DALLAS – A side-by-side comparison of four popular diets – Atkins low-carb, Ornish low fat, the Zone and Weight Watchers – has found no difference in each plan’s weight loss potential. When the fat-free chips are down, the type of diet doesn’t seem to matter nearly as much as whether people actually follow it.

“I was kind of surprised at how similar they all looked,” said Dr. Michael Dansinger, who conducted the research with his colleagues at Tufts-New England Medical Center in Boston. The results appear in Wednesday’s issue of the Journal of the American Medical Association.

No diet, it seems, can escape the laws of thermodynamics. There is no magic: Scale back the calories, or use more of them than you take in, to shed pounds. The four diet approaches studied, Dansinger said, “all end up reducing calories.”

At the start of the research, the Tufts team assigned 160 volunteers to follow one of the four diet approaches. After a year, average weight loss among the 40 participants in each plan was about 5 to 7 pounds.

But dieting, as dieting veterans will understand, proved difficult. About half the participants in the Atkins and Ornish plans dropped out before the year was up, as did about 35 percent of the Zone and Weight Watchers group.

The findings are, in the words of one scientist, the kind of “no duh” conclusions that may nevertheless startle many dieters.

“It’s giving people a slap in the face, one that we scientists have been trying to give for a long time,” said Ruth Ann Carpenter of the Cooper Institute in Dallas. But to those in the trenches, she said, “the results are not at all stunning.”

Carpenter was disappointed that the study did not address the importance of exercise.

Still, it is more diet data than scientists had before.

A second, unrelated study published Tuesday highlighted the dearth of evidence that any commercial weight loss program actually works.

Researchers from the University of Pennsylvania School of Medicine combed the scientific literature for research on some of the country’s most advertised diet programs.

Jenny Craig and LA Weight Loss have no published scientific evidence that address their program’s effectiveness. “The bottom line is that Weight Watchers is by far the best-studied program,” said Thomas Wadden, head of Penn’s Weight and Eating Disorders Program.

That doesn’t mean, however, that everyone should be a Weight Watcher. The encouraging news from the Tufts study, Dansinger said, is that one size doesn’t fit all. With so many options, people can find the one they are more likely to adhere to, which is key to success.

His research found that the more rigorously people stuck to the diets, the more they lost. Those who stayed in the study for the full year, trying to follow the rules, lost about 10 to 12 pounds. And those who followed the diets most strictly lost about 15 pounds on average. In the analysis the researchers assumed that those who dropped out of the study did not lose weight.

One reason people get discouraged, experts say, is that weight is gained slowly, while people want to lose it quickly. That sets many dieters up with unrealistic expectations.

Studies find that people tend to gain weight over years or even decades because they take in only 100 or so calories a day too much. That translates to one extra Coke, a couple of cookies, a few extra globs of creamy dressing. Most obesity experts believe that weight should be lost more akin to way it is gained, a “Low Fad” approach, according to Dr. Robert Eckel of the University of Colorado at Denver. For example, he said, those with a taste for sodas or juice could eliminate the sugary drinks with hardly flinching.

By that equation, “two beverages a day less would be 25 pounds a year,” Eckel said.

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In a commentary published with Wednesday’s study, Eckel calls for more research into lasting results. Few diet studies have spanned more than a year.

And dieting is more complicated that just willpower. Once a person reaches a certain weight, the body starts to defend its size. “The brain thinks that’s where your fat mass should be,” he said. Researchers need to learn more about the internal mechanics of weight gain and loss to help people overcome the physiology that works against them, Eckel said.

In the meantime, Penn’s Wadden said, people wanting to lose weight should consider what they might be able to do over the long term. Like Eckel, he advises starting out with almost unnoticeable changes: smaller portions, more movement during the day, fewer sweet drinks. Even small losses should be considered progress. “Ten pounds counts,” he said. “Usually people focus on what they have not lost rather than what they have.”

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