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If schools reinstated physical education classes, a lot of fat children would lose weight. And they might never have gotten fat in the first place if their mothers had just breast fed them when they were babies. But be warned: obese people should definitely steer clear of crash diets. And they can lose more than 50 pounds in five years simply by walking a mile a day.

Those are among the myths and unproven assumptions about obesity and weight loss that have been repeated so often and with such conviction that even scientists like David B. Allison, who directs the Nutrition Obesity Research Center at the University of Alabama at Birmingham, have fallen for some of them.

Now, he is trying to set the record straight. In an article published online today in The New England Journal of Medicine, he and his colleagues lay out seven myths and six unsubstantiated presumptions about obesity. They also list nine facts that, unfortunately, promise little in the way of quick fixes for the weight-obsessed. Example: “Trying to go on a diet or recommending that someone go on a diet does not generally work well in the long term.”

Obesity experts applauded this plain-spoken effort to dispel widespread confusion about obesity. The field, they say, has become something of a quagmire.

“In my view,” said Dr. Jeffrey M. Friedman, a Rockefeller University obesity researcher, “there is more misinformation pretending to be fact in this field than in any other I can think of.”

Others agreed, saying it was about time someone tried to set the record straight.

“I feel like cheering,” said Madelyn Fernstrom, founding director of the University of Pittsburgh Weight Management Center. When it comes to obesity beliefs, she said, “We are spinning out of control.”

Steven N. Blair, an exercise and obesity researcher at the University of South Carolina, said his own students believe many of the myths. “I like to challenge my students. Can you show me the data? Too often that doesn’t come into it.”

Dr. Allison sought to establish what is known to be unequivocally true about obesity and weight loss.

His first thought was that, of course, weighing oneself daily helped control weight. He checked for the conclusive studies he knew must exist. They did not.

“My goodness, after 50-plus years of studying obesity in earnest and all the public wringing of hands, why don’t we know this answer?” Dr. Allison asked. “What’s striking is how easy it would be to check. Take a couple of thousand people and randomly assign them to weigh themselves every day or not.”

Yet it has not been done.

Instead, people often rely on weak studies that get repeated ad infinitum. It is commonly thought, for example, that people who eat breakfast are thinner. But that notion is based on studies of people who happened to eat breakfast. Researchers then asked if they were fatter or thinner than people who happened not to eat breakfast — and found an association between eating breakfast and being thinner. But such studies can be misleading because the two groups might be different in other ways that cause the breakfast eaters to be thinner. But no one has randomly assigned people to eat breakfast or not, which could cinch the argument.

So, Dr. Allison asks, why do yet another study of the association between thinness and breakfast? “Yet, I can tell you that in the last two weeks I saw an association study of breakfast eating in Islamabad and another in Inner Mongolia and another in a country I never heard of.”

“Why are we doing these?” Dr. Allison asked. “All that time and effort is essentially wasted. The question is: ‘Is it a causal association?’” To get the answer, he added, “Do the clinical trial.”

He decided to do it himself, with university research funds. A few hundred people will be recruited and will be randomly assigned to one of three groups. Some will be told to eat breakfast every day, others to skip breakfast, and the third group will be given vague advice about whether to eat it or not.

As he delved into the obesity literature, Dr. Allison began to ask himself why some myths and misconceptions are so commonplace. Often, he decided, the beliefs reflected a “reasonableness bias.” The advice sounds so reasonable it must be true. For example, the idea that people do the best on weight-loss programs if they set reasonable goals sounds so sensible.

“We all want to be reasonable,” Dr. Allison said. But, he said, when he examined weight-loss studies he found no consistent association between the ambitiousness of the goal and how much weight was lost and how long it had stayed off. This myth, though, illustrates the tricky ground weight-loss programs have to navigate when advising dieters. The problem is that on average people do not lose much – 10 percent of their weight is typical – but setting 10 percent as a goal is not necessarily the best strategy. A very few lose a lot more and some people may be inspired by the thought of a really life-changing weight loss.

“If a patient says, ‘Do you think it is reasonable for me to lose 25 percent of my body weight,’ the honest answer is, ‘No. Not without surgery,’” Dr. Allison said. But, he said, “If a patient says, ‘My goal is to lose 25 percent of my body weight,’ I would say, ‘Go for it.’”

Yet all this negativism bothers people, Dr. Allison conceded. When he talks about his findings to scientists, they often say: “O.K., you’ve convinced us. But what can we do? We’ve got to do something.” He replies that scientists have an ethical duty to make clear what is established and what is speculation. And while it is fine to recommend things like bike paths or weighing yourself daily, scientists must make sure they preface their advice with the caveat that these things seem sensible but have not been proven.

Among the best established methods is weight-loss surgery, which, of course, is not right for most people. But surgeons have done careful studies to show that on average people lose substanial amounts of weight and their health improves, Dr. Allison said. For dieters, the best results occur with structured programs, like ones that supply complete meals or meal replacements.

In the meantime, Dr. Allison said, it is incumbent upon scientists to change their ways. “We need to do rigorous studies,” he said. “We need to stop doing association studies after an association has clearly been demonstrated.”

“I never said we have to wait for perfect knowledge,” Dr. Allison said. But, as John Lennon said, “Just give me some truth.”

Here is an overview of the obesity myths looked at by the researchers and what is known to be true:

MYTHS

Small things make a big difference. Walking a mile a day can lead to a loss of more than 50 pounds in five years.

Set a realistic goal to lose a modest amount.

People who are too ambitious will get frustrated and give up.

You have to be mentally ready to diet or you will never succeed.

Slow and steady is the way to lose. If you lose weight too fast you will lose less in the long run.

Ideas not yet proven TRUE OR FALSE

Diet and exercise habits in childhood set the stage for the rest of life.

Add lots of fruits and vegetables to your diet to lose weight or not gain as much.

Yo-yo diets lead to increased death rates.

People who snack gain weight and get fat.

If you add bike paths, jogging trails, sidewalks and parks, people will not be as fat.

FACTS — GOOD EVIDENCE TO SUPPORT

Heredity is important but is not destiny.

Exercise helps with weight maintenance.

Weight loss is greater with programs that provide meals.

Some prescription drugs help with weight loss and maintenance.

Weight-loss surgery in appropriate patients can lead to long-term weight loss, less diabetes and a lower death rate.