Dieters who want to keep from regaining the pounds they so painstakingly lost would do best to eat low-fat diets rather than curb carbs, new research suggests.

A study presented at a meeting of more than 2,000 obesity experts found that it did not matter what kind of diet people followed to lose weight initially, but keeping from regaining it was another matter.

A low-fat diet "continues to be the key characteristic" of long-term success, said Suzanne Phelan, a Brown University Medical School psychologist who led the work.

Consumers increasingly are losing their enthusiasm for Atkins-style diets as a long-term weight-control strategy and the glut of low-carb products that overloaded grocery store shelves as the diet became a fad in the past few years.

More than half of Americans who have tried a low-carb diet have given up, according to a recent survey by the market research firm InsightExpress. Other published survey information suggests that the number of Americans following such a diet peaked at 9% last February and fell to 6% by June.

The new study involved 2,700 people in the National Weight Control Registry, which tries to mine secrets of success from people who have lost at least 30 pounds and kept it off for at least a year.

The registry was started a decade ago by doctors from the University of Colorado in Denver, the University of Pittsburgh, and Brown University in Providence, R.I.

Doctors compared the diets of those who entered the registry from 1995 through 2003. Their average age was 47, most were women, and they had lost an average of 72 pounds initially.

All reported eating only about 1,400 calories a day, but the portion that came from fat rose, from 24% in 1995 to more than 29% in 2003, while the part from carbohydrates fell, from 56% to 49%.

The number who were on low-carb diets (less than 90g a day) rose from 6% to 17% during the same period.

Those who increased their fat intake over the year after their initial weight loss regained the most weight. That meant they ate less carbohydrates, because the amount of protein in their diets stayed the same, Phelan said.

"Only a minority of successful weight losers consume low-carbohydrate diets," she and the other researchers concluded.

Colette Heimowitz, a nutrition expert and spokeswoman for the Atkins diet organization, noted that the study considered 90g to be low-carb, while Atkins recommends 60g for weight loss and 60 to 120 for weight maintenance.

She said that, for many of the dieters studied, "the carbs aren't low enough for them to be successful." They also should have replaced carbs with more protein rather than fat, she said.

Thomas Wadden, M.D., a University of Pennsylvania weight loss expert who had no role in the study, said it is too soon to say which approach is better. Several longer-term studies of low-carb and low-fat dieters are in the works, he said.

However, he said, "I do think that people who are keeping the weight off are eating a low-fat, high-carb diet."

William Dietz, M.D., director of chronic-disease prevention at the federal Centers for Disease Control and Prevention (CDC), said it is difficult to tell whether these low-carb diets really work, because people find it hard to stick to a strict regimen for long periods.

"My anecdotal experience is that people go on and off these diets," he said. "When their weight goes up, they go back on the diet to lose weight."

Meanwhile, the American Institute for Cancer Research issued a statement in September urging dieters to "come back to common sense."

"Eat a balanced diet weighted toward vegetables and fruits, reduce portion sizes and increase physical activity," the institute said.