Glycemic Index: New Study Finds Popular Weight Loss Tool Worthless

In yet another blow to diets like Atkins and South Beach, research has found that the glycemic index, the tool both diets are based on, is worthless.

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In yet another blow to diets like Atkins and South Beach, research has found that the glycemic index, the tool both diets are based on, is worthless.(1)

South Beach and Atkins have long asserted that high-glycemic foods raise glucose (blood sugar) and insulin, which stimulates appetite and promotes overeating and obesity. So foods with a high glycemic index, even healthful ones like carrots, potatoes, and corn, are banned on these diets.

“No Differences”

But in recent investigations, scientists at Purdue University in Indiana and Universidade Federal de Vicosa in Brazil found “no differences” in the effects of diets containing a lot of high-glycemic foods versus diets focusing on low-glycemic foods.           

The research, published in the September 2005 issue of Diabetes Care, followed 39 healthy adults. For eight days, they were allowed to eat as much as they wanted (ad libitum) of a diet full of foods with high glycemic ratings, all served restaurant-style in the laboratory at Purdue’s Department of Foods and Nutrition.

For another eight-day period, the subjects returned to Purdue for their meals and ate as much as they wanted of a diet plentiful in low-glycemic foods.

On days 1 and 8 of both dietary regimens, the researchers measured glucose and insulin concentrations as well as appetite levels before and two hours after breakfast and lunch.

The results: “There were no significant differences in plasma glucose or insulin responses, appetitive ratings, or food intake between treatments,” reported co-authors Richard D. Mattes, PhD, and Rita C. G. Alfenas, PhD.

2005 Dietary Guidelines

The findings are the latest in a growing body of data questioning the value of the glycemic index for weight control.  The expert committee of scientists appointed by the USDA to develop the 2005 Dietary Guidelines were so skeptical of its value that in their final report outlining dietary recommendations for America, the scientists wrote: “Current evidence suggests that the glycemic index and/or glycemic load are of little utility for providing guidance for Americans.”(2)

The glycemic index, created by University of Toronto scientists in the 1980s, is a tool that ranks individual foods by the effect they have on blood sugar levels. The higher the blood sugar, they theorized, the higher the index.

American Diabetes Association

But today many health organizations, including the American Diabetes Association, do not endorse the glycemic index, arguing that in determining blood sugar levels one’s overall diet is significantly more important than individual foods. A food’s glycemic index can fluctuate depending on what other foods are eaten with it as well as how much is eaten.

And a high glycemic index does not necessarily mean high sugar levels. Carrots, for example, have a high glycemic index, but you’d have to eat over a pound to spike the blood sugar as high as the glycemic index warns.

Ridiculous Choices

Several prominent obesity experts, including Dr. F. Xavier Pi-Sunyer at the Obesity Research Clinic at Columbia University College of Physicians and Surgeons, have stated that using the glycemic index to select food, the primary approach advocated by the Atkins and South Beach diets, would lead to ridiculous choices, like ice cream over carrots, and French fries over baked potatoes.(3)

Diet books advocating the glycemic index tend to refer to studies conducted in the 1980s and early 90s, “but this early research was flawed. It did not reflect real-world living,” points out Dr. James Kenney, Nutrition Research Specialist at the Pritikin Longevity Center® & Spa in Miami, Florida.

In many of these studies, subjects were forced to eat the same number of calories of various foods – 250 calories of white bread, 250 calories of carrots, 250 calories of potatoes, and so on.

But if you try to eat 250 calories of, say, carrots, you’re putting away 1½ pounds of food, which means you’ll have one sorely stuffed stomach. That’s a problem if you’re measuring glycemic index because food from a tightly stretched stomach, like air from a tightly stretched balloon, empties out quickly. Carbohydrates, or glucose, rush into the bloodstream, ratcheting up blood sugar, and therefore the glycemic index.

Real-World Eating

“But outside of a research lab, who’s going to eat 250 calories of carrots in one sitting?” asks Dr. Kenney. “Newer studies tend to report much more accurate findings because scientists are now allowing people to make their own decisions about how much of the various foods they want to eat. They’re no longer being told to stuff themselves with ridiculously huge amounts of food.”

In these real-world settings, as the new study in Diabetes Care found, so-called high-glycemic foods like carrots and potatoes no longer raise blood sugar or insulin levels any more than many other foods. Nor do they enhance appetite or produce excess calorie intake and obesity.

Healthiest Diet

In fact, decades of epidemiological research and well-designed clinical trials have found just the opposite: diets based on fiber-rich, water-rich foods like fruits, vegetables, and whole grains – including carrots, potatoes, and corn – are the diets of the leanest, longest- living populations in the world, and are linked with the lowest rates of heart disease, hypertension, diabetes, and many forms of cancer.

“That’s why the authors of the U. S. Dietary Guidelines, nutrition experts from leading universities nationwide, recommended that Americans center their diet on a wide variety of fiber-rich, unrefined carbohydrates like fruits, vegetables, and whole grains – not on a riddled-with-error tool like the glycemic index,” concludes Dr. Kenney.

1. Diabetes Care, 2005; 28: 2123.
2. Report of the U.S. Dietary Guidelines Advisory Committee on the Dietary Guidelines for Americans, 2005 (
3. American Journal of Clinical Nutrition, 2002; 76: 290(S)-8(S)


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