Dieters everywhere -- yo-yo dieters, crash dieters, fad dieters, dejected and jaded dieters -- may take heart.
A new study offers a flexible take on what it means to diet while underscoring fundamental rules about weight loss.
The study, published in the New England Journal of Medicine, suggests the type of weight-loss diet doesn't matter as much as sticking to it.
Researchers compared a low-fat, average protein diet, a low-fat, high-protein diet, a high-fat, average-protein diet, and a high-fat, high-protein diet in 811 middle-aged obese people over two years. Each group lost an average of 13 pounds after one year, though slowly started to gain it back in the second year, bringing the average lost after two years to nine pounds.
"In the end, all the diets produced weight loss to the same extent," said Dr. Frank Sacks, lead author of the study and professor in the department of nutrition at the Harvard School of Public Health. "It really doesn't matter what particular amounts of fat, carbohydrates or proteins you eat."
Cut Calories, Cut Weight
The diets for each group were calibrated to the calories necessary for each participant to reach their ideal body-mass index -- which meant restricting calories overall.
"There really wasn't magic revealed in the specific composition of the diet," said Dr. Howard Eisenson, executive director of the Duke Diet and Fitness Center. "I thought their conclusions made sense and jibed with what many of us in the field have long been thinking."
In addition to weight loss, subjects reduced their risk of heart disease and diabetes and improved their blood pressure.
The researchers used daily food reports, as well as one-on-one and group sessions with a dietician, to keep participants on track.
"All of that created a kind of synergy for keeping you motivated and informed," said Rudy Termini, 69, one of the study participants who ate a high-fat, moderate-protein diet totaling 1,800 calories each day.
Termini was more successful than the average study participant, dropping to 175 pounds from 195 pounds, a loss he since has maintained.
But the dietary changes weren't easy. Termini was an anomaly for maintaining his diet given that his pre-study diet was very high in protein. One of the study findings was that cutting calories was easier for those who ate fats, carbohydrates and proteins in the same proportions as they usually did.
"You can focus more on reducing calories rather than switching to a completely different diet type," Sacks said. "The more challenging diets were diets that departed a lot from usual intake."
A Cultural Challenge
The challenge proved too great for some. Termini said that about 10 people completed the two-year study in his focus group of 15.
Catherine Loria, a nutritional epidemiologist at the National Heart Lung and Blood Institute and another author on the study, pointed out that only 15 percent of the participants were able to reduce their body weight by 10 percent after the two years and that there were some who gained more weight than they initially lost.
Not everyone was impressed with the study's findings.
An editorial in the same journal challenges the study's optimistic tone, pointing out that subjects were not sticking to the specified calorie allowance based on their eventual weight gain, nor were they eating meals in the specified proportions of nutrients.
Tests showed that, over time, the protein intake between the high-protein diet and the average-protein diet groups, which should have differed by 10 percent, dwindled to a 1 to 2 percent difference. The highest and lowest carbohydrate diet groups, which should have differed in carbohydrate intake by 30 percent, differed by only 6 percent.
Martijn Katan, the author of the editorial, said the data suggest that failure to lose weight is a behavioral problem bolstered by the nutritional environment of the U.S.
"We do not need another diet trial; we need a change of paradigm," Katan said.
While the study authors agreed that creating a healthier food environment is a crucial goal in a country where 78 million people are obese -- 12.5 million of whom are children, according to the Centers for Disease Control and Prevention -- they denied Katan's statement that individual efforts at weight loss are not significant.
"We're never going to have a perfect environment and people have to be willing to make those changes," Loria said. "I don't think you can be successful unless you have both."
In terms of dieting, community support can be critical but choosing a plan is up to an individual, and the hardest part of losing weight may be accepting the hunger factor.
Past studies have shown that calorie restriction is the biggest determining factor for successful weight loss, even greater than exercise.
"There are those with unrelenting optimism, who think [dieting] is going to get easier," said Marlene Schwartz, deputy director of the Rudd Center for Food Policy and Obesity at Yale University. "But if you are eating fewer calories than you are burning, you are going to be hungry."
But the study's findings encourage experimentation, Sacks said, and the flexibility to find the best balance of calorie restriction and satisfaction for individuals.
"The hardest thing, having been born and raised in America, [was that] I thought I was entitled to eat as much as I wanted whenever I wanted and I would stay the same weight," Termini said. "But you can't do that. And it's not healthy."
ABC News' Lauren Cox contributed to this report.
Have more questions on diets, nutrition and exercise? Visit the ABCNews.com OnCall+ Wellness Center to get your answers.