What is the Glycemic Index? A Practical Guide
By Dino Pohilj, Founder, RealFoods
How GI is measured
Researchers feed a test subject a fixed 50g dose of carbohydrate from a single food, draw blood every 15-30 minutes for two hours, and plot the area under the blood-sugar curve. They then compare it to the same subject's response to 50g of pure glucose. The ratio, multiplied by 100, is the GI.
The original GI database, established by Jenkins and colleagues in 1981 and expanded by the University of Sydney since, contains thousands of foods. The methodology is rigorous; the conclusions are widely cited in diabetes management.
High, medium, low
- Low GI (≤55): Most legumes, non-starchy vegetables, intact whole grains, most fruits, dairy.
- Medium GI (56-69): Sweet potatoes, basmati rice, whole-wheat bread, pineapple.
- High GI (≥70): White bread, white rice, instant oats, potatoes, watermelon, sugar-sweetened drinks.
Where GI falls short
We want to be direct about the limitations:
- It ignores portion. Watermelon has a high GI but only ~6g of carbohydrate per 100g serving. The actual insulin response is small. Glycemic load (GL = GI × carbs ÷ 100) corrects for this.
- It assumes single-food meals. Real meals combine carbohydrates with protein, fat, and fibre, all of which flatten the glucose curve. A high-GI food eaten alone is very different from the same food eaten with chicken and avocado.
- It varies between individuals. Continuous-glucose-monitor research has demonstrated that the same food produces different glucose curves in different people, sometimes by a factor of two.
- It excludes most foods. Pure protein and fat foods (chicken, eggs, salmon, butter, cheese) have no GI value because they do not raise blood sugar meaningfully on their own. A diet built only on GI ranks would have no information about the largest part of most fat-loss diets.
Ready to stop guessing what to eat?
Glycemic Load: fixing GI's biggest flaw
Glycemic Load (GL) corrects the most glaring GI weakness by accounting for portion size. The formula is: GL = (GI × grams of carbohydrate per serving) ÷ 100. A food with a high GI but low carbohydrate content per serving ends up with a low glycemic load, meaning its actual blood-sugar impact is modest.
| Food | GI | Carbs per 100g | GL per 100g |
|---|---|---|---|
| Watermelon | 72 (high) | 6g | 4 (low) |
| White rice | 73 (high) | 28g | 20 (high) |
| White bread | 75 (high) | 49g | 37 (very high) |
| Carrot | 39 (low) | 10g | 4 (low) |
We argue glycemic load is a meaningful improvement over raw GI, but still incomplete: GL assumes single-food servings and ignores the protein, fat, and fibre that dominate how the body responds to a real meal. Research has established that combining a high-GL food with adequate protein and fat blunts the blood-sugar response by 30-50%, which GL alone cannot capture.
What to use instead
The variable that matters in practice is not the glycemic index of a single food, it is the insulin response to the meal you actually ate. We argue this is what makes the meal-level Weight Impact Score a more practical tool: it scores the entire plate, accounts for protein and fat buffering, and adjusts for the portion you logged.
White rice has a high GI. White rice eaten with chicken, broccoli, and a tablespoon of olive oil is a 4/10 meal. The first number is what GI tells you. The second number is what your body actually does. We argue the second is more useful.
Continue reading: how blood sugar drives weight gain, the Weight Impact Score of 23 popular foods, or how insulin resistance develops and reverses.
