But can the ketogenic diet help to burn more calories?

There is some evidence that it can. The research is limited and conflicting here too. It may be a very small effect, and not meaningful for weight control. That’s what one study found. In it, 17 obese or overweight volunteers moved into metabolic wards for two months and had every last spoonful of food monitored. (This recounting of the science uses definitional terms like “obese” to be clear about the subjects of research studies.) For the first month, they consumed a high-carb diet; for the second, they had a ketogenic one, with both plans equal in calories.

“We fed them every morsel of food that they ate,” said Kevin Hall, integrative physiology section chief for the National Institute of Diabetes and Digestive and Kidney Diseases’ Laboratory of Biological Modeling. “There were no cheat days.” In the end, though the participants’ insulin levels did decrease while eating the bunless burger, the subjects didn’t lose more fat than when they had bread. The study was limited, though, by having a small sample size, and not having a comparison group that wasn’t on the back-to-back regimens.

For some, a low-carb diet can be appealing. That doesn’t mean that diet is superior, according to a study that followed 609 overweight adults on either a low-carb or a low-fat diet for a year. In the end, both groups shed almost the same amount on average — about 12 to 13 pounds, according to the randomized clinical trial that examined a low-carb diet less restrictive than the keto. The take-home message? “You can succeed on both,” said Christopher Gardner, the lead author and a professor of medicine and nutrition scientist at Stanford Prevention Research Center.

Does the ketogenic diet offer long-term benefits?

It’s not known yet. “If you tell people to go on this diet forever and for a longer term, there is no evidence,” said Dr. Prado, of the University of Alberta who co-authored a narrative review on the ketogenic diet as a possible therapy for cancer.