The glycemic index (GI) ranks carbohydrate foods from 0 to 100 by how quickly they raise blood sugar compared to pure glucose. White bread and cornflakes score high; lentils, oats, and most whole fruits score low, because their fiber and structure slow digestion.
The fine print: GI is measured by feeding people a fixed amount of one food, alone — not the mixed meals real people eat. Portion matters too, which is what glycemic load captures: watermelon has a high GI, but a normal slice carries so little carbohydrate that its real-world effect is small. And anything you pair a carb with — protein, fat, fiber — slows the rise. Use GI as a lens, not a rulebook: it mostly points you back to whole, less-processed carbs, which you already knew were the good ones.