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Photograph by Danny Kim for TIME; Gif by Mia Tramz for TIME

Which diet is better for weight loss: low-fat or low-carb? Ask anyone hip to the headlines, and they’ll likely say the latter. A low-carb diet decreases a hormone called insulin, which helps regulate fat tissue—it’s thought that lowering insulin levels gives you a metabolic, fat-burning edge.

“We wanted to test this theory,” says Kevin Hall, PhD, a metabolism researcher at the National Institute of Diabetes and Digestive and Kidney Diseases. His small but rigorous new trial with the US National Institutes of Health (NIH), published in the journal Cell Metabolism, concludes that the theory is flawed—and that a low-fat diet may have more merits than a low-carb diet.

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Any study trying to accurately answer a nutrition question has to get a little obsessive; nutrition research is notoriously difficult to do well. So Hall and his colleagues wanted to design the most rigorous study they could. They recruited 19 obese people who volunteered to stay at the NIH clinical center in a center where every shred of food and every second of exercise was prescribed and monitored by the scientists. Hall wanted to answer a basic question: How does an obese body adapt to cutting carbohydrates from the diet, versus cutting fat from the diet?

“Unless we do the kind of study that we have done here, where we basically lock people up for an extended period of time, control everything, and make sure we know exactly what they eat…then we don’t have the kind of control that’s required to answer these really basic questions,” says Hall.

So for a pair of two-week stays, the volunteers lived in a metabolic ward where they ate the same thing every day for breakfast, lunch, dinner and snacks. Each person tried two different diets identical in calories: one diet cut 30% of their total calories, all coming from reductions in dietary fat while keeping carbohydrates and protein the same, while the other cut calories from carbohydrates, keeping fat and protein the same. “This is the first time a study has ever just selectively reduced these individual nutrients as opposed to changing multiple nutrients at once,” says Hall. Using special equipment, the researchers were able to see exactly how their bodies were burning both calories and body fat.

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People ended up losing weight on both diets, but they lost slightly more on the reduced-carb diet. That didn’t surprise Hall at all. “We’ve known for quite some time that reduction of dietary carbohydrates causes an excess of water loss,” he says, so the weight loss may be due to water loss. As expected—and in keeping with the theory about carb-cutting—insulin levels went down and fat burning went up.

But on the low-fat diet, people lost more fat, “despite not changing insulin one bit,” Hall says.

How is this possible? The exact mechanism is yet to be determined, but Hall has some ideas. “When we cut fat in people’s diets, the body just doesn’t recognize that we’ve done that…in terms of metabolism, so it keeps burning the same number of calories [and fat] as it did before,” he says. This surprised him; Hall thought that the body would somehow respond to the reduction in fat, but it didn’t.

“Insulin is a hormone that is particularly reactive to changes in carbohydrate,” says Hall. “What I was sort of hoping to find was an analogous hormone that was responsive to changes in fat in the diet and altered metabolism.” But they didn’t find it. “It might not exist,” he says.

What they did find was that cutting 800 calories of fat resulted in the body burning just as much fat as before. In contrast, on a low-carb diet, metabolism changes: insulin levels went, carb-burning went down and fat-burning went up, but only by about 400 calories a day, Hall says. That means that low-carb dieters had a net deficit of about 400 calories per day—but those on the low-fat diet had a net deficit of about 800 calories per day, resulting in slightly less body fat.

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Hall cautions against changing your diet based on the results of his study; the differences in fat loss were small, and so were the number of volunteers in the study, due to expense.

“What happens to 19 people on a metabolic ward may not apply to the general population out in the real world who are trying to lose weight,” says Lydia Bazzano, MD, PhD, professor in nutrition research at Tulane University School of Public Health and Tropical Medicine. (Bazzano, who was not involved in this research, co-authored a study last year that followed people for a year and saw that low-carb dieters lost about eight more pounds than low-fat followers.) “It is also difficult to imagine the the physiology of these 19 people represents the diversity found in the U.S. general population,” she says.

More research is needed, Hall says, but “the takeaway for me is that the theory about metabolism that has previously been used to recommend low-carbohydrate diets probably doesn’t hold water.” “In fact, if anything, the reduced fat diet seemed to offer a slight metabolic advantage.”

If metabolism doesn’t necessarily tip the scale in favor of one diet over another, what else will? In his experiments, Hall is currently exploring the possibility that the brain could respond differently to one diet versus another.

Hall’s bottom line is one agreed upon by many nutrition scientists on both sides of the diet divide: the best diet, whether low-carb or low-fat, is the one you will stick to.

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