Potential problems of using the GI as a dietary tool

The glycaemic index has been criticized on the grounds that values for specific foods are not reproducible from one laboratory to another or in the same individual on different occasions. It has also been suggested that the glycaemic index of a food changes when it is incorporated into meals or diets.

Factors that effect the glycaemic response include particle size, processing, storage, cooking and the presence of fat and protein. For these and other reasons the GI is not a valid, or nutritionally sound tool to use in the formation of diets and eating plans.

Generally speaking the smaller the particle size, the greater the glycaemic response. Soda bread scones made from coarsely milled flour produce smaller glucose and insulin responses than scones made from finer flour milled from the same grist.
The GI of a food is easily changed by actions such as chewing. The more a food is chewed, the greater the glycaemic response.
Similarly the rate of food intake may also play a part with foods consumed quickly initiating a greater GI response than foods consumed slowly.
Fat is known to delay digestion and protein is known to increase insulin production. A given amount of fat may produce different responses depending on whether it has been cooked with the carbohydrate or added after cooking as a spread.
High carbohydrate foods are not consumed in isolation but as part of a mixed diet. Bread is typically eaten with a filling or toping. Add a slice of cheese to white bread and the GI falls.
The cooking and storage of products may effect the GI ranking. When bread cools a starch is produced that is resistant to digestion (resistant starch). Processing can also affect the response as the more highly processed the food is, the higher the GI.