The idea that counting calories is the key to weight loss has long been embedded in the government’s dietary guidelines. It is the driving force behind public health policies like mandatory calorie counts on restaurant menus and food labels. Many experts say that the underlying cause of the obesity epidemic is that Americans eat too many calories of all kinds, prompted by easy access to cheap and highly palatable foods, and that they need to exercise portion control. On its website, for example, the National Institutes of Health encourages people to count calories and warns that dietary fat has more calories per gram than protein or carbs: “You need to limit fats to avoid extra calories,” it states.

But experts like Dr. Ludwig argue that the obesity epidemic is driven by refined carbohydrates such as sugar, juices, bagels, white bread, pasta and heavily processed cereals. These foods tend to spike blood sugar and insulin, a hormone that promotes fat storage, and they can increase appetite. Dr. Ludwig and his colleague Dr. Cara Ebbeling have published studies suggesting that diets with different ratios of carbs and fat but identical amounts of calories have very different effects on hormones, hunger and metabolism. He has also written a best-selling book on lower-carb diets.

Dr. Hall and others disagree. They have published studies disputing the notion that carb-restricted diets accelerate metabolism and fat loss. Dr. Hall said that low-carb diets have many benefits: They can help people with Type 2 diabetes manage their blood sugar levels, for example. But he argues that the carb and insulin explanation for obesity is too simplistic and has been “experimentally falsified” in rigorous studies.

Dr. Hall published a meta-analysis of feeding studies last year that suggested that energy expenditure was actually slightly greater on low-fat diets. But Dr. Ludwig pointed out that those studies were very short, with none lasting longer than a month and most lasting a week or less. He said the process of adapting to a low-carb diet can take a month or longer.

“A few days, or a couple weeks, is not enough time to make any meaningful conclusion about how diets affect metabolism over the long term,” he added.