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The Complete Guide to Glycemic Index: What It Is, How It's Measured, and How to Use It

By the GlycemicWise Data Editorial Team · Published 2026-09-03 · Last reviewed 2026-09-03 · Nutrition Science · 6-8 min read · Dietary reference only

Everything you need to know about the glycemic index — from lab measurement to practical application for diabetes and weight management.

How GI Is Actually Measured

The glycemic index is determined in a clinical laboratory using human participants. Here's the protocol:

1. Ten or more healthy volunteers fast overnight. 2. They consume a test food containing exactly 50 grams of available carbohydrate. 3. Blood samples are taken at 0, 15, 30, 45, 60, 90, and 120 minutes after eating. 4. The area under the blood glucose curve (AUC) is calculated. 5. The same participants repeat the test with 50g of pure glucose (the reference, GI = 100). 6. GI = (AUC of test food ÷ AUC of glucose) × 100.

This is an ISO-standardized methodology (ISO 26642:2010). When GI is measured this way, it is classified as Tier 1 on GlycemicWise — the highest level of data quality. When GI is estimated from category averages rather than directly measured, it is classified as Tier 3 and clearly marked as "ESTIMATED."

Practical GI Use

Despite its limitations (the 50g carb serving issue), GI remains useful for food comparison. Within a food category, lower-GI options generally produce better glycemic responses. Brown rice (GI ~50) vs white rice (GI ~67); whole wheat bread (GI ~55) vs white bread (GI ~75); steel-cut oats (GI ~42) vs instant oats (GI ~65).

The key is to use GI alongside GL — GI tells you which foods to prefer within a category (lower is better). GL tells you whether a specific serving of a specific food will affect your blood sugar. Both numbers are available on every GlycemicWise food page, always with data source transparency.

Dietary reference — not medical advice. Information provided for educational purposes.