I'm sitting in a comfortable chair, in a tastefully lit, cheerfully decorated drug den, watching a steady line of people approach their dealer. After scoring, they shuffle off to their tables to quietly indulge in what for some could become (if it hasn't already) an addiction that screws up their lives. It's likely you have friends and family members who are suffering from this dependence—and you may be on the same path yourself. But this addiction is not usually apparent to the casual observer. It has no use for the drama and the carnage you associate with cocaine and alcohol. It's slower to show its hand, more socially acceptable—and way more insidious.

I'm in a Panera Bread outlet. The company is on Fortune's 2010 list of the 100 Fastest Growing Companies and earned more than $1.3 billion in 2009, mainly from selling flour and sugar by the railcar. Last year, Zagat named it the most popular large chain in the United States and ranked it second in the Healthy Options category. The company responded by touting its "wholesome" food. Sure, Panera sells a few salads. But why do the scones, pastries, baguettes, and bear claws get all the good lighting? Why are the grab-and-go packs of cookies and brownies next to the register? What need is fulfilled by serving soup bowls made of bread, with a mound of bread for dipping, and then offering more bread on the side? How come it's noon and the couple behind me are eating bagels while the guy to my right is sawing into a cinnamon roll with a fork and a knife like it's a steak?

The answer is that fast-burning carbohydrates—just like cocaine—give you a rush. As with blow, this rush can lead to cravings in your brain and intrusive thoughts when you go too long without a fix. But unlike cocaine, this stuff does more than rewire your neurological system. It will short-circuit your body. Your metabolism normally stockpiles energy so you can use it as fuel later. A diet flush with carbohydrates will reprogram your metabolism, locking your food away as unburnable fat. When you get hungry again you won't crave anything but more of the same food that started you down the path to dependency. Think of this stuff as more than a drug—it's like a metabolic parasite, taking over your body and feeding itself.

You aren't supposed to talk this way about carbohydrates. According to USDA dietary recommendations, they are not only healthy but are supposed to make up the majority of the food we eat—45 to 65 percent of all calories. Carbs, which are classified as starches and sugars, make up the essence of bread, cereal, corn, potatoes, cookies, pasta, fruit, juice, candy, beer, and sweetened drinks—basically anything that isn't protein or fat. Our government's recommendations were established in the 1970s and have since been accompanied by an explosion of obesity and diabetes. The advice came about as early nutrition scientists rallied around a misguided maxim that remains embedded in the fabric of our attitudes toward food to this day: Eating too much fat makes you fat. But science never bore out this pre-Galilean view of nutrition. What is now clear is this: At the center of the obesity universe lie carbohydrates, not fat.

"You could live your whole life and never eat a single carbohydrate—other than what you get from mother's milk and the tiny amount that comes naturally in meat—and probably be just fine," says Gary Taubes, the award-winning author of Good Calories, Bad Calories, which is helping to reshape the conversation about what makes the American diet so fattening.

If all you knew about food is what you read in the USDA guidelines, you'd think our bodies conveniently come into the world seeking the one nutrient that is cheap and amenable to commercial mass production: carbohydrates. "Sugars and starches provide energy to the body in the form of glucose, which is the only source of energy for red blood cells and is the preferred energy source for the brain," says the latest edition of the guidelines. Wrong, says Taubes, who just released Why We Get Fat, a layman's version of his influential scientific tome. In the absence of carbs, your body will burn fatty acids for energy. It's how you sleep through the night without eating for eight hours. "The brain does indeed need carbohydrates for fuel," Taubes says, "but the body is perfectly happy to make those out of protein, leafy green vegetables, and the animal fat you're burning." As a pair of Harvard doctors (one an endocrinologist and one an epidemiologist) wrote in the Journal of the American Medical Association last summer, carbohydrates are "a nutrient for which humans have no absolute requirement."