The glycemic index was developed in the early 1980s by Dr David Jenkins at the University of Toronto as a tool for managing blood sugar in people with diabetes. It measures one thing: how fast a specific food raises blood glucose compared to pure glucose.
Pure glucose is the benchmark - it scores 100. Everything else is measured relative to it.
How GI Is Measured
Test subjects eat a fixed amount of a food containing 50g of available carbohydrates. Blood glucose is measured at regular intervals for 2 hours. The area under the resulting blood sugar curve is compared to the curve produced by 50g of pure glucose in the same person.
The result is a GI value - a percentage of the glucose response. GI 70 means the food produces 70% of the blood sugar response of pure glucose.
What the Numbers Mean
| GI Range | Category | Examples |
|---|---|---|
| 55 or below | Low GI | Most vegetables, legumes, oats, whole grain pasta, most fruit |
| 56-69 | Medium GI | Brown rice, sweet potato, pineapple, popcorn, whole grain bread |
| 70 and above | High GI | White bread, white rice, watermelon, rice cakes, most breakfast cereals, sugary drinks |
Low GI foods digest more slowly, produce a gentler blood sugar curve, and are generally more filling. High GI foods digest quickly, spike blood sugar fast, and tend to produce a rapid energy crash afterward.
GI vs Glycemic Load: The More Useful Number
GI has a significant limitation: it doesn't account for how much of a food you actually eat.
Watermelon has a GI of 72 - technically high. But a typical portion of watermelon contains very little carbohydrate, because it's mostly water. Eating a 200g slice only delivers about 12g of carbohydrate. The actual blood sugar impact is minimal.
Glycemic load (GL) fixes this by multiplying GI by the carbohydrate content of a realistic portion:
Glycemic Load = (GI x grams of carbohydrate) ÷ 100
For watermelon: (72 x 12) ÷ 100 = GL of 8.6 (low, despite high GI)
For white bread (2 slices): (75 x 24) ÷ 100 = GL of 18 (high)
GL below 10 is considered low. 11-19 is medium. 20+ is high.
Glycemic load is more accurate for predicting real-world blood sugar impact because it accounts for portion size.
Why GI Isn't the Full Picture
Several other factors influence a food's actual blood sugar impact:
Food combinations. Eating a high-GI food alongside fat, protein, or fibre significantly lowers the overall glycaemic impact of the meal. White rice with chicken and vegetables behaves differently than white rice alone.
Preparation method. Cooking and cooling starchy foods (like rice and potatoes) increases their resistant starch content, lowering their effective GI. Overcooked pasta has a higher GI than al dente pasta.
Individual variation. A landmark 2015 study from the Weizmann Institute found that blood sugar responses to identical foods varied enormously between individuals, driven by gut microbiome composition, genetics, and lifestyle factors.
How to Use GI Practically
You don't need to memorise GI values for every food. The practical takeaway:
- Base meals on low-to-medium GI foods where possible (vegetables, legumes, whole grains)
- Never eat high-GI foods alone - pair them with protein, fat, or fibre
- Don't make GI the only lens - food quality, fibre content, and overall diet pattern matter more than individual scores
See the full guide to foods that spike blood sugar for a practical breakdown.