When nutritionists in the 1960s discussed the pros and cons of sugar and starches, their concern was whether simple carbohydrates were somehow more deleterious than complex carbohydrates of starches. Chemically, simple carbohydrates, as in sugar and highly refined flour, are molecules of one or two sugars bound together, whereas the complex carbohydrates of starches are chains of sugars that can be tens of thousands of sugars long. Complex carbohydrates break down to simple sugars during the process of digestion, but they take a while to do so, and if the carbohydrate is bound up with fiber—i.e., indigestible carbohydrates—the digestion takes even longer. Since the early 1980s, both simple and complex carbohydrates have played a role in determining the glycemic index, which is a measure of how quickly carbohydrates are digested and absorbed into the circulation and so converted into blood sugar. This concept of a glycemic index has had profound consequences on the official and public perception of the risks of starches and sugar in the diet. But it has done so by ignoring the effect of fructose—in sugar and high-fructose corn syrup—on anything other than its ability in the short term to elevate blood sugar and elicit an insulin response.

In the mid-1970s, Gerald Reaven initiated the study of glycemic index to test what he called the “traditionally held tenet” that simple carbohydrates are easier to digest than more complex carbohydrates “and that they therefore produce a greater and faster rise” in blood sugar and insulin after a meal. Reaven’s experiments confirmed this proposition, but he was less interested in blood sugar than in insulin, and so left this research behind. It was taken up a few years later by David Jenkins and his student Thomas Wolever, both of whom were then at Oxford University. Over the course of a year, Wolever and Jenkins tested sixty-two foods and recorded the blood-sugar response in the two hours after consumption. Different individuals responded differently, and the variation from day to day was “tremendous,” as Wolever says, but the response to a specific food was still reasonably consistent. They also tested a solution of glucose alone to provide a benchmark, which they assigned a numerical value of 100. Thus the glycemic index became a comparison of the blood-sugar response induced by a particular carbohydrate food to the response resulting from drinking a solution of glucose alone. The higher the glycemic index, the faster the digestion of the carbohydrates and the greater the resulting blood sugar and insulin. White bread, they reported, had a glycemic index of 69; white rice, 72; corn flakes, 80; apples, 39; ice cream, 36. The presence of fat and protein in a food decreased the blood-sugar response, and so decreased the glycemic index.