FROM THE 1950S ONWARD, researchers worldwide set out to test Ancel Keys’s hypothesis that coronary heart disease is strongly influenced by the fats in the diet. The resulting literature very quickly grew to what one Columbia University pathologist in 1977 described as “unmanageable proportions.” By that time, proponents of Keys’s hypothesis had amassed a body of evidence—a “totality of data,” in the words of the Chicago cardiologist Jeremiah Stamler—that to them appeared unambiguously to support the hypothesis. Actually, those data constituted only half the evidence at best, and the other half did not support the hypothesis. As a result, “two strikingly polar attitudes persist on this subject, with much talk from each and little listening between,” wrote Henry Blackburn, a protégé of Keys at the University of Minnesota, in 1975.

Confusion reigned. “It must still be admitted that the diet-heart relation is an unproved hypothesis that needs much more investigation,” Thomas Dawber, the Boston University physician who founded the famous Framingham Heart Study, wrote in 1978. Two years later, however, he insisted the Framingham Study had provided “overwhelming evidence” that Keys’s hypothesis was correct. “Yet,” he noted, “many physicians and investigators of considerable renown still doubt the validity of the fat hypothesis…. Some even question the relationship of blood cholesterol level to disease.”

Understanding this difference of opinion is crucial to understanding why we all came to believe that dietary fat, or at least saturated fat, causes heart disease. How could a proposition that incited such contention for the first twenty years of its existence become so quickly established as dogma? If two decades’ worth of research was unable to convince half the investigators involved in this controversy of the validity of the dietary-fat/cholesterol hypothesis of heart disease, why did it convince the other half that they were absolutely right?

One answer to this question is that the two sides of the controversy operated with antithetical philosophies. Those skeptical of Keys’s hypothesis tended to take a rigorously scientific attitude. They believed that reliable knowledge about the causes of heart disease could be gained only by meticulous experiments and relentlessly critical assessments of the evidence. Since this was a public-health issue, and any conclusions would have a very real impact on human lives, they believed that living by this scientific philosophy was even more critical than it might be if they were engaged in a more abstract pursuit. And the issue of disease prevention entailed an unprecedented need for the highest standards of scientific rigor. Preventive medicine, as the Canadian epidemiologist David Sackett had observed, targets those of us who believe ourselves to be healthy, only to tell us how we must live in order to remain healthy. It rests on the presumption that any recommendation is based on the “highest level” of evidence that the proposed intervention will do more good than harm.