Diet researchers reported Tuesday that patients who had recently lost weight seemed to burn calories more efficiently in the crucial weight-maintenance phase when they ate a low-carbohydrate, Atkins-style diet than they did when they ate a traditional, higher-carblow-fat diet.

The small but intensive study, which was published in the Journal of the American Medical Assn., showed through various measurements of energy expenditure that patients burned about 300 more calories a day on a low-carb diet than they did on thelow-fat diet. But the research did not reveal the underlying reason why.

“The physiological basis for the difference ... remains subject to speculation,” wrote the study’s co-authors.

But Dr. Robert Lustig, a professor of clinical pediatrics at UC San Francisco who says that the modern diet is far too heavy on sugar — and has suggested that authorities should tax the sweet stuff — told The Times he suspected there was one simple cause.


“To borrow a phrase from Bill Clinton: It’s the insulin, stupid,” said Lustig, who wasn’t involved in the JAMA study. “The reason any diet will work is because it lowers insulin. And a diet that doesn’t, like the traditional low-fat diet, won’t work.”

Of the three diet types the team fed to study participants, the low-carbohydrate diet (modeled after the Atkins Diet) lowers insulin the most, Lustig said — and gets the body burning fuel more efficiently. The higher the insulin level, the more energy is stored into fat, the more tired a person feels, and the more he’s likely to eat in an effort to compensate.

Insulin “affects both sides of the equation,” Lustig said.

The research team “gets a big gold star,” he added. “The study is great because it proves the nature of the problem. Insulin is the bad guy.”