Glycaemic Index - hi or Low

WHY DO SOME CARBOHYDRATES HAVE A LOW GI WHILE OTHERS DO NOT

It is the physical properties of the individual starch molecules in a food that determine whether it has a low, moderate or high GI factor. Such properties include:

-the degree of starch gelatinisation

-particle size

-the relative amounts of amylose and amylopectin

-fat & protein content

-sugar content

-fibre content

The Degree of Starch Gelatinisation
When cooked, starch granules are expanded to different degrees by the effects of water and heat. Some types of starch granule expand so much that they actually burst and free the starch molecules that they contain. If most of the starch granules in a particular food have swollen and burst during cooking, that particular food is said to contain fully gelatinised starch. Such swollen granules and free starch molecules are incredibly easy to digest as the starch digesting enzymes in the small intestine have a larger surface area to work on. When digestion enzymes are able to work quickly, as in the case of highly gelatinised starch granules, the effect on blood-sugar is a rapid and high rise after consumption of the food. Therefore, a food containing highly gelatinised starch will have a high GI factor. An interesting and surprising example of this in practice is biscuits, once assumed to contain "simple" sugars and therefore to have a blood-sugar raising effect. Due to the presence of sugar and fat and very little water, starch gelatinisation in biscuits is very difficult. Consequently, biscuits are digested relatively slowly and thus have a intermediate GI factor, raising blood-sugar levels only moderately.

Particle Size
The size of starch granules in foods influences the gelatinisation of the molecules. When cereals are ground or milled, the size of the starch granules is greatly reduced making it much easier for water to be absorbed to bursting point. Again, this increases the surface area available for digestion enzymes to attack. This is why cereal foods from fine flours tend to have high GI factors as the steel rollers used in mills from the 19th century greatly reduced the particle size of starch. The larger the particle size, the lower the GI factor of the food.

Amylose and Amylopectin
These are the two sorts of starch that are present in carbohydrate foods. The ratio of one type to another in a particular food has a great effect on the GI factor of the food.

Amylose is a straight chain molecule which lines up in rows and forms tight compact clumps. Such a close structure makes it very difficult for water molecules to enter and thus very difficult for the starch to become gelatinised and be digested.

Amylopectin consists of a string of glucose molecules with lots of open branching points. Consequently, its structure is much larger, with a greater surface area for gelatinisation and digestion.

Obviously, a food with little amylose and plenty of amylopectin in its starch composition with have a high GI factor, whereas foods with plenty of amylose and little amylopectin will have a lower GI factor.

Fat & Protein
Foods high in fat and protein are digested slowly as fats and proteins slow the rate of stomach emptying. High fat and high protein foods therefore tend to have low GI factors.

Sugar
Despite the belief that sugars raise blood-sugar levels rapidly, refined sugar was found to have a moderate GI factor of only 65. This is again due to its structure. Refined sugar is a disaccharide composed of one glucose molecule coupled to one fructose molecule. Fructose is absorbed and taken directly to the liver where it is mostly converted to glucose. Blood sugar response to pure fructose is very small therefore when we consume refined sugar we are in effect only consuming half as much glucose.

Fibre
If the fibre surrounding a starch granule is still intact it can act as a physical barrier to digestion thus maintaining a low GI factor. Viscous fibre also thickens the mixture in the digestive tract slowing down the passage of food and thus slowing down digestion. Fibre therefore has a dual role in keeping the GI factor of a food low and the blood-sugar response to a minimum.