Glycemic Index

Glycemic index (GI) is a numerical scale from 0 to 100 that ranks carbohydrate-containing foods by how quickly and how high they raise blood glucose in the two hours after eating them. Pure glucose is the reference at 100; white bread is often used as an alternate reference at the same 100 value.

How GI is measured

The standard protocol: 10 fasted subjects eat a test food containing 50 g of available carbohydrate. Blood glucose is measured at standard intervals for 2 hours. The area under the resulting glucose curve (iAUC) is compared to the same person's iAUC after 50 g of pure glucose. Average across subjects gives the GI value for that food.

The GI scale

• Low GI (< 55): most non-starchy vegetables, legumes, most fruits, whole oats, most dairy.
• Medium GI (55–69): whole-grain bread, brown rice, sweet potato, banana.
• High GI (≥ 70): white bread, white rice, instant oats, processed breakfast cereals, jelly beans.

Why real-meal GI diverges from the lab number

GI is measured on isolated foods eaten in a fasted state. Real meals include:

• Fat — slows gastric emptying, reduces post-meal glucose spike.
• Protein — similar effect, plus modest insulin response.
• Fibre — particularly soluble, reduces glucose absorption rate.
• Food form — al-dente pasta has substantially lower GI than overcooked pasta. Whole fruit has lower GI than juiced fruit.
• Ripeness — a ripe banana has higher GI than a green one.

So the "GI of X" number from a published table is a ceiling estimate for that food eaten in isolation. In a mixed meal, the effective GI is usually lower.

Does GI matter for weight and body composition?

Evidence is mixed. Low-GI dietary patterns correlate with improved satiety per calorie in some studies, with no effect in others. For body-composition outcomes in calorie-controlled diets, GI alone is a weak predictor. Total calories, protein intake, and food-environment variables (palatability, processing, energy density) consistently outperform GI as explanatory factors.

Where GI is genuinely useful

• Type 1 and type 2 diabetes management — matching carbohydrate timing and composition to insulin dose or endogenous insulin response.
• Pre-exercise nutrition — low-GI pre-workout for sustained energy, higher-GI during and post-workout for rapid glycogen repletion.
• Continuous glucose monitor (CGM) n=1 tracking — users with CGMs often find that their personal glucose response to specific foods diverges from published GI tables. Personal data beats population averages here.

GI vs glycemic load

GI is a percentage-based comparison; it does not factor in serving size. A high-GI food eaten in a small portion raises glucose less than a medium-GI food eaten in a large portion. Glycemic load (GL = GI × grams of available carbohydrate per serving ÷ 100) corrects for portion and is the more practical metric for real-world dietary assessment.