Here's how Phinney explains it: When you cut carbs, your body first uses available glycogen as fuel. When that's gone, the body turns to fat and the pancreas gets a break. Blood sugar stabilizes, insulin levels drop, fat burns. That's why the diet works for diabetics and for weight loss.
Beyond the fat-burning effects of ketosis, people lose weight on low-carb diets because fat and protein increase satisfaction and reduce appetite. On the flip side, simple carbs cause an insulin surge, which triggers a blood sugar drop, which makes you hungry again.
"At my obesity clinic, my default diet for treating obesity, Type 2 diabetes and metabolic syndrome is a low-carb diet," says Dr. Eric Westman, director of the Lifestyle Medicine Clinic at Duke University Medical Center, and co-author of the new Atkins book. "If you take carbohydrates away, all these things get better."
Though the movement to cap carbs is growing, not all nutritional scientists have fully embraced it. Dr. Ronald Krauss, senior scientist at Children's Hospital Oakland Research Institute and founder and past chair of the American Heart Assn.'s Council on Nutrition, Physical Activity and Metabolism, says that while he fundamentally agrees with those advocating fewer dietary carbs, he doesn't like to demonize one food group.
That said, he adds, those who eat too many calories tend to overconsume carbohydrates, particularly refined carbohydrates and sugars. "It can be extremely valuable to limit carbohydrate intake and substitute protein and fat. I am glad to see so many people in the medical community getting on board. But in general I don't recommend extreme dietary measures for promoting health."
Joanne Slavin, professor of nutrition at the University of Minnesota and a member of the advisory committee for the 2010 Dietary Guidelines for Americans, is less inclined to support the movement. The committee, she says, "looked at carbohydrates and health outcomes and did not find a relationship between carbohydrate intake and increased disease risk."
Most Americans need to reduce calories and increase activity, Slavin adds. Cutting down on carbs as a calorie source is a good strategy, "but making a hit list of carbohydrate-containing foods is shortsighted and doomed to fail, similar to the low-fat rules that started in the 1980s."
As nutrition scientists try to find the ideal for the future, others look to history and evolution for answers. One way to put our diet in perspective is to imagine the face of a clock with 24 hours on it. Each hour represents 100,000 years that humans have been on the Earth.
On this clock, the advent of agriculture and refined grains would have appeared at about 11:54 p.m. (23 hours and 54 minutes into the day). Before that, humans were hunters and gatherers, eating animals and plants off the land. Agriculture allowed for the mass production of crops such as wheat and corn, and refineries transformed whole grains into refined flour and created processed sugar.
Some, like Phinney, would argue that we haven't evolved to adapt to a diet of refined foods and mass agriculture — and that maybe we shouldn't try.