Like Joslin, Himsworth considered all carbohydrates to be equivalent, sugar included; they could all be treated under one nutritional category when comparing diet and disease trends in populations. So Himsworth’s strongest argument was also the Japanese/American comparison. Whereas Joslin used it to exonerate sugar and high-carbohydrate diets, Himsworth used it to implicate fat and low-carbohydrate diets. Himsworth found the correlation between trends in diabetes mortality and the rising tide of fat consumption in England and Wales to be “striking” (the same word that Emerson had used to describe the correlation between trends in diabetes mortality and sugar consumption in the United States). “The progressive rise in diabetic mortality in Western countries during the last fifty years coincides with a gradual change towards higher fat and lower carbohydrate diets,” Himsworth wrote. “The diabetic mortality rate is high in countries whose diets tend to be high in fat and poor in carbohydrate; and low where the opposite tendency prevails. The fall in diabetic mortality in World War I was related to a fall in fat and rise in carbohydrate intake…. Diabetic mortality rises with economic position and, simultaneously, dietary habits change so that a greater proportion of fat and less carbohydrate is taken.” All of these observations, however, could also have been explained by variations in the consumption of sugar and white flour.

To defend his theory, Himsworth had to render irrelevant the conflicting evidence—the experience of isolated populations, for example, eating their traditional diets. “There appears to be unanimous agreement,” he wrote, “that the incidence of diabetes mellitus is very low in the lower social grades of coloured races resident in their native lands, but there is evidence that when these races are transplanted to westernized countries the diabetic mortality rate rapidly rises.” Himsworth’s interpretation was that the original diets of these populations were fat-poor and carbohydrate-rich and became higher in fat when these people moved into urban environments. Himsworth acknowledged that the Masai ate a diet that “contained the highest proportion of fat of any recorded diet” and did not appear to suffer from diabetes, but he considered this evidence “so scanty that no opinion can be expressed.”