According to many popular fad diets, foods that rapidly raise blood sugar levels are responsible for making you fat. Specifically, the four low-carbohydrate, high-protein plans reviewed by AICR attempt to blame overweight and obesity on high insulin levels. They maintain that by avoiding foods that trigger the body to release insulin in great amounts, it is possible to lose fat quickly and permanently.
To make this claim, the fad diet authors ignore the substantial amount of research that runs counter to their "theory." More importantly, however, they have taken a controversial nutritional concept—the Glycemic Index—out of the context for which it was intended. To understand how they have manipulated a scientific idea to bolster their claims, it is important to understand what the Glycemic Index actually is. The Glycemic Index was first developed in 1981 by scientists led by Dr. David Jenkins at the University of Toronto. In an effort to rethink dietary guidelines for diabetics, which are based on an intricate system of "exchanges," Jenkins and his colleagues charted how quickly various foods affect blood sugar levels. The basic idea of the Glycemic Index has been embraced by many diabetes organizations around the world. The American Diabetes Association, however, has not yet endorsed glycemic indexing. Its refusal to do so raises questions about its practical importance. Specifically, the ADA believes that the differences in rates of digestion ranked by the Glycemic Index are not sufficient to warrant changes in the diet. The ADA also holds that these individual differences fade even further once different foods are eaten together, at meals. The range of Glycemic Index numbers are derived by comparing the rate of a given food's digestion to that a food known to be rapidly digested. The reference food (different methods use either pure glucose or white bread) is assigned a Glycemic Index of 100, and the tested food is charted against this standard.
Foods with a high Glycemic Index (70 and above) are those that break down quickly and cause a spike in blood sugar levels. Foods with a low Glycemic Index (55 and below) break down more slowly and steadily, providing a sustained supply of energy. Popular notions regarding the rates of absorption of "simple" and "complex" carbohydrates are refuted by a glance at the Glycemic Index tables. For years, it was thought that sugary foods like candy and ice cream—composed of simple carbohydrates—would be absorbed immediately. Similarly, it was believed that starchier foods like bread and potatoes provided steady, long-term energy. But these assumptions were made solely by studying the chemistry of the foods in question. In the body, they act quite differently: Most sugary foods actually cause very small changes in blood sugar levels, while many starchy foods—white bread and potatoes in particular—cause sharp spikes. These are some of the important subtleties and contradictions ignored by authors of fad diets. By urging a sweeping ban on all carbohydrates, these books display only a passing familiarity with a nutritional concept whose real-world applications remain to be seen.
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