We spend lots of time and money trying to lose weight. By some estimates, 45 million people in the U.S. alone go on a diet every year and spend $33 billion on weight loss products doing it.

But despite all our effort and expense, we’re often not successful. Even if we do lose weight, the pounds may come back after just a few months, and we’re left wondering if the problem is our choice of diet. Is it better to stick with a diet that’s low-fat, high carb, plant-based and filled with lots of vegetables? Or, should we opt for an animal-based, low carb diet that’s packed with protein?

Is it better to stick with a diet that’s low-fat, high carb, plant-based and filled with lots of vegetables? Or, should we opt for an animal-based, low carb diet that’s packed with protein?

That’s a question the researchers at the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) wanted to answer. In order to figure it out, they housed 20 adults for four weeks in the National Institute of Health’s (NIH) Clinical Center’s Metabolic Clinical Research Unit. The non-diabetic volunteers (11 men and 9 women) received either a plant-based and low-fat diet or a low-carb, animal-based, “keto” diet. Then, after two weeks, they switched and ate the alternate diet. Participants in both groups ate three meals a day, plus snacks, and could eat as much as they wanted during the study.

The results? Folks on the low-fat diet ate 500 to 700 calories less per day than those on the low-carb diet. But even though there was a difference in calories, they reported no differences in feelings of hunger, enjoyment of fare or satiation. What’s more, both sets of volunteers lost weight. The biggest difference? Only the low-fat diet resulted in significant loss of body fat.

“Despite eating food with an abundance of high glycemic carbohydrates that resulted in pronounced swings in blood glucose and insulin, people eating the plant-based, low-fat diet showed a significant reduction in calorie intake and loss of body fat which challenges the idea that high-carb diets per se lead people to overeat,” Kevin Hall, the study’s lead author, said in a statement. “On the other hand, the animal-based, low-carb diet did not result in weight gain despite being high in fat.”

Since the researchers are quick to note that their study was not designed to make diet recommendations, they only concluded that their findings suggest that factors such as overeating and weight gain are more complicated than the amount of fat or carbohydrates in our diet.

“Interestingly, our findings suggest benefits to both diets at least in the short term,” Hall added. “While the low-fat plant-based diet helps curb appetite, the animal-based low-carb diet resulted in lower and more steady insulin and glucose levels.”

More research is probably on the table.

The study is published in Nature Medicine.