LAS VEGAS – 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 yesterday 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% 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 90 grams a day) rose from 6% to 17% during the same period.

The type of diet – low-fat, lowcarb, 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, Ms. 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 Wadden, 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.”