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Unscramble the Number

Deciphering the glycemic index

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Unscramble the Number

Until relatively recently, experts incorrectly assumed that all simple carbohydrates digested quickly, causing a rapid rise in blood sugar, and that all starches digested slowly, causing a gradual rise in blood sugar. High blood sugar and insulin levels have been associated with insulin resistance, hypertension, strokes, and cardiovascular disease.

Carbohydrates are one of three major components of food (the other two are fat and protein). Carbs have the greatest effect on blood sugar levels because they are a rich source of the body’s primary fuel, glucose.

Carbohydrates are split into two groups based on their molecular structure. Simple carbohydrates (sugars) are comprised of one or more sugars such as glucose and fructose. Complex carbohydrates (starches) consist of long chains of glucose.

Some carbohydrates release their sugar too quickly during digestion, causing a rapid, prolonged rise in blood sugar. Over time, these foods can cause weight gain, insulin resistance, and cardiovascular disease. The glycemic index (GI) is a gauge of a carbohydrate’s blood sugar-raising potency in the first few hours after ingestion.

Until relatively recently, experts incorrectly assumed that all simple carbohydrates digested quickly, causing a rapid rise in blood sugar, and that all starches digested slowly, causing a gradual rise in blood sugar. High blood sugar and insulin levels have been associated with insulin resistance, hypertension, strokes, and cardiovascular disease.

The Highs and Lows

In 1979 the glycemic index was developed by Dr. David Jenkins, Dr. Alexandra Jenkins, and Dr. Thomas Wolever. It resulted from research conducted at the Universities of Toronto and Oxford to determine which foods were best for control of blood sugar in diabetics. The study found that some heavily eaten starches, such as white bread, rice, and potatoes, cause blood sugar levels to rise almost as fast as glucose and faster than table sugar. Foods such as meats, poultry, fish, and many leafy vegetables have a GI of zero since they contain little or no carbohydrates.

The two GI foods with the greatest impact in the Canadian diet are potatoes and wheat products.

Don’t Try This at Home

Many factors can affect the glycemic index. To ensure consistent, accurate results, a food must be tested under carefully controlled conditions using valid scientific methods. Only a few laboratories in the world perform legitimate testing.

Testing a food for GI requires eight to 10 test subjects. Each subject must be separately tested for both the test food and a reference (glucose), using equal amounts of digestible carbohydrate. The effect of the test food is compared to that of the reference as a percentage. In other words, if the reference has a value of 100, and the test food’s GI is 50, it is expressed as 50 percent.

Aim Low

The objective in using the GI is to lower the average GI of meals. Snacks should always be low in GI. However, not all foods with a low GI are healthy, nor are all foods with a high GI unhealthy. In choosing nutritious foods it is important to consider other factors such as fibre content and type and amount of fat. Mastering a low GI diet can take some effort. In his GI Diet series of books, Canadian Rick Gallop has made it easy to get up to speed with red, yellow, and green ratings that consider fat as well as GI (see gidiet.com).

Not on the Label

Just as information on the quantity and type of fat is important to healthy food choices, GI information is essential in choosing the right kind of carbohydrate. Australia, New Zealand, the UK, and the US permit and promote GI labelling.

In Canada, however, the glycemic index value is currently not part of the Nutrition Facts label on packaged foods. In fact, the Canadian Food Inspection Agency has said that product statements about the glycemic index are not acceptable. Proponents of GI labelling can write their MPs, food regulators, and members of the food industry requesting that GI labelling become mandatory on packaged foods.

For the past 10 years, Jennie Brand Miller at the Human Nutrition Unit of Sydney University in Australia has been at the forefront of glycemic index research. Her team has determined the GI values of more than 400 foods. The University of Sydney Glycemic Index website lists the GI values of hundreds of foods. See glycemicindex.com.

Glycemic Index Values

When glucose is used as the reference (that is, the numerical value for glucose is 100), carbohydrate values in food may be rated as high, intermediate, or low, as follows:

  • High GI 70 or higher
  • Intermediate GI 56 to 69
  • Low GI 0 to 55
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