Regardless of how they shed pounds in the first place, big losers stayed that way by limiting fat rather than carbohydrates, according to new research that could add fuel to the backlash against low-carb diets.

Dieters already have been turning away from Atkins-style plans as a long-term weight-control strategy, and the new study gives them more reason: Low-fat plans seem to work better at keeping weight off.

"People who started eating more fat ... regained the most weight over time," said Suzanne Phelan, a Brown Medical School psychologist who presented results of the study Monday at a meeting of the North American Association for the Study of Obesity.

The study used the National Weight Control Registry, a decade-old effort to learn the secrets of success from people who had lost at least 30 pounds and kept them off for at least a year. The registry run by doctors from the University of Colorado in Denver, the University of Pittsburgh and Brown University in Providence, R.I.

They studied 2,700 people 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. Doctors compared their diets to see whether one type or another made a difference in how much weight they had lost and how much they had regained a year later.

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

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

The type of diet — low-fat, low-carb or in between — made no difference in how people lost weight initially.

But those who increased their fat intake over a year 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 90 grams to be low-carb, while Atkins recommends 60 grams 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.

Dr. Thomas Waddell, 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.

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

The dietary establishment has long been skeptical of the long-term safety and effectiveness of low-carb diets, and consumers increasingly are losing their enthusiasm for 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 percent last February and fell to 6 percent by June.

The American Institute for Cancer Research used those trends to issue 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.

Dr. William Dietz, director of chronic-disease prevention at the federal Centers for Disease Control and Prevention, 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."

Other research at the conference underscored the many health and personal problems obese people face.

Duke University doctors said two-thirds of obese people seeking treatment at the Duke Diet & Fitness Center reported not enjoying or wanting to have sex, and having problems with sexual performance. Only 5 percent of normal-weight people from the surrounding community who completed the same quality-of-life survey reported such problems.