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updated 2/13/2013 12:49:47 PM ET 2013-02-13T17:49:47

Many weight-loss and obesity-prevention beliefs thought to be gospel truth are actually false or yet unproven, according to a study published in the Jan. 31 issue of the New England Journal of Medicine.

Some of these false assumptions might even surprise medical doctors: Breast-feeding protects a child against obesity… Physical education in schools prevents childhood obesity… Gradual weight loss is better than rapid loss… You burn hundreds of calories during sex... These are just some of the obesity myths identified by an international team of doctors led by David Allison at the University of Alabama at Birmingham.

These so-called facts are pervasive on websites, in the news media, and even in the scientific literature despite contradicting scientific evidence, the researchers said. The team identified a total of seven myths, six additional presumptions not yet proven true or false, and nine evidence-supported facts that are relevant for sound public health policy.

Myth 1:Small changes in energy intake or expenditure will produce large, long-term weight changes. This seemingly logical belief is based on a rule that a loss or gain of one pound comes from expending or consuming 3,500 calories. The problem is that this applies only to the short term. Additional pounds are harder and harder to shed once you begin losing weight. That's because, in part, as you lose weight your body has lower energy requirements, meaning it can maintain its weight (without any loss) on a lower calorie count. [7 Diet Tricks That Really Work]

Myth 2:Setting realistic goals in obesity treatment is important because otherwise patients become frustrated and lose less weight. How can you argue with this logic? Well, according to Allison, data suggest that people do better with more ambitious goals. Think big to lose big.

Myth 3:Gradually losing weight is better than quickly losing pounds because quick weight losses are more likely to be regained. This myth, likely dating back to the 1960s, has no biological basis, the researchers said. In fact, studies reveal that people who lose more weight rapidly are more likely keep off the pounds even several years later.

Myth 4:Patients who feel "ready" to lose weight are more likely to make the required lifestyle changes. Being "ready" doesn't hurt. But studies have shown that the degree of readiness or willingness to diet doesn't predict the magnitude of weight loss or adherence to a program.

Myth 5:Physical-education classes, in their current form, play an important role in reducing or preventing childhood obesity. Studies actually have shown, perhaps surprisingly, that increasing the number of days that phys-ed is offered does not reduce childhood obesity. The researchers speculate this is because the type of exercise — in terms of frequency, intensity and duration — isn't enough to promote sustained weight loss. Or, obese kids who need it most aren't moving in pace with other students.

Myth 6:Breast-feeding protects the breastfed offspring against future obesity. Breast-feeding has many positive effects for the child, such as increased IQ and immunity, but obesity protection is not among the benefits. Although the belief is passionately defended, large studies in recent years have found that one's breast-feeding history is not a determinant for obesity.

Myth 7:One episode of sex can burn up to 300 calories per person. Well, maybe it's worth a try. Allison's group crunched the numbers and calculated that a 150-pound man would burn about 250 calories per 60 minutes of vigorous and continuous sex. Studies reveal, however, that most sexual intercourse lasts only six minutes, which translates to 20 calories burned — not much more than sitting on a chair and watching television. [10 Most Surprising Sex Statistics]

Among presumptions yet to be proven true or false, the researchers said, are that snacking contributes to weight gain; that regularly eating (and not skipping) a healthful breakfast protects against weight gain; that building sidewalks and walking and biking trails reduces obesity rates; and that yo-yo dieting is associated with premature death.

Among proven dieting facts, the researcher said, are the points that exercise improves health regardless of whether weight is lost, and that genetics is not destiny — that is, anyone can maintain a healthy weight.

Not that deadly?

Published the same day as the Allison-led paper, in the current issue of the American Journal of Medicine, obesity researcher Charles Hennekens of Florida Atlantic University reported on what some consider to be another obesity myth: that obesity is not that deadly.

Hennekens said that "the dangers of obesity have been grossly underestimated" and that obesity is approaching smoking as the leading cause of preventable premature death, mainly as a contributor to type 2 diabetes and cardiovascular disease.

"Unless Americans lose weight and increase their levels of physical activity, cardiovascular disease will remain the leading killer in the U.S.," Hennekens said.

Recent studies have given the impression that carrying a few extra pounds can be healthy. Allison explained that the misconception might stem from the fact that certain populations, such as older people, can tolerate extra weight. The extra weight can be beneficial if that older person develops a disease such as cancer, in which the patient loses weight during the treatment.

This does not imply that being overweight throughout your life is healthy, Allison said. Hennekens added that childhood obesity is of particular concern, increasing the risk of developing type 2 diabetes and heart disease before age 30.

Fortunately, as Allison's group has helped publicize, diets — particularly those that reduce energy intake — very effectively reduce weight, and environmental changes such as exercise can help prevent weight gain.

Christopher Wanjek is the author of a new novel, " Hey, Einstein! ", a comical nature-versus-nurture tale about raising clones of Albert Einstein in less-than-ideal settings. His column, Bad Medicine, appears regularly on LiveScience.

© 2012 LiveScience.com. All rights reserved.

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