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Researchers at a New York City hospital several years ago conducted a test of the widely accepted notion that skipping breakfast can make you fat.

For some nutritionists, this idea is an article of faith. Indeed, it is enshrined in the Dietary Guidelines for Americans, the federal government’s advice book, which recommends having breakfast every day because “not eating breakfast has been associated with excess body weight.”

As with many nutrition tips, though, including some offered by the Dietary Guidelines, the tidbit about skipping breakfast is based on scientific speculation, not certainty, and indeed, it may be completely unfounded, as the experiment in New York indicated.

At 8:30 in the morning for four weeks, one group of subjects got oatmeal, another got frosted corn flakes and a third got nothing. And the only group to lose weight was ... the group that skipped breakfast. Other trials, too, have similarly contradicted the federal advice, showing that skipping breakfast led to lower weight or no change at all.

“In overweight individuals, skipping breakfast daily for 4 weeks leads to a reduction in body weight,” the researchers from Columbia University concluded in a paper published last year.

A closer look at the way that government nutritionists adopted the breakfast warning for the Dietary Guidelines shows how loose scientific guesses — possibly right, possibly wrong — can be elevated into hard-and-fast federal nutrition rules that are broadcast throughout the United States.

This year, as the Dietary Guidelines are being updated, the credibility of its nutritional commandments has been called into question by a series of scientific disputes. Its advisory committee called for dropping the longstanding warning about dietary cholesterol, which had long plagued the egg industry; prominent studies contradicted the government warnings about the dangers of salt; and the government’s longstanding condemnation of foods rich in saturated fats seems simplistic, according to critics, given the ever more intricate understanding of the nutrition in fatty foods.

The Dietary Guidelines are important because they shape the contents of school lunches and other federally subsidized programs, and because amid widespread obesity, so many people look to them for sound eating advice.

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The notion that skipping breakfast might cause weight gain entered the Dietary Guidelines in 2010, during one of the reviews conducted every five years by experts to update its findings.

In preparation, a government-convened panel known as the Dietary Guidelines Advisory Committee collected research on skipping breakfast. Some of it did, indeed, suggest that breakfast skippers may be more likely to gain weight.

One of the key pieces of evidence, for example, examined the records for 20,000 male health professionals. Researchers followed the group for 10 years and published results in 2007 in the journal Obesity. They showed that after adjusting for age and other factors, the men who ate breakfast were 13 percent less likely to have had a significant weight gain.

“Our study suggests that the consumption of breakfast may modestly lower the risk of weight gain in middle-aged and older men,” the researchers said.

The advisory committee cited this and similar research, known as "observational studies," in support of the notion that skipping breakfast might cause weight gain. In "observational studies," subjects are merely observed, not assigned randomly to “treatment” and “control” groups as in a traditional experiment.

Observational studies in nutrition are generally cheaper and easier to conduct. But they can suffer from weaknesses that can lead scientists astray.

One of the primary troubles in observational studies is what scientists refer to as “confounders” — basically, unaccounted factors that can lead researchers to make mistaken assumptions about causes. For example, suppose breakfast skippers have a personality trait that makes them more likely to gain weight than breakfast eaters. If that’s the case, it may look as if skipping breakfast causes weight gain even though the cause is the personality trait.

In analyzing the results of observational studies, scientists make statistical adjustments to adjust for the potential confounding factors that they can measure — age, alcohol consumption, exercise, employment, and the like. Breakfast skippers in the health professionals study, for example, tended  to drink more, smoke more, and exercise less. The scientists adjusted their statistics accordingly. But the adjustments are imprecise, and there is no guarantee that the groups are not different in some other unmeasured way.

Relying on observational studies has drawn fierce criticism from many in the field, particularly statisticians.

S. Stanley Young, former director of bioinformatics at the National Institute of Statistical Sciences has estimated that for observational studies in the medical field, “over 90 percent of the claims fail to replicate” — that is they cannot be replicated later by more exacting experiments.

“Wow. Is this really science?” he said during a talk at the Sigma Xi, the science research society. “Every observational study could be challenged."

Because of the weaknesses in observational studies, many scientists prefer true experiments, or randomized controlled trials, which they often say provides the "gold standard" in evidence.

