The GI Factor

The glycemic index (GI) is a measure of the effects of carbohydrates on blood sugar levels. In the early 1980s, Dr. David Jenkins (professor of nutrition at the University of Toronto) and his colleagues developed the glycemic index (GI) in their research to determine which foods were best suited for control of blood sugar levels in people with diabetes. In was concluded that carbohydrates which break down quickly during digestion and release glucose rapidly into the bloodstream have a high GI, while carbohydrates that break down more slowly and release glucose gradually into the bloodstream, have a low GI.

There are three major components of food; proteins, carbohydrates, and fats. Because carbohydrates are a rich source of the body’s primary fuel (glucose), they have the greatest effect on blood sugar levels. (Glucose is what the body uses for energy.)

Carbohydrates are split into two groups:

Simple carbohydrates are naturally occurring single sugars such as glucose and fructose. Table sugar, honey, natural fruit sugars and molasses are also simple carbohydrates. Processed foods such as baked goods, white flour and white rice behave as sugar in the body. Consistent consumption of these types of foods wreaks havoc with the body’s blood sugar levels. The simple carbohydrates contained in them release their sugar too quickly during digestion, causing a rapid, prolonged rise in blood sugar and can lead to obesity, insulin resistance and diabetes.

Complex carbohydrates are rich in starches that are mainly found in plants. They include whole grains, legumes, vegetables and fruits. Complex carbohydrates are digested more slowly than simple carbohydrates, allowing a sustained and level amount of sugar (and other nutrients) into the bloodstream.

All carbohydrates are eventually broken down into glucose, which is absorbed directly into the bloodstream, however complex carbohydrates break down much more slowly. Complex carbohydrates also provide a good source of fibre. A diet high in fibre has been shown to enhance elimination, help the body detoxify, balance blood sugar levels, boost energy and improve immunity.

A number of factors can influence the digestion and absorption rate of food. The GI of a food can be greatly influenced by a variety of factors such as the kind of starch or sugar the food contains; the degree to which a food is processed and prepared; how long the food is cooked; the acidity of the food and so on. Generally, anything that speeds the rate at which a food is digested and absorbed will raise its glycemic index.

Examples of foods that rate high on the glycemic index are products made from white flours such as bread and baked goods, processed cereals, snack foods such as chips and pretzels, baked and mashed potatoes, french-fries, and short-grain white rice. Some examples of foods that rate lower on the glycemic index include most vegetables and fruit, legumes, sweet potatoes, boiled potatoes and whole grains products.

In clinically controlled settings, the glycemic index is determined by assessing the increase in blood glucose level caused by ingestion of (up to) 50 grams of a carbohydrate-containing food compared to the increase in blood glucose levels that results from consuming the same amount of glucose. (Sometimes a slice of white bread is used.) The rise in blood sugar is measured and graphed and the results are indexed. The index rates foods on a 100-point scale. Foods that cause the quickest increase in glucose have the higher scores. Foods with a GI of 70 to 100 are considered “high” on the index, foods with a GI of 56 to 69 are considered “medium” and foods with a GI of 0 to 55 are considered “low”.

Just because certain foods have a high glycemic index doesn’t mean they should not be consumed as part of a balance diet. Some high GI foods are good sources of nutrients. But for people attempting to prevent or control type 2 diabetes or to diminish the effects of insulin resistance, food choices should lean towards a selection of mainly low-GI foods.

Just as the calories, quantity and type of fat, and sodium content listed on the Nutrition Facts label on packaged foods is important to healthy food choices, the GI information should be equally essential in choosing the right kind of carbohydrate. The USA, UK, Australia and New Zealand permit and promote GI labeling. Perhaps sometime in the near future here in Canada we will see the glycemic index of foods included on the Nutrition Facts label also. The GI factor is definitely worth knowing about when making healthy food choices.

For more information on the glycemic index and the GI values of hundreds of foods, visit http://www.glycemicindex.com/.