The Glycaemic Index: What It Measures and When It's Useful

How the Glycaemic Index Is Measured

The GI of a food is determined in a laboratory setting. Volunteers consume a portion of the test food containing exactly 50 grams of available carbohydrate, then researchers track blood glucose levels over two hours. The resulting curve is compared against the response to pure glucose (GI = 100). The average score across multiple participants becomes that food's GI rating.

This controlled process reveals something genuinely useful: carbohydrates are not metabolically identical. Lentils, white rice, and a candy bar all contain carbohydrates, but they behave very differently in the bloodstream. GI gives researchers and clinicians a standardized way to describe those differences.

For a broader look at how nutrients are described on packaged foods, see our guide on reading a nutrition label without getting lost.

Why Real-World GI Is More Complicated

Laboratory conditions rarely match how people actually eat. Several factors shift a food's blood sugar impact significantly:

• Cooking and processing: Al dente pasta has a lower GI than overcooked pasta. Rolled oats have a lower GI than instant oats.
• Ripeness: A ripe banana has a higher GI than a green one, because starches convert to sugars as fruit matures.
• Food combinations: Eating fat, protein, or fiber alongside carbohydrates slows digestion, lowering the overall blood sugar response — even if the carbohydrate source has a high GI on its own.
• Individual variation: Gut microbiome composition, metabolic health, and even stress levels affect how each person responds to the same food.

This is why GI rankings, while informative, can feel disconnected from real eating patterns. A chocolate bar may technically have a lower GI than some fruit — because its fat content slows glucose absorption — yet that comparison tells you nothing about the nutritional quality of either food.

Glycaemic Load: A More Complete Picture

Because GI is measured using a fixed 50-gram carbohydrate portion, it can distort comparisons for foods eaten in small amounts. Watermelon, for example, has a high GI score — but a typical serving contains relatively few carbohydrates, so its actual blood sugar impact is modest. This is where glycaemic load (GL) becomes valuable.

GL is calculated by multiplying the food's GI by the grams of carbohydrate in a typical serving, then dividing by 100. A GL of 10 or below is considered low; 20 or above is high. Using GL alongside GI gives a much more grounded sense of how a meal is likely to affect blood sugar.

Understanding how different sugars fit into this picture is equally important. Our article on sugar in food: separating the science from the noise explores how added and natural sugars differ and what the research actually shows.

When the Glycaemic Index Is — and Isn't — Useful

GI has legitimate clinical applications. People managing type 2 diabetes or insulin resistance may find it a helpful framework for comparing carbohydrate choices within an otherwise balanced diet. Research has shown that consistently choosing lower-GI foods can help with blood glucose stability, though it is most effective as part of a broader dietary approach rather than a standalone strategy.

Where GI falls short is as a general guide to healthy eating. It says nothing about protein, fat, fiber, vitamins, or minerals. A food's GI score does not tell you whether it supports long-term health — only how quickly it raises blood sugar under controlled conditions. Dietary patterns, portion sizes, and food variety matter far more than any single ranking system.

This article is for general informational and educational purposes only. It is not a substitute for professional medical or dietary advice. If you have diabetes, blood sugar concerns, or any related health condition, please consult a qualified healthcare provider before making changes to your diet.