The 2010 Dietary Guidelines committee did cite one randomized controlled trial on the question of breakfast. That experiment, conducted on children in Mexico,  "found no relationship with breakfast alone" and weight gain, according to a committee summary.

But the committee looked beyond that trial and gave weight to several observational studies.

“Modest evidence suggests that children who do not eat breakfast are at increased risk of overweight and obesity,” the advisory committee said. “The evidence is stronger for adolescents.” As for adults, the evidence was described as “inconsistent.”

It was hardly a ringing endorsement of what might be called the breakfast hypothesis, but it was enough to get the federal agencies who write the guidelines — the U.S. Department of Agriculture and the Department of Health and Human Services — to buy in. The breakfast recommendation and the link to obesity became part of the 2010 Dietary Guidelines, under its advice for losing weight.

“Eat a nutrient-dense breakfast," it said. "Not eating breakfast has been associated with excess body weight, especially among children and adolescents. Consuming breakfast also has been associated with weight loss and weight loss maintenance.”

Asked how the government could recommend daily breakfast as a means of losing weight given the contrary evidence, Angela Colson, a spokeswoman for the Department of Health and Human Services, did not answer directly. Instead, she pointed by e-mail to pages in the introduction of the Dietary Guidelines that in broad terms lists the types of evidence that were taken into consideration.

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Linda Van Horn, a professor of nutrition and preventive medicine at Northwestern University, was chair of the 2010 advisory committee. She explained in an e-mail that the amount of evidence available at the time was “limited” and that more research has been conducted in the intervening five years. She maintained, too, that the contents of breakfast foods may be especially good for you.

“Regardless of the evidence though, it might be important for you to recognize the value of eating breakfast due to its frequent inclusion of higher fiber containing foods,” her e-mail said. “As you are no doubt aware, Americans eat only about half of the recommended amount of dietary fiber.”

The notion that eating breakfast was good for losing weight was not new — a year before the Dietary Guidelines, the popular WebMD site featured an article called “Skip Breakfast, Get Fat.” But the federal adoption of the idea gave it added prominence. In the months after the guidelines release, the notion was picked up in newspapers around the country.

The Tampa Tribune offered this advice to men trying to lose weight: “Lots of guys skip breakfast. Don't.”

The Baltimore Sun said “Eating breakfast is an important strategy for weight control.”

And in Salt Lake City, the Deseret Morning News reported that “Breakfast might actually be the most important meal of the day and eating it regularly can help a person to lose weight.”

The trouble with all these pronouncements is, aside from raising doubts about the credibility of other dietary advice from the government, that they might actually cause people to eat breakfast when they otherwise wouldn’t, potentially leading to weight gain.

David Allison, of the University of Alabama-Birmingham, has become one of the leading critics of what he sees as the misuse of nutritional research. He recently compiled a list of the randomized controlled trials that investigated links between breakfast and obesity.

He found five, including a variety of different breakfasts, and none offered clear evidence that skipping breakfast leads to weight gain. (The New York research was funded by Quaker Oats, a unit of Pepsi Co, though the results could hardly have been what the breakfast food company would have hoped for.) Mostly, it seemed, skipping breakfast made no difference. A sixth study, published this month in Obesity, also showed no differences in weight loss between those who ate a breakfast and those who skipped, though subjects who had a high-protein breakfast gained less body fat.)

Allison attributes the widespread adoption of the breakfast hypothesis at least in part on researchers who read too much into observational studies, and wrongly ignore the stronger evidence from the randomized controlled trials. In addition, he speculates that there “maybe some sense that eating breakfast is ‘moral’ and ‘upstanding,’” and that makes people more willing to believe it’s good for you.

When in the coming months the government unveils the 2015 Dietary Guidelines, it is unclear the advice on breakfast and weight gain will be included. The 2015 advisory committee issued a report that steered clear of the subject of skipping breakfast and weight.

“I just don’t think it surfaced as a priority question,” said Barbara Millen, president of Millennium Prevention, a life sciences start-up company, and chair of the 2015 advisory committee. “The sentiment was we don’t have to say anything further about it. We didn’t want to focus on a laundry list of foods and meals. We were focusing on overall dietary patterns.”