- Because GI can only be measured by feeding volunteers enough of a food that it provides them exactly 50 grams of carbohydrate, and because that carbohydrate must be consumed rapidly, foods that have very low carbohydrate densities cannot be accurately measured--one would have to consume pounds of beef to get 50 grams of carbohydrate (for example).
- Glycemic Index is intended to be a parameter that is independent of the amount of carbohydrate ingested (glycemic load), but in the real world, both matter. For example, a single teaspoon of sugar with a high protein breakfast is unlikely to have much effect upon blood glucose whereas a plateful of sweet potatoes likely would even though the former carbohydrate is high GI and latter only moderate.
- GI is measured by feeding fasting volunteers a pure food alone. It is not at all clear how other foods affect the overall effect upon glucose levels in the blood.