Glycemic index and glycemic load, explained with everyday foods
The glycemic index (GI) ranks carbohydrate foods from 0 to 100 by how sharply a fixed 50 g carbohydrate dose raises blood glucose, with pure glucose scoring 100. Foods at 55 or under count as low GI, 70 or over as high. Useful, but incomplete: GI ignores how much carbohydrate a real portion contains. Glycemic load (GL) fixes that by multiplying GI by the carbs actually on your plate, and it flips several famous verdicts. Watermelon scores a scary GI around 76 yet a harmless GL of about 5 per slice, because a slice is mostly water.
Speed rating of the carbohydrate itself.
GI times the carbs in a real portion, divided by 100.
GL under 10 is low, over 20 is high.
The watermelon paradox, with real numbers
To test watermelon at its published GI, volunteers had to eat 50 g of its carbohydrate, which means roughly 700 g of fruit. Nobody eats a meal-sized dose that way. A normal 300 g portion carries about 21 g of carbohydrate, giving a glycemic load of only 5, firmly in the low range. Meanwhile white bread, GI around 75, packs 52 g of carbohydrate per 100 g, so two slices reach a GL near 17. Same index, completely different real-world impact. Whenever GI makes a watery fruit or vegetable look dangerous, the portion arithmetic is missing.
| Food (Dietly label) | Typical GI | Carbs per portion | Glycemic load |
|---|---|---|---|
| Watermelon, 300 g portion | ~76 (high) | 21 g | ~5 (low) |
| White bread, 2 slices (92 g) | ~75 (high) | 48 g | ~17 (medium-high) |
| Basmati rice, 150 g cooked | ~52 (low) | 42 g | ~14 (medium) |
| Orange juice, 250 ml | ~50 (low) | 28 g | ~14 (medium) |
| Cooked lentils, 150 g | ~32 (low) | 11 g | ~4 (low) |
What changes a food's GI in real life
GI is measured on single foods eaten alone after fasting, which is not how anyone eats. In a mixed meal, fat and protein slow stomach emptying, fibre blunts absorption, and acidity (vinegar, sourdough fermentation) lowers the response further. Cooking matters too: pasta cooked al dente scores lower than soft, and potatoes cooked then cooled form resistant starch that digests more slowly. Even the same person retested on the same food can vary by 20 points or more. So treat published GI numbers as rough categories, not constants. The low GI of juice is also a good reminder that GI does not measure healthfulness: orange juice scores "low" while delivering 28 g of fast-drinking sugar per glass, a trade-off covered in is fruit too high in sugar.
Who should actually care
For people with diabetes or impaired glucose control, low-GI and low-GL eating patterns produce modest but real improvements in HbA1c in randomised trials, and the concept is endorsed as one tool among several by diabetes organisations. If that is you, it is worth learning alongside professional guidance rather than instead of it.
For everyone else, the honest reading of the evidence is that GI adds little once diet quality is accounted for. The foods that low-GI advice pushes you toward, such as legumes like lentils, intact grains, vegetables and dairy, are the same foods every other framework recommends, largely because their fibre and protein make them filling, as explained in the satiety explainer. If choosing between two similar carbs, check their fibre and sugar per 100 g in the comparison tool; that comparison captures most of what GI would tell you.
Bottom line
GI rates the carbohydrate; GL rates the portion; only GL describes your actual meal. High-GI watery foods like watermelon are fine, low-GI liquids like juice are not automatically virtuous, and mixed meals blur everything toward the middle. Use GL if you manage blood glucose; use fibre, protein and portion awareness if you do not.
Sources
Common questions
What is the glycemic index?
The glycemic index (GI) ranks carbohydrate foods from 0 to 100 by how much a 50 g carbohydrate dose raises blood glucose over two hours, compared with pure glucose at 100. Low is 55 or under, high is 70 or over.
What is the difference between glycemic index and glycemic load?
GI describes the quality of the carbohydrate; glycemic load (GL) multiplies GI by the carbs in an actual portion. Watermelon has a high GI near 76 but a low GL of about 5 per slice because a portion holds little carbohydrate.
Should healthy people track GI?
Mostly no. Mixed meals, fibre, fat and protein change the response, and overall diet quality predicts health better. GI is most useful for people managing diabetes, alongside professional advice.