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The GI is a valid and potentially useful concept, but is also deceptively complex…Detailed knowledge is required [that’s] probably beyond the needs and abilities of most people.” Thomas M.S. Wolever, MD, PHD, Faculty of Medicine, University of Toronto


New Light on the Glycemic Index

When it comes to diet, I don’t like counting. I’ve long discouraged counting calories. If you eat the right kind food, your body does the counting for you. Most successful dieters eat by concept, not count. (See article 14, Weight Control Masters) Calorie counting makes weight control more tedious than it needs to be. The same reasoning may apply to the Glycemic Index (GI).

Dr. David Jenkins, a professor of nutrition at the University of Toronto, developed the Glycemic Index in 1981. It’s a measure of how rapidly carbohydrates are digested. The GI assigns a number based on the extent to which a given food causes blood glucose, or “blood sugar” to rise. Foods with a high GI rating contain carbohydrates that cause a dramatic rise in blood-glucose levels, while those with a low number contain carbohydrates with much less impact.

Needless to say, researchers have been busy in the intervening years; there have been many surprises and not a little controversy. Jennie Brand-Miller (PhD), Thomas M.S. Wolever (MD, PhD), Kaye Foster-Powell (M Nutr.& Diet.) and Stephen Colagiuri (MD) collect the latest information in The New Glucose Revolution (Marlow  & Company, 2003), a completely revised and expanded edition of their earlier book published in 1999.

Weight Control

“The glycemic index can play an important role in weight control by helping control appetite and insulin levels,” Brand-Miller and her co-authors write. Low glycemic foods fill you up and keep you satisfied longer. They also moderate insulin response, which helps you burn more fat and less muscle.

“After energy density [volume to calorie ratio],” say the authors, “the second best predictor of satiety” is a food’s GI rating. More than seventeen studies confirm foods with a low GI value keep you satisfied longer than high GI foods, according to the authors.

Brand-Miller and her colleagues say several mechanisms are at work.

Low GI foods, such as apricots and black beans, remain in the small intestine longer, “triggering receptors that tell the brain there’s food still in the gut to be digested.” On the other hand, high GI foods, like papaya and white bread, are digested readily causing a “rapid rise and then fall” in blood sugar, triggering a hunger signal to the brain.

“Stress hormones like adrenalin and cortisol are released when glucose levels” spike after eating high GI foods, say the authors. “Both hormones tend to stimulate appetite.”

Finally, low-GI foods may be more satisfying “simply because they are often less energy dense than their high-GI counterparts,” the authors write. “The naturally high fiber content of many low-GI foods” takes up more room “without increasing their energy content.” In short, foods with low GI values frequently have low energy density as well.

  Insulin Response

  What’s more, according to the authors, low-GI foods, such as lentils, pasta, oatmeal and corn, result in lower levels of insulin than high-GI foods (boiled potatoes and instant rice). This is important to weight watchers, the authors explain, because “high levels of insulin mean the body is forced to burn carbohydrates rather than fat.” Even if total calories are the same, low-GI foods result in more fat burn.

Obese individuals tend to be sugar burners. Carbohydrate or sugar (glycogen) stored in their muscles and liver is their major source of fuel. Their fat (in food they eat and fat deposits) is untouched.

“The next meal restores glycogen to its former level (especially if the food has a high GI value) and the cycle repeats itself,” the authors write. Their fat stores remain intact and usually grow larger.      

Obviously, it would be helpful to people trying to control their weight to know the GI value of various foods. Unfortunately, that’s easier said than done.

  Some Surprises

“Even an experienced scientist with a detailed knowledge of a food’s chemical composition finds it difficult to predict a foods glycemic index value,” the authors acknowledge.

To determine GI value, the amount of food containing a standard amount of carbohydrate (usually 25 or 50 grams) must be fed to real people, blood drawn over 2 or 3 hours, and the glucose measured in the laboratory.

Surprises -- and confusion -- are not unusual.

