When Emerson presented his evidence that rising sugar consumption was the best explanation for the rise in diabetes incidence, Joslin rejected it. He said that increased sugar consumption had been offset in America by decreasing apple consumption, and that the carbohydrates in apples were effectively identical to table sugar as far as diabetics were concerned. (This wasn’t the case, but Joslin had little reason to believe otherwise in the 1920s.) Emerson countered with U.S. Department of Agriculture data reporting an actual increase in apple consumption in the relevant decades, but Joslin was unyielding.

Joslin found it inconceivable that sugar or any other refined starch could have a unique property that other carbohydrates did not. They all broke down to glucose after digestion, or glucose and fructose, in the case of table sugar. The insulin-releasing cells of the pancreas (known as ß cells), which are dysfunctional in diabetes, respond only to the glucose. Early on in his career, Joslin, like Ancel Keys thirty years later, found the Japanese diet to be compelling evidence for the salubrious nature of carbohydrate-rich diets. “A high percentage of carbohydrate in the diet does not appear to predispose to diabetes,” he wrote in 1923, since the Japanese ate such a diet and had an extremely low incidence of diabetes. He acknowledged that the rising death rate from diabetes in the United States coincided with rising sugar consumption, and that diabetes mortality and sugar consumption “must stand in some relation,” but the Japanese experience argued against causality. He considered a rising incidence of obesity to be one factor in the increasing prevalence of diabetes, and decreasing physical activity, caused by the increasing mechanization of American life, to be another. A third factor, as the Japanese experience suggested, was a diet that was fat-rich and carbohydrate-poor.

Joslin effectively based his belief primarily on the work of a single researcher: Harold Himsworth of University College Hospital, London. To Joslin, Himsworth’s “painstakingly accumulated” data constituted compelling evidence that a deficiency of carbohydrates and an excess of fat bring on diabetes. It was Himsworth’s research and Joslin’s faith in it that led a half-century of diabetologists to believe unconditionally that diabetes is not caused by the consumption of sugar and refined carbohydrates.