Cleave’s nutritional education was furthered by the experience of his brother, Surgeon Captain H. L. Cleave, who spent the war years imprisoned by the Japanese in Hong Kong and then Tokyo. In the Hong Kong prison, peptic ulcers were a plague. The diets in these camps were predominantly white rice. Until vitamin-B supplements were distributed, beriberi was also a problem. After two years, many of the prisoners, including Cleave’s brother, were transferred to a camp outside Tokyo, where the ulcers vanished. In the Tokyo POW camps, the rice was brown, lightly milled, with unmilled barley and millet added.

In the decades after the war, Cleave became an obsessive letter-writer, corresponding with hundreds of physicians around the world, requesting information on disease rates and the occurrence and appearance of specific diseases. His 1962 book on peptic ulcers contained page after page of testimony from physicians reporting the relative absence of ulcers in those populations where sugar, white flour, and white rice were hard to come by.

Cleave’s intuition was to reduce the problem of nutrition and chronic disease to its most elementary form. If the primary change in traditional diets with Westernization was the addition of sugar, flour, and white rice, and this in turn occurred shortly before the appearance of chronic disease, then the most likely explanation was that those processed, refined carbohydrates were the cause of the disease. Maybe if these carbohydrates were added to any diet, no matter how replete with the essential protein, vitamins, minerals, and fatty acids, it would lead to chronic diseases of civilization. This would explain why the same diseases appeared after Westernization in cultures that lived almost exclusively on animal products—the Inuit, the Masai, and Samburu nomads, Australian Aborigines, or Native Americans of the Great Plains—as well as in primarily agrarian cultures like the Hunza in the Himalayas or the Kikuyu in Kenya.

Cleave would later be disparaged for suggesting that all chronic diseases of civilization have a single primary cause, but he insisted that it was naïve to think otherwise. Though it may seem odd, he considered dental cavities the chronic-disease equivalent of the canary in the mine. If cavities are caused primarily by eating sugar and white flour, and cavities appear first in a population no longer eating its traditional diet, followed by obesity, diabetes, and heart disease, then the assumption, until proved otherwise, should be that the other diseases were also caused by these carbohydrates.