For example, early press reports said that table sugar is given a value of 100 and used as the reference point, because it’s pure glucose and rapidly absorbed. That was a logical assumption, but it’s wrong. The authors explain that sugar (sucrose) is actually half glucose and half fructose. After being absorbed, fructose is taken directly to the liver for immediate oxidation (burning for energy); its GI value is actually very low, at 19. The GI rating of table sugar is, therefore, the average of glucose and fructose, or 60. Glucose is, in fact, the standard, with a GI of 100.

I remember being surprised and actually questioning the GI value of carrots, originally reported to be 92. People were excluding carrots from their diet, because of the high GI score. Turns out, that was a mistake. Only five people were used in the first study and, according to Brand-Miller and her co-authors, the variation among them was “huge.” “When carrots were assessed more recently,” the authors write, “ten people were included, the reference food was tested twice, and a mean value of 32 was obtained with narrow variation.”

The variation must not have been too narrow, because the comprehensive tables in the back of the book – the “largest, most reliable list of GI values in the world” -- give four different values for carrots (one each from Romania and Canada, and two from Australia), ranging from 16 to 92. The condensed tables give a value of 49 for peeled, boiled carrots. Makes you wonder, doesn’t it?

  Glycemic Load

A major criticism of the glycemic index, according to the authors, is that it doesn’t predict the effect of a normal serving of food. That’s because foods contain different amounts of carbohydrate, while the GI is a comparison of the same amount of carbohydrate. To solve that problem, researchers at Harvard University came up with a companion value called “Glycemic Load.”

Glycemic load attempts to predict the blood glucose response to foods containing carbohydrate, rather than the carbohydrate alone. The glycemic load is calculated by multiplying the GI value of a food by the amount of carbohydrate per serving and dividing by 100. For example, an apple has a GI value of 40. One serving, however, contains only 15 grams of carbohydrate, making the glycemic load only 6 (40 x 15 divided by 100). A potato has a GI value of 90 and 20 grams of carbohydrate per serving. Therefore, it has a glycemic load of only 18 (90 x 20 divided by 100).

For each food, we now have two glycemic values to consider. As if one wasn’t confusing enough! But wait, there’s more.

A related complaint is that carbs are usually consumed as part of a meal, rather than an individual food. The authors also give a multi-step formula for calculating the GI value of a total meal. They acknowledge that it’s complicated, but argue that most people don’t need to make these sorts of calculations. “Many studies have shown a very close relationship between the predicted blood-glucose response (based on published GI values of the relative effects of different foods and meals) and the actual observed blood-glucose response,” they write.

  Hunger Complex

Maybe so, but I’m beginning to side with experts who say the glycemic index is becoming too complicated to be of much value to most people. “[Hunger is] not that simple,” Kathleen Zellman, RD, a spokesperson for the American Dietetics Association, is quote as saying on WebMD. “It’s also affected by meal size, whether there’s any fat in the meal, and overall health status.”

In the same article, Susan Roberts, PhD, of the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University adds: “The best research indicates that [the] glycemic index is probably only one of several factors that influence how hungry and satisfied we are.” Her suggestion: Think high protein, high fiber, low calorie, and low glycemic index when looking for a hunger-satisfying food. “If you can focus on a variety of foods with these properties you will certainly feel more satisfied and less hungry than if you eat a highly refined, low-fiber, high-fat diet,” she says.    

  Remember Concept, Forget Index

The concept of the glycemic index is helpful. By all means, keep in mind that refined carbohydrate foods leave you hungry and make you fat. But no one wants to carry glycemic tables around in their pocket or purse. I certainly don’t. It’s not necessary.

Walter Willett, MD, Harvard School of Public Health, agrees. "I do think this is an important concept for people to understand, but I don't think they need to worry about numbers," he told Daniel Q. Haney, Associated Press Medical Editor. "In practice, it means consuming refined starches--white bread, white rice, white pasta--sugary foods and beverages and potatoes sparingly." 

Build your meals around high fiber, unrefined, whole foods – vegetables, fruit and whole grains -- and add some lean protein and good fat. You’ll be fine.

For many more details, read The New Glucose Revolution and the discussion of “The Glycemic Index Conundrum” in Challenge Yourself.

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