Low-fat diets, and diets that restrict one category of nutrients in general, do a poor job of reducing weight and keeping it off for a year or more, according to a new review of clinical trials.

"Over the last several years and decades (low-fat) had been recommended as the way to lose and keep off weight, but obesity rates only increased," said lead author Dr. Diedre K. Tobias of Brigham and Women's Hospital in Boston. "It was clearly not doing much to change the obesity epidemic."

Evidence of the success of diets based on nutrient restriction, like low-fat and low-carb diets, has been mixed, Tobias told Reuters Health.

"Our study shows that it needs to move beyond discussion of low-fat and low-carb," she said.

Over one year or more, low-carb diets yield a couple more pounds of weight loss on average, but that's not meaningful for most people, she said.

The reviewers included 53 randomized controlled trials comparing low-fat and higher-fat dietary interventions that collected weight change data for at least one year. The studies included a total of more than 68,000 people.

Low-carb diets tended to lead to about 1.15 kilograms, or 2.5 pounds, more weight loss than low-fat diets in 18 comparisons. Low-fat diets only won out in weight loss when compared to a group of people who were not dieting.

Low-fat diets were no more effective than other higher-fat diets. Low-fat and higher-fat interventions also had similar effects in weight maintenance trials, according to the results published in The Lancet.

"They restricted diets to prescriptions at least one year in duration, and on that timeframe there was rather disappointing long-term weight loss," said Dr. Kevin D. Hall of the National Institute of Diabetes and Digestive and Kidney Diseases in Bethesda, Maryland, who wrote an editorial accompanying the results.

"People start diets and do relatively well when they first begin over a month or two, then slowly lose steam as they relapse back to the way they used to eat," Hall told Reuters Health.

People should try to choose a diet they can stick to long-term rather than treating it like a short-term fix, he said.

"It's not that low-fat diets are particularly bad, but they're no more effective than others," he said.

However, within any one study, some individuals were very successful while others did not lose any weight, or even gained weight, so the overall average weight loss was small, he noted.

"Moving beyond measuring percent of calories from fat or carbs, which we essentially found was irrelevant, will lead us to a discussion of overall foods," Tobias said.

"It will hopefully steer us away from processed foods and limit red and processed meats, so then we don't need to focus so much on fats and carbs," she said.

Focusing on restricting certain nutrients over the long-term can be hard to maintain, which makes low-fat and low-carb diets less successful, she said.

"Originally, when the low-fat message was promoted there wasn't a lot of talk about what do we replace fat with, and we replaced them with a slew of sugar and sodium," when they should have been replaced with fruits, vegetables and whole grains, Tobias said.

"Bad fats," like the saturated fat in dairy and meat and trans fats in shelf-stable pastries and muffins, are still bad fats and should be avoided, she said.

"That is really important for heart health," Tobias said.

But avoiding all fats doesn't seem to be the best strategy for long-term weight loss, she said.