Glycemic Index vs Glycemic Load: What's the Difference?

3 min read

The short answer

Glycemic index (GI) ranks carbohydrate foods by how much they raise blood glucose compared with pure glucose, always using the same amount of carbohydrate. Glycemic load (GL) applies that to the carbohydrate in a real serving. GI tells you how fast. GL tells you how fast and how much.

How GI is measured

Ten or more healthy people eat a portion of the food containing 50 g of available carbohydrate. Their blood glucose response over two hours is compared with their response to 50 g of glucose. A GI of 55 or less is low; 70 or more is high.

How GL is calculated

GL = GI ÷ 100 × grams of available carbohydrate in the serving. Per serving, 10 or less is low, 11 to 19 is medium and 20 or more is high. On FatTonic, available carbohydrate is total carbohydrate minus fiber.

Why the two disagree

GI is fixed for a food; GL moves with the serving. A food with a high GI can have a low GL when a normal serving holds little carbohydrate, and a food with a moderate GI can have a high GL when the serving is large. That is why the portion matters as much as the food.

The limits

Both are averages from laboratory tests. GI differs between varieties and with processing and cooking, for real reasons such as the type of starch. Meat, fish, eggs, tofu, most nuts and seeds, avocado and many vegetables have no GI: they contain too little carbohydrate to test. Neither number predicts one person's response on one day, and neither is a clinical measure.

How FatTonic uses them

Every food in the Food Index shows its GL per serving, the main input to the FatTonic Score. The formula, the source of each input and the downloads are on the data page.