Portions, foods in a diet won't determine weight loss, study says

From staff and wire reports

DENVER, Colorado (CNN) -- A preliminary study of three popular diets found that the composition of the eating plans didn't have much to do with how much weight participants lost, the U.S. Department of Agriculture announced Thursday.

The USDA study, which is ongoing, is investigating the long-term effects of several popular diets, including the Atkins, Sugarbusters and Dean Ornish diets.

The diets studied represent a low-carbohydrate, high-protein model like the Atkins diet, which allows users more food but fewer calories; very low-fat diets such as the Ornish diet, which claim to help prevent heart disease; and moderate fat diets, like those promoted by Weight Watchers and Jenny Craig.

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The U.S. government estimates that 18 percent of Americans are obese and 55 percent are overweight. The Centers for Disease Control and Prevention defines obesity as having a body mass index of 30 or higher -- or generally being about 30 pounds overweight.

Diets like the ones being studied have gained in popularity as the U.S. waistline continues to expand. People in the United States spend about $30 billion a year trying to lose weight.

Nutritionists agree the only surefire way to lose weight is to get more exercise and cut calories. But which calories to cut is still up for debate.

Dr. Robert Atkins, who developed the high-protein Atkins diet, says his plan helps redirect the body's metabolism so it burns fat first instead of glucose. He doesn't forego carbohydrates entirely, but insists a low carbohydrate diet helps curb hunger.

At the other end of the spectrum, Dr. Dean Ornish, developer of the low-fat Ornish diet, keeps his plan focused on beans, legumes, fruits, vegetables and grains. He stays away from meat, oils, nuts and other high-fat foods.

USDA Undersecretary Eileen Kennedy said preliminary findings from the study show many factors determined how much weight participants lost, but the foods and amounts included in the diets were not among them.

Researchers also found that people who "self select" a low carbohydrate diet tend to consume fewer calories. "Self selecting" means choosing to go on a diet without being told to do so by a doctor.

Kennedy said researchers still need to examine the long-term effects of weight loss based on each diet, and consider issues like the ability to stick with a diet.

The preliminary study results, which were presented at the American Dietetic Association meeting in Denver, Colorado, are from the first section of the scientific review. The next section's study begins next year, with conclusive results expected in about three years.

CNN Medical Editor Debra Goldschmidt and Reuters contributed to this report.

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U.S. Department of Agriculture

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