Starch and the Glycemic Index

Starch digestion generally occurs in the small intestine by the action of α-amylase, dextrinase, and glucoamylase in non-ruminant animals. Starch granules interact with other chemical components such as lipids and proteins adhered to starch in the endosperm after reaching the small intestine of mono-gastric animals. Chewing also makes a difference on starch digestibility, increases carbohydrate availability by reducing food structure. It also mixes food fragments with saliva and converts them into a well-lubricated semi-solid bolus form and helps to swallow food for effective digestion. Only a limited extent of starch digestion takes place in segments of the alimentary canal before the small intestine.

The interactions of starch with other compounds and the effect of structural features of starch on starch digestibility are not fully revealed yet. Several in vitro studies have revealed a negative correlation between the amyloseamylopectin ratio with starch digestion (Bornet et al., 1990; Xue et al., 1996; Topping et al., 1997; Zhou and Kaplan, 1997; Akerberg et al., 1998; Ankrah et al., 1999; Bednar et al.,2001; Ito et al., 1999; Saito et al., 2001; Abdel-Aal et al., 2002,).

Surface proteins of starch granule might be considered other factor influencing starch digestibility. As there is a relationship exists between surface area and starch volume, thus size of starch granules might also affect starch digestibility in raw materials. The contact between substrate and enzyme decreases as size of the granule increases. It has been reported that cereals with small granules such as oats and rice have greater starch digestibility than larger starch granules such as maize, wheat, and potato (Manelius and Bertoft, 1996; Bednar et al., 20...

... middle of paper ...

...r meal (Ebbeling CB & Ludwig DS, 2001). It has been observed in the previous study that regular consumption of high–glycemic index meals results in higher average 12-hour blood glucose and insulin levels, higher glycosylated hemoglobin concentrations and 24-hour higher C-peptide excretion in nondiabetic and diabetic individuals compared with isoenergetic and nutrient-controlled low–glycemic index meals (Jenkins et al., 1987; Miller JC 1994).

Another concept similar to GI was glycemic load (GL) which was introduced by Harvard researchers in order to quantify the overall glycemic effect of a portion of food. The term GL of a typical serving food can be calculated by the following formula:

Amount of available carbohydrate in the food X GI of the food

GL =

100

The higher the GL, the greater the elevation in blood glucose and in the insulinogenic effect of the food.