"Saturated fat does not cause heart disease"—or so concluded a big study published in March in the journal Annals of Internal Medicine. How could this be? The very cornerstone of dietary advice for generations has been that the saturated fats in butter, cheese and red meat should be avoided because they clog our arteries. For many diet-conscious Americans, it is simply second nature to opt for chicken over sirloin, canola oil over butter.

The new study's conclusion shouldn't surprise anyone familiar with modern nutritional science, however. The fact is, there has never been solid evidence for the idea that these fats cause disease. We only believe this to be the case because nutrition policy has been derailed over the past half-century by a mixture of personal ambition, bad science, politics and bias.

Our distrust of saturated fat can be traced back to the 1950s, to a man named Ancel Benjamin Keys, a scientist at the University of Minnesota. Keys was formidably persuasive and, through sheer force of will, rose to the top of the nutrition world—even gracing the cover of Time magazine—for relentlessly championing the idea that saturated fats raise cholesterol and, as a result, cause heart attacks.

As the director of the largest nutrition study to date, Keys was in an excellent position to promote his idea. The "Seven Countries" study that he conducted on nearly 13,000 men in the U.S., Japan and Europe ostensibly demonstrated that heart disease wasn't the inevitable result of aging but could be linked to poor nutrition.

Critics have pointed out that Keys violated several basic scientific norms in his study. For one, he didn't choose countries randomly but instead selected only those likely to prove his beliefs, including Yugoslavia, Finland and Italy. Excluded were France, land of the famously healthy omelet eater, as well as other countries where people consumed a lot of fat yet didn't suffer from high rates of heart disease, such as Switzerland, Sweden and West Germany. The study's star subjects—upon whom much of our current understanding of the Mediterranean diet is based—were peasants from Crete, islanders who tilled their fields well into old age and who appeared to eat very little meat or cheese.

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As it turns out, Keys visited Crete during an unrepresentative period of extreme hardship after World War II. Furthermore, he made the mistake of measuring the islanders' diet partly during Lent, when they were forgoing meat and cheese. Keys therefore undercounted their consumption of saturated fat. Also, due to problems with the surveys, he ended up relying on data from just a few dozen men—far from the representative sample of 655 that he had initially selected. These flaws weren't revealed until much later, in a 2002 paper by scientists investigating the work on Crete—but by then, the misimpression left by his erroneous data had become international dogma.

Other studies ensued. A half-dozen large, important trials pitted a diet high in vegetable oil—usually corn or soybean, but not olive oil—against one with more animal fats. But these trials, mainly from the 1970s, also had serious methodological problems. Some didn't control for smoking, for instance, or allowed men to wander in and out of the research group over the course of the experiment. The results were unreliable at best.

But there was no turning back: Too much institutional energy and research money had already been spent trying to prove Keys's hypothesis. A bias in its favor had grown so strong that the idea just started to seem like common sense. As Harvard nutrition professor Mark Hegsted said in 1977, after successfully persuading the U.S. Senate to recommend Keys's diet for the entire nation, the question wasn't whether Americans should change their diets, but why not? Important benefits could be expected, he argued. And the risks? "None can be identified," he said.

In fact, even back then, other scientists were warning about the diet's potential unintended consequences. Today, we are dealing with the reality that these have come to pass.

One consequence is that in cutting back on fats, we are now eating a lot more carbohydrates—at least 25 percent more since the early 1970s. Consumption of saturated fat, meanwhile, has dropped by 11 percent, according to the best available government data. Translation: Instead of meat, eggs and cheese, we're eating more pasta, grains, fruit and starchy vegetables such as potatoes. Even seemingly healthy low-fat foods, such as yogurt, are stealth carb-delivery systems, since removing the fat often requires the addition of fillers to make up for lost texture—and these are usually carbohydrate-based.

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