The body cells in general -- and brain cells in particular -- require a constant supply of glucose in order to maintain function. Since cells obtain their glucose from the bloodstream, the body works hard to maintain constant blood sugar. Low glycemic-index foods, which are absorbed slowly into the blood, help to achieve this goal.
Significance
Glucose is a carbohydrate, meaning it's a chemical compound composed of carbon, hydrogen and oxygen on a one-two-one ratio. Ubiquitous in food and in nature, dietary glucose comes from table sugar and starch. The body takes up glucose from the bloodstream and can either store it or chemically burn it in oxygen to yield energy, notes Dr. Lauralee Sherwood in her book "Human Physiology." Taking up glucose from the blood requires the hormone insulin.
Glycemic Index
The glycemic index is a measure of the rate at which the glucose from a carbohydrate-containing food enters the bloodstream as compared to the rate at which a sample of pure glucose enters the bloodstream. Low glycemic index foods remain in the digestive tract for a longer time, while high glycemic index foods are absorbed quickly by the intestine, resulting in a rapid increase in blood sugar and rapid insulin response, notes Dr. Sherwood.
Considerations
A food's glycemic index is higher if the food contains large quantities of glucose relative to other nutritional and indigestible molecules, and lower if the food contains protein, fat, or fiber. Protein, fat, and fiber slow the rate at which enzymes in the intestine can absorb ingested glucose, explains Dr. Gary Thibodeau in his book "Anatomy and Physiology." As a result, blood sugar increases slowly over time, and the pancreas releases insulin slowly.
Identification
Low glycemic index foods are those without large quantities of isolated glucose. For instance, whole grain carbohydrate, because it contains fiber, has a lower glycemic index than refined--meaning white--carbohydrate. Many fruits and vegetables also have a lower glycemic index than refined carbohydrate or sources of pure sugar. Fat- and protein-containing foods, including cheese and meat, have a low glycemic index and their glucose is absorbed relatively slowly from the digestive tract.
Expert Insight
In addition to a reduced rate of glucose absorption, low glycemic index foods have other metabolic benefits. First, explains Dr. Sherwood, the rapid insulin release triggered by ingestion of high glycemic index foods increases fat storage, and decreases the amount of glucose available for immediate energy use. Second, low glycemic index foods stay in the digestive tract longer and promote a feeling of fullness, in addition to avoiding the hypoglycemia, or low blood sugar, triggered by rapid insulin release.
References
- "Human Physiology"; Lauralee Sherwood, Ph.D.; 2004
- "Anatomy and Physiology"; Gary Thibodeau, Ph.D.; 2007
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