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When I was a kid, the milkman came right to our back door. He brought us bright glass bottles of rich whole milk and thick sweet cream. We drank a lot of milk. Nobody had heard of skim. On weekends my dad cooked up breakfasts of eggs fried in butter, piles of bacon, delicious German sausages. For dinner, we had big chunks of fatty meat every night.

That was in the 1950s. Nobody was fat, except for one lone girl at school who everybody picked on. Most kids ate like horses and were skinny as rakes.

Then the experts came along and declared that all that fat was killing us. Whole milk was banished from children's diets so that they would not develop clogged arteries and heart disease in later life. To keep our cholesterol in check, we began to ration eggs and treat butter like a toxic substance. We gave up our juicy, marbled steaks and switched to pasta. Ever since the 1960s, the authorities have told us that a healthy diet is a low-fat diet.

The results were not what they had hoped. Obesity rates soared, but heart disease did not subside. And now, a mountain of new evidence says the experts were all wrong. One Harvard study found that people who had consumed the most dairy fat were far less likely to develop heart disease. Researchers at Oxford University discovered that the biggest consumers of saturated fat in Europe – the French – also have the healthiest hearts. Last year, a major review in The BMJ, a leading medical journal, found that "saturated fats are not associated" with mortality, heart disease, strokes or Type 2 diabetes. As Ian Leslie, writing in The Guardian, puts it, "The promotion of low-fat diets was a 40-year fad, with disastrous outcomes, conceived of, authorized, and policed by nutritionists."

The modern history of nutrition science is fraught with controversy, flawed theory, faulty research, vested interests, suppression of evidence, and vicious battles between the old guard and the insurgents. They're still fighting. But it's clear that a lot of what your Food Guide says is flat-out wrong.

The biggest villain of modern diets isn't fat. It's sugar and carbohydrates.

As far back as 1972, a mild-mannered British nutrition scientist named John Yudkin challenged the conventional wisdom, arguing that sugar, not dietary fat, was what was making people fat and sick. His reasoning was in part grounded in history: Humans have been carnivores forever, but carbohydrates, and especially sugar, are very recent additions to the human diet.

The Yudkin theory made sense, and is undergoing a revival. But in the meantime, as Mr. Leslie writes, he and his work were brutally suppressed. By then the North American dietary establishment was firmly in the grip of the fat hypothesis, which had been developed by a forceful and ambitious American nutritionist named Ancel Keys. He had all the institutional power, and he used it to trash his rivals.

Dr. Keys, who died in 2004, also seems to have suppressed inconvenient evidence. In the late 1960s and early seventies, he and a research team conducted a massive investigation into the effects of diet on thousands of mental patients. One group was fed a "heart healthy" diet low in saturated fats; the other ate a more typical American diet. The special diet did indeed reduce blood cholesterol, what the researchers called a "favourable trend." But the published results were incomplete. The full results were published for the first time last week in the BMJ, they tell quite a different story. Patients on the special diet, especially those over 64, had a higher mortality rate than those on the regular diet.

The Keys theory is on the way to being thoroughly debunked, not least because of the investigative work of journalist Nina Teicholz (author of The Big Fat Surprise, who is persona non grata among the nutrition establishment). Yet the establishment is still deeply embedded in the status quo. Reputations and careers are at stake; plenty of leading doctors have diet empires of their own.

What's so devastating about this story, as Mr. Leslie points out, is the answer it supplies to one simple question: Who made us fat? And no, it wasn't the usual villains – big-food interests, the sugar lobby – although they certainly played a role. It was the scientific authorities and the governments that believed them.

So Vive la France, and pass the butter. There's no time to waste.

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