Carbohydrate
The macronutrient (starches and sugars) that has the largest direct effect on blood glucose.
Carbohydrates are sugars, starches, and fiber. Apart from fiber, they break down into glucose, which is why they move blood sugar more directly than protein or fat.
How much they move it depends on the amount and type: fiber, fat, and protein in the same meal all slow the rise, the idea captured by glycemic index and load.