What You Should Know About the Glycemic Index

The “Glycemic Index” has become a huge dieting buzzword. Around the world, the Canadian G.I. Diet and American Glucose Revolution books are selling like hotcakes (you should excuse the expression) – buoyed by the promise of helping dieters avoid the “dreaded blood sugar rush blamed for weight gain.” But is the GI diet approach just another empty fad? The answer is yes, and no. Read on for details.

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What is the GI?

The Glycemic Index (GI) was actually created as a system to help people with diabetes judge how different foods would affect their blood sugar levels. Researchers at the University of Sydney, Australia, set out to scientifically test and record the impact of individual foods on blood glucose (BG) levels. In fact, you can find the “GI value” of hundreds of foods in their database at www.glycemicindex.com. You can even submit samples of a food, along with a check, and the research team will measure its GI value for you.

This is accomplished by a simple experiment. To set the GI value of, say, a slice of bread, 10 volunteers eat 50 grams (about 2 ounces) of the bread in the morning after fasting. Their BG is then measured over the next two hours, and the total rise in glucose during this time period is calculated. Several days later, the same 10 volunteers drink 50 grams of pure glucose, and have their BG measured in the same way over the next two hours. The two glucose sums are compared, and the difference in value between the test food and the ingested glucose becomes the GI value for that food.

What’s important for you to know here is that foods with lower GI values have a lesser impact on BG in the first two hours after you’ve eaten them. In other words, these foods need longer to absorb into your system, so they don’t cause a BG spike. Typically, high GI foods are sweets and starches – high-carbohydrate, lower-fiber foods which are also obvious foods to avoid when you are attempting to lose weight. For this reason, a low-GI diet sounds like the ideal weight-loss plan.

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Trouble in Paradise

Yet as easy as it sounds, the Glycemic Index becomes a little complicated when you try to apply it in real life. Foods don’t always affect your BG the way you would expect them to based on their GI value, due to a number of variables: most of your food is not eaten directly after fasting, as it was during the GI testing, and you also eat foods in various combinations and amounts.

Note that the American Diabetes Association (ADA) does not endorse the Glycemic Index. The organization points out that the GI impact of a food differs tremendously on its type (country of origin, fresh or processed), its age or ripeness, the length of time it was stored, method of cooking, and a half-dozen other variables. Also, the effect of GI values vary from person to person, and even in a single individual from day to day. What's more, the GI values are calculated based on 50g/carb portions, which is rarely the amount normally eaten. So heck, does this Index have anything valuable to tell us at all?

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Real Food, Real Value

The Glycemic Index diet has not actually been shown to affect weight loss, despite the belief that controlling BG will control appetite and insulin, and therefore control food intake and fat storage. The International Food Information Council recently summed it up by stating: "Current evidence suggests that the Glycemic Index … is of little utility for providing dietary guidance for Americans."

That said, it is true that carbohydrates are central to most diets, providing your most essential source of energy and also the biggest impact on your BG levels. Thus, it’s important to be mindful of how carbohydrates affect your diabetic metabolism, both in terms of quantity and quality. A rule of thumb akin to avoiding “high GI” foods goes like this: “If it’s white, it’s not right.” This helps remind people to limit their intake of potatoes, white rice, and breads and pastas made with white flour – all of which have a high GI value and do indeed make your BG levels immediately soar.

In the final analysis, most experts agree that using the Glycemic Index or relying on the recommendation to eat more fiber, fruits, and vegetables, will lead you in the same general direction. Unprocessed or “whole” foods will always be better for you than any fabricated “low-carb” product. That stuff is often extremely high in fat and chemicals, and has in some cases even been altered specifically to be indigestible, so that your so-called “free” chocolate will create a nasty stomachache.

Probably the most useful thing you can do as a diabetic is to get a feel for the carbohydrate content – and impact on your BG levels – of the usual foods you enjoy eating regularly. Only by knowing this can you really begin to understand the impact of your diet on your day-to-day glucose control and longer-term A1c results.

Amy Tenderich is creator of the popular web log www.diabetesmine.com and co-author of the new book, “Know Your Numbers, Outlive Your Diabetes.”