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Americans have long been told that routinely eating breakfast is a simple habit that helps prevent weight gain.

Skipping breakfast, the thinking goes, increases hunger throughout the day, making people overeat and seek out snacks to compensate for missing that first – and some would say most important – meal of the day.

“Eating a healthy breakfast is a good way to start the day,” according to the Web site of the United States surgeon general, “and may be important in achieving and maintaining a healthy weight.”

But new research shows that despite the conventional weight-loss wisdom, the idea that eating breakfast helps you lose weight stems largely from misconstrued studies.

Only a handful of rigorous, carefully controlled trials have tested the claim, the new report, published in The American Journal of Clinical Nutrition, found. And generally they conclude that missing breakfast has either little or no effect on weight gain, or that people who eat breakfast end up consuming more daily calories than those who skip it.

But those trials have been largely overlooked, and their findings drowned out by dozens of large observational studies that have found associations between breakfast habits and obesity but no direct cause and effect, said Dr. David B. Allison, director of the Nutrition Obesity Research Center at the University of Alabama at Birmingham. Dr. Allison and his colleagues scoured the medical literature and found that the only long-term, carefully controlled trial that randomly assigned people to routinely eat or go without breakfast and then measured the effect on their body weight was published in 1992.

That seminal study, carried out over 12 weeks at Vanderbilt University, had mixed results. Moderately obese adults who were habitual breakfast skippers lost an average of roughly 17 pounds when they were put on a program that included eating breakfast every day. And regular breakfast eaters who were instructed to avoid eating breakfast daily lost an average of nearly 20 pounds.

Both programs included an identical amount of calories, and each caused people to lose more weight than a program in which a person’s typical breakfast habits did not change.

The study was fairly small and limited, involving only 52 overweight adult women, but it suggested that as far as breakfast is concerned, the most important factor in weight loss may be how drastically you change your routine. “Those who had to make the most substantial changes in eating habits to comply with the program achieved better results,” the authors wrote in their paper.

Dr. Allison said that the findings “showed no effect over all of eating versus skipping breakfast, that people do equally well on either one.”

“You would think at this point that you would either abandon the idea or do some more randomized controlled trials,” he added. “But instead the association studies started.”

Through the years, the equivocal findings were wildly misinterpreted. Dr. Allison and his colleagues found about 50 subsequent articles on breakfast and body weight in the medical literature that cited the Vanderbilit research. Of those papers, 62 percent cited the findings inaccurately, and they were almost exclusively biased in favor of the idea that eating breakfast protects against weight gain.

Another study that became the basis of widespread misinformation was published in 2002. In that study, researchers looked at data from the National Weight Control Registry, which tracks thousands of people who have lost weight and kept it off for at least a year.

Data from the registry showed that after their weight loss, about 80 percent of people reported regularly eating breakfast. “There was no difference in reported energy intake between breakfast eaters and non-eaters,” the registry showed, “but breakfast eaters reported slightly more physical activity than non-breakfast eaters.”

The research showed only that eating breakfast was a common behavior among people who were actively trying to avoid regaining weight, just as diet soda might be a common drink of choice among dieters but not necessarily the cause of their weight loss.

But of 72 subsequent research articles on breakfast and weight loss that cited the registry study, about half overstated its findings, Dr. Allison found, and roughly a quarter suggested that it showed a causal relationship between breakfast habits and obesity.

In the real world, when people form an opinion, they tend to seek out evidence that supports it and discard anything that contradicts it, a phenomenon academics refer to as confirmation bias.

“Scientists are humans, and they’re susceptible to confirmation bias too,” Dr. Allison said.

In the meantime, a small number of randomized trials has continued to cast doubt on the protective effect of breakfast. At Cornell University, for example, scientists have showed in experiments that in some cases, but not all, depriving people of breakfast can lead them to eat more calories at lunch. But those extra calories do not make up for the calories they missed at breakfast, so at the end of the day, they still end up eating fewer calories over all.

The Cornell researchers have argued that for some adults, skipping breakfast may actually be a good way to reduce weight – not gain it.

Dr. Allison said that the true relationship between eating breakfast and body weight, if there is one, was still an open question. But observational studies that tout an association between the two are churned out “just about every week,” despite doing nothing to actually test or prove the claim.

“At some point, this becomes absurd,” he said. “We’re doing studies that have little or no value. We’re wasting time, intellect and resources, and we’re convincing people of things without actually generating evidence.”

As for why the subject has created something of an echo chamber of observational research, Dr. Allison said that unlike randomized controlled trials, which are expensive and difficult to carry out, sifting through large sets of observational data to find tantalizing associations is fairly low cost and easy to do.

“Just like bakers bake bread, scientists write papers,” he said, “and we get rewarded for writing and publishing papers.”