Glycemic Index - Factors Affecting Rank
It is not just the carbohydrate content of a food which will affect its GI ranking. A number of other factors can also influence GI ranking.
Type Of Starch
Starch is a carbohydrate which consists of two types of molecules: amylopectin and amylose. Amylose accounts for about 20-30% of the starch molecules and amylopectin about 70-80%.
Amylose absorbs less water; the molecules form tight clumps which slows the rate of ingestion. Kidney beans are an example of a low GI food with higher levels of amylose (GI 28).
By contrast, amylopectin the other type of molecule in starch absorbs more water. Amylopectin molecules are more open and lead to a higher rate of digestion.
Gluttonous rice is an example of a high GI ranking food (GI 98).
Physical entrapment of the food item can affect its GI ranking. Physical entrapment refers to the way food is encased and the ability of the food to be absorbed by the body.
For example, bran has a physical barrier which slows down the enzymatic activity of the internal starch layer during digestion. Bran has a low GI of 38.
By contrast, corn flakes a high GI food (GI 92), do not have a pronounced physical barrier like bran and are more quickly absorbed.
Viscosity of fiber
Viscosity is the measurement of the thickness of a liquid. A high viscosity item has a thick sticky consistency somewhere between a liquid and a solid.
The viscosity of fiber can have a large influence on a foods GI ranking as viscous soluble fibers can transform the contents of the intestine into a gel-like /gooey matter. The gooey consistency of this matter slows down the enzymatic activity on starch and so slows down the rate of digestion.
A slower rate of digestion reduces the rate of rise in blood glucose levels. Apples (GI 40) and rolled oats (GI 51) are examples of low GI foods which have a high viscosity of fiber.
By contrast, whole wheat bread (GI 73) and the breakfast cereal, Cheerios (GI 74) are higher GI foods which have a low viscosity of fiber.
The sugar content of the food item also affects its ranking. Foods with a higher sugar content are more likely to be ranked higher as sugar is a food item very similar to glucose.
For example, the breakfast cereal, Golden Grahams has a GI of 71, whilst Frosted Flakes has a lower GI of 55.
Fat and Protein Content
Fats and proteins can slow gastric emptying. Gastric emptying is the process by which food exits the stomach and enters the duodenum. The duodenum, the first section of the small intestine, aids digestion by secreting enzymes into the material passing down from the stomach.
By slowing down gastric emptying the food matter is absorbed more slowly by the body which reduces blood glucose levels. Examples of foods with high fat and protein content include peanut M&M's (GI 33), and potato chips (GI 54).
Like fats and proteins, acid also slows down gastric emptying. Kidney beans are a low acid vegetable with a GI of 28.
Highly processed foods have a higher GI compared to their unprocessed equivalents.
Processed foods require less digestive processing and give rise to higher levels of blood glucose levels once absorbed by the body.
For example, traditional rolled oats have a low GI of 51 whilst process quick 1-minute oats have a higher GI of 66.
The cooking process swells the starch molecules in food and softens it. This speeds up the rate of digestion giving rise to higher levels of blood glucose.
Al dente spaghetti (cooked but firm to touch) boiled for 10 -15 minutes has a GI of 44 whilst the same over cooked spaghetti boiled for 20 minutes has a higher GI of 64.