Risk factors for heart disease, such as cholesterol and blood sugar problems, improved as well. Experts say this shows the exact nutrients in a diet aren't important; they can all be effective if you limit calories and stick with the program.
As the world grapples with rising obesity, millions have turned to popular diets like Atkins, Zone and Ornish that tout the benefits of one nutrient over another.
Some previous studies have found that low carbohydrate diets like Atkins work better than a traditional low-fat diet. But the new research found that the key to losing weight boiled down to a basic rule -- calories in, calories out.
"The hidden secret is it doesn't matter if you focus on low-fat or low-carb," said Dr. Elizabeth Nabel, director of the National Heart, Lung and Blood Institute, which funded the research.
Limiting the calories you consume and burning off more calories with exercise is key, she said.
The study, which appears in Thursday's New England Journal of Medicine, was led by Harvard School of Public Health and Pennington Biomedical Research Center in Louisiana.
Researchers randomly assigned 811 overweight adults to one of four diets. Though the diets were twists on commercial plans, the study did not directly compare popular diets. The four diets contained healthy fats, were high in whole grains, fruits and vegetables and were low in cholesterol.
Nearly two-thirds of the participants were women. Each dieter was encouraged to slash 750 calories a day from their diet, exercise 90 minutes a week, keep an online food diary and meet regularly with diet counselors to chart their progress.
There was no winner among the different diets; reduction in weight and waist size were similar in all groups.
All groups saw their weight creep back up after a year. At two years, the average weight loss was about 9 pounds while waistlines shrank an average of 2 inches. Only 15 percent of dieters achieved a weight-loss reduction of 10 percent or more of their starting weight.
Dieters who got regular counseling saw better results. Those who attended most meetings shed more pounds than those who did not -- 22 pounds compared with the average 9 pound loss.
Lead researcher Dr. Frank Sacks of Harvard said a restricted calorie diet gives people greater food choices, making the diet less monotonous.
"They just need to focus on how much they're eating," he said.
Sacks said the trick is finding a healthy diet that is tasty and that people will stick with over time.
Before Debbie Mayer, 52, enrolled in the study, she was a "stress eater" who would snack all day and had no sense of portion control. Mayer used to run marathons in her 30s, but health problems prevented her from doing much exercise in recent years.
Mayer tinkered with different diets -- Weight Watchers, Atkins, South Beach -- with little success.
"I've been battling my weight all my life. I just needed more structure," said Mayer, of Brockton, Mass., who works with the elderly.
Mayer was assigned to a low-fat, high-protein diet with 1,400 calories a day. She started measuring her food and went back to the gym. The 5-foot Mayer started at 179 pounds and dropped 50 pounds to 129 pounds by the end of the study. She now weighs 132 and wants to shed a few more pounds.
Another study volunteer, Rudy Termini, a 69-year-old retiree from Cambridge, Mass., credits keeping a food diary for his 22-pound success. Termini said before participating in the study he would wolf down 2,500 calories a day. But sticking to an 1,800-calorie high-fat, average protein diet meant no longer eating an entire T-bone steak for dinner. Instead, he now eats only a 4-ounce steak.
"I was just oblivious to how many calories I was having," said the 5-foot-11-inch Termini, who dropped from 195 to 173 pounds. "I really used to just eat everything and anything in sight."
Dr. David Katz of the Yale Prevention Research Center and author of several weight control books, said the results should not be viewed as an endorsement of fad diets that promote one nutrient over another.
The study compared high quality, heart healthy diets and "not the gimmicky popular versions," said Katz, who had no role in the study. Some popular low-carb diets tend to be low in fiber and have a relatively high intake of saturated fat, he said.
Other experts were bothered that the dieters couldn't keep the weight off even with close monitoring and a support system.
"Even these highly motivated, intelligent participants who were coached by expert professionals could not achieve the weight losses needed to reverse the obesity epidemic," Martijn Katan of Amsterdam's Free University wrote in an accompanying editorial.
The Associated Press contributed to this report.