What is the glycaemic index, and does it matter?
The glycaemic index (GI) ranks carbohydrate-containing foods by how quickly and how much they raise blood glucose, compared with a reference — usually pure glucose at 100.
How it is measured. Volunteers eat a portion containing a fixed amount of available carbohydrate — typically 50g — and blood glucose is measured over two hours. The area under the curve is compared with the reference.
The classification is conventionally low (55 or below), medium, and high (70 or above).
The immediate problem: GI ignores portion size. Watermelon has a high GI but contains little carbohydrate per serving, so a normal portion barely affects blood glucose. This makes the raw index misleading for real eating.
Glycaemic load (GL) corrects for this by accounting for the actual carbohydrate in a typical serving. GL is the more useful figure, and the one worth looking at if you look at either.
What changes a food's GI:
Processing and particle size. Finely milled flour raises glucose faster than intact grains. This is why bread made from wholemeal flour can have a surprisingly high GI.
Ripeness — a ripe banana is higher than a green one.
Cooking time, with overcooked pasta higher than al dente.
Cooling. Cooked and cooled potatoes, rice and pasta form resistant starch, lowering the response.
What it is eaten with. Fat, protein, fibre and acidity all slow gastric emptying and blunt the response — which is the practical point, because almost nobody eats a food alone.
Where GI genuinely matters: managing diabetes, where glucose response is clinically relevant; sports nutrition, where fast absorption is sometimes wanted and sometimes not; and reactive hypoglycaemia.
Where it matters less than claimed: for general weight management in people without glucose regulation problems, the evidence for low-GI diets outperforming others of equal calories is weak. Satiety and diet quality explain most of any observed benefit.
The large caveat: individual responses vary enormously. Research using continuous glucose monitoring has found people respond very differently to identical meals, which undermines the idea of a universal index.
General information, not medical or dietary advice — anyone managing diabetes should follow clinical guidance.