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.

“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.

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 in 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 other researchers said.