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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.
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