The study was small, consisting of just 34 overweight adults who either ate the recommended diet for three months; ate the recommended diet and exercised regularly; or ate pretty much what they usually eat.
All meals were prepared for participants, who were instructed to eat as much as they wanted. They also were told to return any uneaten food, which the researchers said enabled them to calculate calorie intake.
Many doctors dispute whether people can lose weight without reducing their food intake, and at least one questioned the study's accuracy.
But the diet is more compatible with conventional notions of healthful eating than the fatty, low-carbohydrate Atkins and South Beach diets.
Participants on the recommended diet lost about 7 pounds without cutting calories and without exercise, and almost 11 pounds with 45 minutes of stationary bike-riding four times weekly. The control group lost no weight.
The findings appear in Monday's Archives of Internal Medicine.
Gary Foster, clinical director of the University of Pennsylvania's Weight and Eating Disorders Program, said he suspects participants who lost weight ate less than what was reported. He said that while he recommends a low-fat, high carb diet to patients, without calorie reduction it would be "a public health disaster."
"The whole idea that you could lose weight without reducing energy intake flies in the face of 100 years of data," Foster said.
Lead author William Evans of the University of Arkansas for Medical Sciences stood by his findings.
"Calories in, minus calories out, does not always determine the amount of weight loss," Evans said. "This is because we metabolize fats and carbohydrates very differently."
American Dietetic Association spokeswoman Cindy Moore agreed and said with low-carb diets hogging the spotlight, "it may be a reminder that we can lose weight in a variety of different ways."
Foods on the successful diets included high-fiber cereal, vegetarian chili, whole-wheat spaghetti, many fruits and vegetables, and skim milk. Daily calories totaled about 2,400, similar to participants' usual consumption.
The control group also received prepared meals with similar calories, but the foods included sausage, scrambled eggs, macaroni and cheese, French fries, whole milk and fewer fruits and vegetables.
The successful diet was not tested against Atkins and other low-carb regimens, which contain more fat and fewer carbs than the control group diet.
By Lindsey Tanner
By Lindsey Tanner