In this book my aim is to look critically at a straightforward question to which most of us believe we know the answer: What constitutes a healthy diet? What should we eat if we want to live a long and a healthy life? To address this question, we’ll examine the evidence supporting both the prevailing wisdom and this alternative hypothesis, and we’ll confront the strong possibility that much of what we’ve come to believe is wrong.

This scenario would not be uncommon in the history of science, although, if it happened in this case, it would be a particularly dramatic and unfortunate example. If it is true, it would be because medical researchers had a relatively easy, reliable test for blood levels of cholesterol as early as 1934, and therefore fixated on the accumulation of cholesterol in the arteries as the cause of heart disease, despite considerable evidence to the contrary. By the time they developed reliable methods for measuring what are known as blood lipids, such as triglycerides, and for measuring blood levels of insulin and a condition known as insulin resistance—indicators that may be more reliable and important—a critical mass of clinicians, politicians, and health reporters had decided that dietary fat and high cholesterol levels were the cause of heart disease, and that low-fat, high-carbohydrate diets were the solution.

In science, researchers often evoke a drunk-in-the-streetlight metaphor to describe such situations: One night a man comes upon a drunk crawling on hands and knees on the pavement under a streetlight. When the man asks the drunk what he’s doing, the drunk says that he’s looking for his keys. “Is this where you lost them?” asks the man. “I don’t know where I lost them,” says the drunk, “but this is where the light is.” For the past half-century, cholesterol was where the light was.

By critically examining the research that led to the prevailing wisdom of nutrition and health, this book may appear to be one-sided, but only in that it presents a side that is not often voiced publicly. Since the 1970s, the belief that saturated fat causes heart disease and perhaps other chronic diseases has been justified by a series of expert reports—from the U.S. Department of Agriculture, the Surgeon General’s Office, the National Academy of Sciences, and the Department of Health in the U.K., among others. These reports present the evidence in support of the fat-cholesterol hypothesis and mostly omit the evidence in contradiction. This makes for a very compelling case, but it is not how science is best served. It is a technique used to its greatest advantage by trial lawyers, who assume correctly that the most persuasive case to a jury is one that presents only one side of a story. The legal system, however, assures that judge and jury hear both sides by requiring the presence of competing attorneys.