Several factors influence how fast a particular carbohydrate food raises blood sugar. These factors include: the chemical and physical structure of the carbohydrate-food in question; how refined the carb is; how the carb is cooked and also the presence of other substances which reduce either the potency of the body’s digestive enzymes, or the speed of digestion.

Chemical Structure of the Carbohydrate
The body processes glucose very efficiently. (The glycemic index of glucose is 100.) But the body cannot easily metabolize fructose, a common monosaccharide in fruits, which is why fructose has a low GI of 23. Ordinary table sugar (sucrose), is a disaccharide made up of one molecule of glucose linked to one of fructose. Hence the glycemic index of table sugar is 65, midway between 23 and 100 in the medium-glycemic-index range

Physical Structure of the Carbohydrate
The physical structure of the carbohydrate also affects its glycemic index value. For example, most breads are in the high range – not due to the chemical nature of wheat starch, but for two physical reasons.
(1) The fine particle size of wheat flour gives digestive enzymes great surface area to attack and metabolize the bread.
(2) The surface area of bread is also increased by its puffed-out, fluffy structure. Result? The glycemic value of bread is significantly raised by these structural attributes.

How Refined is the Carbohydrate
One of the most important factors that determines the glycemic index of carbohydrate foods, is how refined or processed the carbs are. In general, refined or processed carbs have had most of their ‘natural’ fiber and other ‘inconvenient’ constituents (eg. which may affect the food’s shelf-life) removed. Result? The carbohydrate is incapable of resisting the digestive enzymes and is rapidly metabolized into glucose.

How Carbohydrates are Cooked or Prepared
Pasta has a glycemic-index value of 40-50. This can be further reduced by cooking it less (al dente). This is because al dente pasta resists the effect of digestive enzymes more than regular cooked pasta and so has a lower GI.

Fibre Slows Down Metabolism of Carbs and Their Digestion
Fibre (either in the carbohydrate itself or in the stomach) protects the starchy carbohydrate from rapid attack by digestive enzymes, or slows digestion in the digestive tract. Either of these consequences will slow down the conversion of the carbohydrate to glucose.

Fat and/or Acid Slows Down Metabolism of Carbs and Their Digestion
The more fat or acid a carbohydrate food contains, (or, the more fat or acid in the stomach, during digestion) the slower the carbohydrate food is converted to glucose and absorbed into the bloodstream. The presence of fat and/or acid retards the emptying of the stomach. Example: Vinegar, lemon juice, pickles, sourdough bread, etc., all contribute to lowering the GI of a meal.