Several factors influence how fast a particular carbohydrate food raises blood glucose levels. These factors include: the chemical and physical structure of the carbohydrate-food in question, how refined the carbohydrate is, how the carbohydrate 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.
How Refined is the Carbohydrate
One of the most important factors that determine the GI of carbohydrate foods is how refined or processed the carbohydrates are. In general, refined or processed carbohydrates have had most of their ‘natural’ fibre removed. The carbohydrate becomes incapable of resisting the digestive enzymes and is therefore rapidly metabolized into glucose.
Chemical Structure of the Carbohydrate
The body processes glucose very efficiently. (The GI 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 the GI value. For example, most breads are in the high GI 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 greater surface area to attack and metabolize the bread.
(2) The surface area of bread is also increased by its puffed-out, fluffy structure, particularly in white bread. This results in GI of white bread being significantly raised by these structural attributes.
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 and has a lower GI. However, cooking pasta for longer accelerates starch gelatinisation, increasing the GI.
Fibre Slows Down Metabolism of Carbohydrates 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. For example: adding vinegar, lemon juice, pickles, will help to lower the GI of a meal. Fermenting foods or the sourdough method of baking bread also lower the GI.
What about the GI of mixed meals and the effect of extra protein and fat in the food on GI and blood glucose response?
Eaten alone, protein and fat have little effect on blood glucose levels, but that’s not to say they don’t affect your blood glucose response when they are combined with a carb-rich food. Protein will stimulate additional insulin secretion, resulting in lower blood glucose levels. Protein and fat both tend to delay stomach emptying, thereby slowing the rate at which carbohydrate can be digested and absorbed. So a high fat meal will have a lower glycemic effect than a low fat meal even if they both contain the same amount and type of carbohydrate.
A low GI diet is not a fad diet but a way of eating that is sustainable in the long term and backed by over 35 years of scientific evidence. Consuming good quality low GI carbohydrates helps to facilitate the management of diabetes and reducing the risk of developing type 2 diabetes, diabetes complications and other chronic lifestyle diseases. Following a low GI diet can also assist with weight loss and importantly healthy weight maintenance. In fact, a low GI diet provides health benefits for everybody across all stages of life.