Diet, redefined

For more than a quarter-century, one set of principles has dominated the otherwise fractious, bewildering field of human nutrition and health. The official word on the diet most likely to keep you thin and prevent chronic illnesses such as heart disease and diabetes is that it be low in fat, include lots of fruits and vegetables and be moderate in total calories. Scientists know this. Your doctor and your government tell you the same. It makes sense. There's just one problem.

It doesn't work.

Americans are getting fatter and sicker. Two-thirds of Americans are overweight, and a third of them are clinically obese. Ten percent of Americans suffer from Type II diabetes. Past age 60, that soars to 20 percent. So what's wrong with our concept of a healthful diet?

Just about everything, says Gary Taubes in a watershed new book, "Good Calories, Bad Calories: Challenging the Conventional Wisdom on Diet, Weight Control, and Disease." Deeply researched and profoundly unsettling, the book proposes a seismic paradigm shift that could well undo our perceptions about the relationship between food and health. It could also literally change the way you eat, the way you look and how long you live.

This is not another diet book -- there are no recipes, no menus, no lists of foods to embrace or shun. It's a sober, meticulous review of the vast universe of nutritional science, much of which turns out to be flawed or inconclusive, yet also lucid and lively, the easiest nearly 500 pages of science you'll ever digest. It's an unwavering challenge to conventional thinking; Taubes leaves no sacred cow unbutchered.

The culprits: refined carbs

What's wrong with the American diet has nothing to do with fat, cholesterol or even calories, Taubes writes. It's not overeating, nor a lack of willpower among the overweight. The problem is the consumption of carbohydrates, especially easily digested, refined carbs like processed grains and sugars. We're talking bread, cereal, white rice, pizza, pasta, cookies, snack foods, soda, beer, anything formulated with high-fructose corn syrup -- the stuff clogging your pantry and, Taubes contends, your arteries.

He made a similar argument in 2002, in a controversial New York Times story in which he reported that there was simply no evidence that eating a diet high in fat is bad for you. The article sparked a furor among researchers and nutritionists -- and also touched off a resurgence of the low-carb Atkins Diet, which had been popular 30 years earlier.

For much of the 19th century and the first half of the 20th, it was widely understood that carbohydrates caused obesity and that restricting their consumption was an effective way to lose weight. Scientists and missionaries had also reported that obesity, diabetes, heart disease and many forms of cancer were virtually nonexistent among aboriginal populations in the undeveloped world -- until native people began consuming a Western diet high in carbs, whereupon all those health problems immediately appeared. These illnesses became known as "diseases of civilization."

Taubes blasts U of M's Keys

In the late 1940s, the story changed when Ancel Keys, the famed University of Minnesota physiologist, proposed that increased fat and cholesterol in the American diet were driving an explosive epidemic of heart disease. Keys, inventor of the K-ration, emerges as the book's main villain.

Keys and other researchers who agreed with him recognized that there was inadequate proof of the dangers of dietary fat. But they were so convinced that proof would some day materialize that it seemed to them prudent to advise Americans to adopt a low-fat diet while waiting for the final evidence.

This seems reasonable. But Taubes shows there were problems with this policy. For one, the evidence didn't develop because there is no connection between the consumption of dietary fat and obesity, heart disease, or any other chronic disorder. Not only that, when Americans did attempt to reduce fat intake, especially saturated fats, they gained weight and were more likely to develop diabetes. Finally -- and this is a critical point -- Taubes points out that, just as there was no evidence a low-fat diet is good for you, there was also no way to be sure it might not be bad for you -- which, Taubes argues, it is.

It started with farming

The problem with a low-fat diet is that it substitutes carbohydrates for foods high in fat and protein. Most of the carbs in the modern diet come from sources that were not human food during the thousands of millennia over which our species evolved before the advent of agriculture. Not surprisingly, then, the normal operating conditions of the human body are thrown out of whack by carb consumption.

Here's how Taubes explains it: When you consume carbohydrates, your body responds by flooding your bloodstream with the hormone insulin. Fat cells, highly responsive to insulin, readily convert calories to additional fat in its presence -- and resist giving those calories back as fuel. At the same time, persistently high levels of insulin reduce the responsiveness of nonfat cells, creating an insulin resistance that is a precursor for diabetes. Meanwhile, certain carbohydrates -- especially fructose -- are transformed by the liver into triglycerides, which strongly correlate with heart disease.

What emerges from this vicious cycle is a unified theory of nutrition and health. Obesity, heart disease, diabetes -- and probably cancer and Alzheimer's disease -- are manifestations of a single underlying disorder known as Metabolic Syndrome, which Taubes demonstrates results from constantly elevated insulin levels.

Besides its practical value, "Good Calories, Bad Calories" is an entertaining debunking of many popular misconceptions. Some of these he blames on the media, Jane Brody in particular. Brody, the influential food and nutrition writer for the New York Times, has preached the virtue of a low-fat, high carbohydrate diet. Taubes describes her as a credulous, agenda-driven reporter whose advice on nutrition has been a detriment to public health.

(And perhaps her own. Brody recently informed her readers that, despite decades of strenuous exercise and a low-fat diet, she's recently begun taking a statin drug to treat her rising cholesterol.)

Flouting dogma

Taubes' most elegant and surprising arguments examine long-held assumptions. Although it is nutritional dogma, all calories are not the same. The source of a calorie determines how it is metabolized by the body. Nor is obesity caused by overeating, Taubes says. Rather, it's the other way around. As carbohydrates sequester abnormal amounts of calories in your fat tissue, your body signals that it is starved for energy -- and you therefore eat more. Hunger, he says, is not a behavioral issue or character defect. It's a metabolic response -- like breathing hard when you run -- that is not, except in the short term, controllable through willpower.

Of course, Taubes' point is that volition in what we eat still matters. Deciding to reduce carbohydrate consumption will, he says, help to prevent illness and control your weight -- without leaving you hungry. "Good Calories, Bad Calories" is thus a prescription for health. The question, despite Taubes' exhaustive and persuasive case, is whether we're ready to take our medicine.

William Souder writes about science and nature. His most recent book, "Under A Wild Sky: John James Audubon and the Making of the Birds of America," was a finalist for the 2005 Pulitzer Prize.