Losing weight is hard, but for many people, keeping off the pounds over the longer term is even more difficult.

Now, new research has attempted to shed some light on why: It found that certain popular diets are better than others at boosting the rate at which the body burns calories. And that, the researchers argued, could have implications for how successful people are at keeping extra pounds at bay.

"The results indicate that from a metabolic perspective, all calories are not alike," paper co-author Dr. David Ludwig, director of the New Balance Foundation Obesity Prevention Center at the Boston Children's Hospital told The Huffington Post.

For the new study, published on Tuesday in the Journal of the American Medical Association, researchers recruited 21 overweight and obese young adults who had already shed 10 percent to 15 percent of their body weight. The researchers randomly placed participants for four weeks at a time on a succession of three popular eating plans: a low-fat diet, a low-carb diet and a low-glycemic index diet. With a low-glycemic diet, someone eats only certain types of carbohydrates to help regulate blood sugar levels.

Prior research has suggested that weight loss can decrease a person's rate for burning calories. According to the new study's authors, this might help explain why only 1 in 6 overweight or obese adults who have lost 10 percent of their body weight can maintain that reduction for a year.

For this study, the low-fat plan triggered the biggest decline in an overall calorie-burning rate, which could mean less weight loss. On that plan, participants received 60 percent of their daily calories from carbs, 20 percent from fat and 20 percent from protein.

"The research subjects burned about 350 calories per day more on the low-carb diet than on the low-fat, even though they were consuming the same number of calories," Ludwig explained. "That's roughly equal to an hour of moderate physical activity without lifting a finger. On the low-glycemic [plan], they burned about 150 calories per day more than the low-fat diet."

But the study also showed negative effects associated with the low-carb diet, which limited participants to receiving 10 percent of their daily calories from carbs.

The low-carb diet increased levels of the hormone cortisol, which can lead to insulin resistance and heart disease. It also boosted the levels of certain proteins that have been linked to heart disease over the long term.

Ludwig cautioned that any diet plan that drastically reduces a major class of nutrients like fat or carbs might be difficult to stick to because it is so restrictive, thereby undermining long-term maintenance of a lower weight.

The new findings suggest benefits, however, to paying attention to the quality of fats and carbohydrates consumed, he said.

"The dogma in nutrition today is that it's calories in, calories out and all calories are alike," Ludwig said. "This is saying something quite novel. It's saying that the quality of the calories going in affects the number of calories going out."

But the findings should be regarded with caution.

Karen Reznik Dolins, a professor of nutrition and physical education at Columbia University, said the new research does not definitively prove the idea that different food plans lead to significant differences in energy expenditure. The study, which she called “provocative,” was limited in size and duration but did reveal differences between individuals in how a diet affected energy expenditure, she said.

For now, further research is necessary before people go changing their eating patterns, Dolins said.

"What I worry about with studies like this, is that they get misinterpreted and people say, 'Very low carb is the way to go,'" Dollins said. "I don't think this study shows that. They're showing that weight loss was maintained regardless of treatment, and that the markers we have for cardiovascular disease were actually higher [in the low-carb] group."