Carbohydrates: Digestion and its Absorption

A small amount of digestion of cooked starch commences in the mouth where there is a salivary amylase,but carbohydrates are mainly digested in the small intestine by enzymes (alphamylase) secreted by the pancreas and also present in the wall of the intestine.

The resultant monosaccharides are absorbed into the bloodstream and carbohydrate absorption can be tracked by measuring blood glucose levels at intervals after a meal.The blood glucose level rises rapidly for the first 30 minutes or so after a meal and returns to baseline in about 2 hours as it is taken into tissues under the influence of insulin and other hormones. It is now recognized that the rate of rise in blood glucose and subsequent fall to baseline levels (glycaemic response) is not the same for different sources of carbohydrates.Many factors influence this; these include:

(a) whether the carbohydrate is given as a simple solution such as glucose,or in a more complex         food form.

(b) the relative proportions of amylose and amylopectin in a starchy food...

(c) the presence of non-starch polysaccha rides in the food.
(d) the form of the food, e.g. finely ground versus large particles of grain.
(e) the cooking method.
(f) the presence of other nutrients in the food, e.g. fat.

Extensive trials have compared the glycaemic response to different carbohydrate sources, and the concept of the glycaemic index has been developed. The glycaemic index predicts the rate at which blood glucose will rise after a particular food compared with the rate at which it would rise after an equivalent amount of white bread. If the glycaemic index of white bread is taken as 100, then wholemeal bread is 52, white spaghetti 32, sucrose 58,baked beans 48, and soya beans 18.

These indices are especially useful if incorporated into the diet of people with diabetes mellitus, where it is important to control the levels of blood glucose between defined limits in order to prevent complications.

Low glycaemic index diets can help to achieve this.Non-starch polysaccharides are generally not digested by the human amylases because the enzymes cannot break the beta linkages between the molecules, and are part of what has more commonly been known as dietary fibre.