For more than half a century, the conventional wisdom among nutritionists and public health officials was that fat is dietary enemy number one – the leading cause of obesity and heart disease.

It appears the wisdom was off. And not just off. Almost entirely backward.

According to a new study from the National Institutes of Health, a diet that reduces carbohydrates in favor of fat – including the saturated fat in meat and butter – improves nearly every health measurement, from reducing our waistlines to keeping our arteries clear, more than the low-fat diets that have been recommended for generations. "The medical establishment got it wrong," says cardiologist Dennis Goodman, director of Integrative Medicine at New York Medical Associates. "The belief system didn't pan out."

It's not the conclusion you would expect given the NIH study's parameters. Lead researcher Lydia Bazanno, of the Tulane University School of Public Health, pitted this high-fat, low-carb diet against a fat-restricted regimen prescribed by the National Cholesterol Education Program. "We told both groups to get carbs from green, leafy vegetables, because those are high in nutrients and fiber to keep you sated," says Bazanno. "We also told everyone to stay away from trans fats." The fat-restricted group continued to eat carbs, including bread and cereals, while keeping saturated fat – common in animal products – below 7 percent of total calories. By contrast, the high-fat group cut carbs in half and did not avoid butter, meat, and cheese. Most important, both groups ate as much as they wanted – no calorie counting, no going hungry.

One year later, the high-fat, low-carb group had lost three times as much weight – 12 pounds compared with four – and that weight loss came from body fat, while the low-fat group lost muscle. Even more persuasive were the results of blood tests meant to measure the risk of heart disease and diabetes. The high-fat group, despite eating nearly twice as much saturated fat, still saw greater improvements in LDL cholesterol, HDL cholesterol, and triglycerides. This was enough to improve their scores on the Framingham Risk Calculator, a tool for predicting 10-year risk of heart attack. The low-fat group, by contrast, saw no improvement on their Framingham scores. "I think the explanation lies in how the low-fat dieters filled the hole left by fat – they just ate more carbs," says Bazanno.

How a fatty pork chop can trump pasta begins with the fact that our bodies don't process calories from fat, protein, and carbohydrates in the same way. "When we eat carbs, they break down into sugar in the blood; that's true of whole grains, too, though to a lesser extent," says Jeff Volek, a leading low-carb researcher at Ohio State University. The body responds with the hormone insulin, which converts the extra blood sugar into fatty acids stored in the body fat around our middles. Our blood sugar then falls, and that body fat releases the fatty acids to burn as fuel. But carb-heavy diets keep insulin so high that those fatty acids aren't released, Volek says. The body continues to shuttle sugar into our fat cells – packing on the pounds – but we never burn it. Dietary fat, meanwhile, is the only macronutrient that has no effect on insulin or blood sugar. "This means it's likely excessive carbs, not fat, that plump us up," he adds. Low-carb diets stop that vicious cycle, keeping insulin levels low enough to force the body to burn fat again.

But isn't too much saturated fat bad for your heart? "The evidence for that has really disintegrated," says Dr. Eric Westman, a bariatric physician and director of the Duke Lifestyle Medicine Clinic. It is true that saturated fat can raise cholesterol. But as we know, there is good cholesterol and bad cholesterol. And it turns out that a diet rich in saturated fat increases the former while decreasing the latter. Carbs, on the other hand, do exactly the opposite. In fact, a new Annals of Internal Medicine review of 72 studies and hundreds of thousands of subjects found no strong evidence that saturated fat causes heart disease.

The NIH report actually adds to research that's been accumulating for years. "It's something like the 25th clinical trial in the last 15 years to come out saying this, with almost none going the other way," says Westman.

High-fat diets have been slow to catch on mostly because of two long-held theories. The first is the calorie-counting theory of weight gain, which came about in the 1950s. "It looks at the human body as a mathematical counting machine," says Gary Taubes, author of Why We Get Fat: And What to Do About It. "Fat has more calories per gram than carbs or protein, so eating fat must make you fatter. It's a naive view of human physiology." The second idea, the lipid hypothesis, blamed saturated fat for clogging arteries. This notion emerged from vast population studies in the 1970s that found loose correlations between fat consumption, total cholesterol, and heart disease. Just because two things occur together, however, does not mean that one causes the other. But the lipid hypothesis became so popular at the USDA and the American Heart Association that, says Westman, "there was no money to fund research into anything other than low-fat, low-calorie diets for 20 years."

The AHA now acknowledges that refined carbs like flour and sugar threaten your waistline and your cardiovascular health. "We no longer think low-fat diets are the answer," says Dr. Linda Van Horn of the AHA Nutrition Committee. But, she says, the AHA still recommends keeping saturated fat below 6 percent of total daily calories, or half what the low-carb dieters consumed in the NIH study. "There just haven't been any controlled clinical trials yet showing us how much saturated fat is safe," says Van Horn.

There also haven't been low-carb clinical trials running long enough to reach "hard end points" – heart attack, stroke, or death. That means no one can say with certainty that a high-fat diet will make you live longer. That might be why so few doctors recommend them. Goodman cites another possible reason: "The idea that fat kills got so ingrained, it became folklore. Your mother told you, your grandmother told you. It's going to take years to get people to believe that was wrong," he says. "We're in a transition, and on the cutting edge. It may take a while, but you'll see new guidelines."

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