The idea that calorie counting is the key to weight loss has long been embedded in the government's nutritional guidelines. It is the driving force of public health policies such as mandatory calorie counting in restaurant and food labels. Many experts say the underlying cause of the obesity epidemic is that Americans consume too many calories of all kinds, caused by easy access to cheap and particularly delicious foods and that they need to control portion. In its website, for example, National Institutes of Health encourage people to count calories and warns that dietary fat has more calories per gram than protein or carbohydrate: "You need to reduce fat to avoid extra calories."
But experts such as Dr. Ludwig argue that the obesity epidemic is driven by sophisticated carbohydrates such as sugar, juices, bagels, white bread, pasta and grains that have been greatly processed. These foods tend to increase blood sugar and insulin, a hormone that promotes fat storage and can increase appetite. Dr. Ludwig and his colleague Dr. Cara Ebbeling has published studies suggesting that diets with different carbohydrate and fat proportions but the same amounts of calories have very different effects on hormones, hunger and metabolism. He has also written a best-selling book on low carbohydrate diets.
Dr. Hall and others disagree. They have published studies challenging the idea that carbohydrate-restricted diets accelerate metabolism and fat loss. Dr. Hall said low carbohydrate diets have many benefits: They can help people with type 2 diabetes manage their blood sugar levels, for example. However, he argues that the explanation for obesity by carbin and insulin is too simplistic and has been experimentally falsified in rigorous studies.
Dr. Hall published a meta-analysis of diet studies last year that suggested that energy expenditure was actually slightly higher in low fat diets. But Dr. Ludwig pointed out that these studies were very short, with no duration lasting longer than one month and most of them lasting for a week or less. He said the process of adapting to a low carbohydrate diet may last for a month or more.
"A few days, or a few weeks, it is not enough time for us to make sense of how diets affect long-term metabolism," he added.
To make the new study, Dr. Ludwig and his colleagues worked with Framingham State University, about 20 miles out of Boston, where overweight students, staff members and teaching staff were recruited. Each participant passed two phases of the study. First, they were placed in strict diets that reduced their body weight by about 12 percent, which was designed to highlight their metabolism.
"At this point their bodies are trying to regain weight," said Dr. Ludwig. "It pushes the body and predisposes to regain weight."