The much more contentious issues were how the quantity and type of fat influenced cholesterol levels, and, ultimately more important, whether cholesterol is even the relevant factor in causing heart disease. Keys and his wife had measured only total cholesterol in the blood, and he was comparing this with the total amount of fat in the diet. Through the mid-1950s, Keys insisted that all fat—both vegetable and animal—elevated cholesterol. And if all fat raised cholesterol, then one way to lower it was to eat less fat. This was the basis of our belief that a healthy diet is by definition a low-fat diet. Keys, however, had oversimplified. Since the mid-1950s, researchers have known that the total amount of dietary fat has little effect on cholesterol levels.

In 1952, however, Laurance Kinsell, director of the Institute for Metabolic Research at the Highland–Alameda County Hospital in Oakland, California, demonstrated that vegetable oil will decrease the amount of cholesterol circulating in our blood, and animal fats will raise it. That same year, J. J. Groen of the Netherlands reported that cholesterol levels were independent of the total amount of fat consumed: cholesterol levels in his experimental subjects were lowest on a vegetarian diet with a high fat content, he noted, and highest on an animal-fat diet that had less total fat. Keys eventually accepted that animal fats tend to raise cholesterol and vegetable fats to lower it, only after he managed to replicate Groen’s finding with his schizophrenic patients in Minnesota.

Kinsell and Edward “Pete” Ahrens of Rockefeller University then demonstrated that the crucial factor in controlling cholesterol was not whether the fat was from an animal or a vegetable, but its degree of “saturation,” as well as what’s known as the chain length of the fats. This saturation factor is a measure of whether or not the molecules of fat—known as triglycerides—contain what can be considered a full quotient of hydrogen atoms, as they do in saturated fats, which tend to raise cholesterol, or whether one or more are absent, as is the case with unsaturated fats, which tend, in comparison, to lower it. This kind of nutritional wisdom is now taught in high school, along with the erroneous idea that all animal fats are “bad” saturated fats, and all “good” unsaturated fats are found in vegetables and maybe fish. As Ahrens suggested in 1957, this accepted wisdom was probably the greatest “handicap to clear thinking” in the understanding of the relationship between diet and heart disease. The reality is that both animal and vegetable fats and oils are composed of many different kinds of fats, each with its own chain length and degree of saturation, and each with a different effect on cholesterol. Half of the fat in beef, for instance, is unsaturated, and most of that fat is the same monounsaturated fat as in olive oil. Lard is 60 percent unsaturated; most of the fat in chicken fat is unsaturated as well.