The evidence supporting the second and third propositions—that eating less fat, or less saturated fat, makes for a healthier and longer life—has remained stubbornly ambiguous. The message of the 1984 consensus conference and the ensuing expert reports was that the benefits of low-fat diets were effectively indisputable, and so pursuing further research on these questions was unnecessary. This in turn led to the ubiquitous belief in the validity of Keys’s hypothesis and the unwholesome nature of saturated fat, but the reality is that since the early 1980s the evidence has become progressively less compelling.

Keys’s own experience stands as an example. In the early 1950s, Keys had based his dietary-fat hypothesis of heart disease to a great extent on the congruence between the changing-American-diet story and the appearance of a heart-disease epidemic. By the early 1970s, however, he had publicly acknowledged that the heart-disease epidemic may indeed have been a mirage. There was “no basis” to make the claim, he admitted, that trends in heart-disease mortality in the United States reflect changes in the consumption of any item in the diet.

In the late 1950s, Keys supported his fat hypothesis with the disparity in fat consumption, cholesterol levels, and heart-disease mortality he found among Japanese men living in Japan, Hawaii, and Los Angeles. This association was then confirmed, more or less, in his Seven Countries Study, in which the Japanese villagers still had remarkably little fat in their diets, low cholesterol levels, and fewer heart-disease deaths over ten years than any other population with the exceptions of those of the islands of Crete and Corfu and the village of Velika Krsna in what is now Serbia. By the mid-1990s, however, the Japanese contingent of the Seven Countries Study, led by Yoshinori Koga, reported that fat intake in Japan had increased from the 6 percent of calories they had measured in the farming village of Tanushimaru thirty-five years earlier, to 22 percent of calories. “There have been progressive increases in consumption of meats, fish and shellfish and milk,” they reported. Mean cholesterol levels rose in the community from 150 mg/dl to nearly 190 mg/dl, which is only 6 percent lower than the average American values (202 mg/dl as of 2004). Yet this change went along with a “remarkable reduction” in the incidence of strokes and no change in the incidence of heart disease. In fact, the chance that a Japanese man of any particular age would die of heart disease had steadily diminished since 1970. “It is suggested that dietary changes in Tanushimaru in the last thirty years have contributed to the prevention of cardiovascular disease,” Koga and his colleagues concluded.

In the late 1950s, Keys had dismissed the possibility that misdiagnosis might have contributed to the extremely low heart-disease death rates in Japan they had observed initially. In 1984, Keys reversed himself, saying that the Japanese cardiologists who had worked with his Seven Countries Study “might have been misled by the local physicians who signed the death certificates and provided details.”