GI testing is conducted on a group of healthy adult volunteers (free of metabolic disorders like diabetes or insulin resistance) to ensure consistent and reliable baseline results.

Testing Principles

• The test food is standardized by 50 grams of digestible carbohydrates (excluding dietary fiber, which is not broken down into glucose).
• The weight of the test food is not fixed — the amount of food needed to provide 50 grams of digestible carbs varies widely between different foods (e.g., a large amount of watermelon vs. a small bowl of rice).

Measurement Process

1. Volunteers fast for 8–12 hours to establish a stable baseline blood glucose level.
2. Volunteers consume the test food (containing 50g of digestible carbs) within 15–30 minutes.
3. Blood glucose levels are measured at regular intervals over a 2-hour period (typically at 0, 15, 30, 45, 60, 90, and 120 minutes).

Core Logic

• Researchers calculate the Area Under the Blood Glucose Response Curve (AUC) for the test food (only the area above the baseline is included).
• The GI value is determined by comparing the test food's AUC to the AUC of the reference food (glucose or white bread), then multiplying by 100 to get a percentage.

IV. Differences Between Low, Medium, and High GI Foods

Characteristics of Low GI Foods

• Slow to digest and absorb in the gastrointestinal tract
• Cause a gentle, steady rise in blood glucose levels with no sharp spikes
• Provide long-lasting energy and satiety

Characteristics of High GI Foods

• Quickly digested and absorbed into the bloodstream
• Cause a rapid, steep spike in blood glucose levels, followed by a sharp drop
• Lead to quick energy bursts followed by fatigue and cravings

Common Food Examples

V. What Factors Affect the GI of a Food?

1. Food Processing and Refinement

Processing breaks down the natural structure of food and removes dietary fiber, making carbohydrates easier to digest and raising the GI value. For example, refined grains (white rice, white bread) have a much higher GI than their whole-grain counterparts (brown rice, whole wheat bread), which retain the fiber-rich bran and germ layers.

2. Cooking Methods

The duration and intensity of cooking directly impact a food's GI. Generally, the longer a food is cooked, the higher its GI. This is because prolonged cooking increases starch gelatinization (the process where starch absorbs water and swells), making it more accessible to digestive enzymes and faster to break down into glucose. For example, overcooked pasta has a higher GI than al dente pasta.

3. Food Structure

• Dietary fiber: Soluble and insoluble fiber slows gastric emptying and carb absorption, lowering the GI (e.g., apples with skin have a lower GI than peeled apples).
• Presence of fat and protein: When carbs are eaten alongside fat or protein, digestion is slowed, which reduces the overall GI of the meal (e.g., rice with chicken and avocado has a lower GI than plain rice).

4. Individual Differences

• Digestion speed: Variations in gut transit time and enzyme activity affect how quickly carbs are broken down.
• Insulin sensitivity: People with higher insulin sensitivity clear glucose from the bloodstream faster, leading to different GI responses compared to those with insulin resistance.

VI. Common Misconceptions About GI

VII. The Difference Between GI and Glycemic Load (GL)

Why GL Is More Useful in Real-Life Diets

GI is a laboratory metric that uses a fixed 50g carb portion, which is often larger than a typical serving. GL, by contrast, accounts for how much of the food you actually eat. For example, watermelon has a high GI (~72) but a low GL (~5) because a normal serving contains very little digestible carbs. This makes GL a more practical tool for planning daily meals and managing blood glucose in real-world scenarios.

VIII. How to Use GI Reasonably in Daily Life

• Focus on overall meal combinations: Prioritize meals that include carbs, protein, healthy fats, and fiber, rather than single foods.
• Control portion sizes: Even low GI carbs will raise blood glucose if eaten in excess. Use appropriate portion sizes for staple foods like rice and pasta.
• Pair carbs with other nutrients: Always pair carbohydrate-rich foods with protein (e.g., chicken, tofu) and healthy fats (e.g., avocado, nuts) to slow digestion.
• Improve meal order: Eat non-starchy vegetables first, then protein/fat, and finally carbs — this can reduce post-meal blood glucose spikes by up to 30%.

Important Reminder: A healthy diet is not about "chasing low GI" foods. Instead, it is about creating a balanced, sustainable, and individualized dietary pattern that aligns with your body's needs, lifestyle, and taste preferences.

References & Data Sources

• Jenkins, D. J., et al. (1981). Glycemic index of foods: a physiological basis for carbohydrate exchange. American Journal of Clinical Nutrition. 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. View on Diabetes Care
• University of Sydney Glycemic Index Research Service (GIS). (2023). GI Database of Common Foods. Access GI Database
• Brand-Miller, J., & Foster-Powell, K. (2022). The New Glucose Revolution. New York: Penguin Random House. View on Amazon