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Americans who eat a low-fat diet often don't lose weight because they eat too many carbohydrates out of ignorance about the relative importance of fat and calories, researchers say.

"When offered high-fat diets, people tend to consume more calories than when offered low-fat diets," said James Hill of the University of Colorado in Denver. But that does not mean that people on a low-fat diet can eat as much as they want and still lose weight, he said."I would argue we're not going to treat obesity by reducing dietary fat alone," Hill said. A lot of people remain overweight because "they took our advice and ate a low-fat diet, but they're eating too much of it," Hill said.

"People are putting forth simplistic messages," said Barbara Rolls, a nutrition professor at Pennsylvania State University. "Is it calories? Is it fat? It's both."

Researchers also say nutrition labels on food might actually be worsening Americans' diets as people who make low-fat choices some of the time overcompensate by eating too much food at other times.

"There's no question people are doing that," said Richard Mattes, a nutrition professor at Purdue University.

Low-fat foods are presumed to be potentially beneficial for people trying to lose weight and cut the risk of disease. However, researchers have not adequately studied the implications of providing nutrition information, Mattes said.

He studied 17 men and women who were given a fixed meal at lunch, then asked to keep a record of what they ate the rest of the day. When they were told the lunch was a low-fat meal, they increased their consumption during the rest of the day, Mattes found. When told it was a normal lunch, they ate less during the rest of the day, though the content of the lunch didn't change.

Rolls did a separate study in which she varied the fat and calorie content of a yogurt snack given before lunch.

When she told people they were getting a low-fat snack, they increased what they ate at lunch.

The point of the studies is that the amount people eat is determined partly by their knowledge of their food, not simply by the body's regulation of hunger and satiety.