The glycemic index of a carbohydrate is influenced by its rate of intestinal absorption, which in turn is influenced by its composition, tertiary structure, type of starch, and susceptibility to enzymic digestion.
Complex carbohydrates are polymeric chains of repeating monosaccharide units. Starches comprise repeating glucose units. The glycemic indexes of different starches are determined by their susceptibility to enzymic digestion, not chain length. White bread and pasta have similar chain lengths, but bread has a higher glycemic index due to its tertiary structure and solubility that ensures greater exposure to salivary and pancreatic amylases.
Short-chain carbohydrates are rapidly absorbed; however, when they contain nonglucose sugars, the glycemic index is lowered proportionally. The disaccharides sucrose and lactose consist of 50% glucose and 50% fructose or galactose, respectively, and both have a lower glycemic index than maltose, the disaccharide formed from two molecules of glucose.
The starches in cereal grains, rice, potatoes, and all green plants are composed of repeating glucose units arranged in straight (amylose) and branched-chained (amylopectin) polysaccharides. The absorption rate, and hence the glycemic index, of these starches is influenced by the ratio of amylose to amylopectin. The more compact structure of amylose than amylo-pectin results in a smaller surface area being available for amylase digestion. Amylose-enriched starches therefore have lower glycemic indexes than those enriched in amylopectin.
Relationship of Insoluble and Soluble Nonstarch Polysaccharides (Fiber) to Glycemic Index
Nondigestable complex carbohydrates are commonly known as dietary fiber; the more correct terminology is nonstarch polysaccharides (NSPs). NSPs are either soluble or insoluble. Clinical studies have shown that diets rich in soluble fiber/NSPs, such as guar gum, pectin, and sugar beet fibers, lower postprandial blood glucose and insulin levels. Guar gum, a /3-glactomannan from the Indian locust bean, also reduces postprandial lipemia. Nonsoluble NSPs have no effect on dietary glycemic index.
Soluble NSPs, such as pulse vegetables, whole fruits, oats, and barley, form gelatinous gels within the stomach that delay gastric emptying and enzy-mic digestion, the latter by forming a physical barrier around the carbohydrate. Insoluble NSPs have little effect on gastric emptying and no effect on glucose absorption. High-fiber/high-NSP diets are therefore not necessarily synonymous with low gly-cemic foods. Cellulose is the most widely used NSP in household cereals, whole meal bread, and brown rice, and since it is insoluble, these foods have the same glycemic index whether replete or deplete of their dietary fiber/NSPs. For unknown reasons, Albran is an exception, and despite its high insoluble fiber content, it has a low glycemic index.
The solubility of dietary fiber/NSPs have benefits on postprandial glycemia and hyperinsulinemia. The reason for this are multifactorial including slowing of gastric emptying, a physical barrier to amylase, possible thickening of the unstirred lear and possa-tive effects on gut incretin hormones such as GLP-1 and GIP. The lack of effect on increasing non-soluble fiber NSPs on glucose and insulin should not detract from important effects on bowel function and bowel pathology.
Cell Structure, Food Preparation, and Processing
Cooking and food preparation can modify the gly-cemic index. Highly processed convenience foods tend to have high glycemic indexes. When cooking and processing disrupt the cell wall, the starch granules are broken open, optimizing amylase digestion and increasing the glycemic index. Cooked pulse vegetables have low glycemic indexes because their cell walls are resistant to cooking. The intact cereal grains of pumpernickel rye bread, granary bread, and bulgur wheat all have low glycemic indexes. However, when granary bread is processed to wholemeal bread, these grains are disrupted and the glycemic index rises. Cooling can paradoxically lower the glycemic index of certain starches, such as potatoes, due to the formation of retrograde starches that are resistant to amylase digestion.
For many foods, their glycemic index is determined by the process of chewing and swallowing. Chewing can reduce food particle size, which increases absorption rates. This explains why boiled and mashed potatoes have different glycemic indexes. Chewing can also change the constituency of the food such that with bread the particle size is reduced to such an extent that it behaves more as a fluid on swallowing and is therefore very rapidly absorbed. In contrast, pastes retain their structure on swallowing and are more slowly absorbed. The rate of gastric emptying also influences the glycemic index, with lower glycemic index foods being retained in the stomach for longer periods than high glycemic index foods.
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