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Busted: Seven myths about obesity, and why they're wrong

Sex doesn’t burn 300 calories – it’s probably more like 14. And there’s no reason to tell people they need to lose weight slowly and steadily. And guess what? There’s no proof that PE classes help kids lose weight.Some top obesity researchers joined forces to bust the most pervasive myths about weight loss in a paper published in the New England Journal of Medicine on Wednesday. They say
How many calories does a romp between the sheets really burn? Not so many, diet experts say.
How many calories does a romp between the sheets really burn? Not so many, diet experts say.Lev Dolgachov / FeaturePics stock / Today

Sex doesn’t burn 300 calories – it’s probably more like 14. And there’s no reason to tell people they need to lose weight slowly and steadily. And guess what? There’s no proof that PE classes help kids lose weight.

Some top obesity researchers joined forces to bust the most pervasive myths about weight loss in a paper published in the New England Journal of Medicine on Wednesday. They say they don’t want people to stop trying, but they do fear there are some misguided policies out there.

Plus they want people to learn how to do a better job of evaluating claims before they go around repeating them on the Internet, in diet books and to each other.

“Everyone’s trying to do something about obesity,” says David Allison of the University of Alabama at Birmingham, who led the study. “But if they are sending messages to the public that are based on erroneous understanding, arguably they may be wasting time and money and not pursuing the right things.”

They combed through Internet posts, books and government health advisories, and settled on seven common beliefs that they say are not only incorrect, but often illogical, even though they’ve been endorsed by government or other experts:

  • Small, sustained changes in energy intake or expenditure will produce large, long-term weight changes. It’s not only untrue, the researchers say, but illogical.
  • Setting realistic goals for weight loss is important, because otherwise patients will become frustrated and lose less weight. So-called realistic goals may actually be too modest, the researchers say.
  • Large, rapid weight loss is associated with poorer long-term weight-loss outcomes, as compared with slow, gradual weight loss. In fact, some people can successfully lose a lot of weight very quickly, the researchers said.
  • It is important to assess the stage of change or diet readiness in order to help patients who request weight-loss treatment. The researchers note that studies show people who sign up for weight-loss programs may say they are ready, but often actually lose little weight.
  • Physical-education classes, in their current form, play an important role in reducing or preventing childhood obesity. The researchers say most PE classes do little actually to get kids moving much.
  • Breast-feeding is protective against obesity. Studies have shown that in fact, there’s little correlation.
  • A bout of sexual activity burns 100 to 300 calories for each participant.

None of these seven myths has been shown to be true, Allison says, and he found studies that directly refute most of them. The last finding, about sex, is both eye-catching and instructive, he said.

“You will find very many websites that talk about it,” Allison said in a telephone interview.

One actually did a fairly complicated calculation, combining assumptions underlying another of the myths – that burning off a few extra calories every day can add up to real pounds. “He said if you have two romps a day for 6 months you’ll lose 21 pounds,” Allison said.

“It’s absurd … as soon as you just stop a minute and think, ‘okay how would someone know that? What would it take to acquire that information?’ “

One of the research centers taking part in the New England Journal of Medicine report is the Pennington Biomedical Research Center at Louisiana State University, which has a lab where they measure how many calories people actually burn while exercising. So does the University of Alabama, where Allison works. It isn’t easy to do.

“You’d have to get a group of people to volunteer to be measured while having sex,” Allison said. “You’d have to measure all the oxygen they use while having sex. We have a big metabolic chamber here at UAB. We could bring couples in, close the door and let them have sex and measure all the oxygen they consume. The fact that they know they are being measured, how would that affect their performance?”

The only study the team could find that came even close to a scientific measurement of calories burned during sex was done in 1984 on 10 men -- hardly a representative sample.

“What that study showed was the average duration of the sex act was about 6 minutes,” Allison said.

"A man in his early-to-mid-30s might expend approximately 21 calories during sexual intercourse. Of course, he would have spent roughly one third that amount of energy just watching television, so the incremental benefit of one bout of sexual activity with respect to energy expended is plausibly on the order of 14 calories,” the researchers wrote.

Not every diet expert is happy with the study. 

"I think it’s weird. I don’t get it," said Marion Nestle of New York University. "I can’t understand the point of the paper unless it’s to say that the only things that work are drugs, bariatric surgery, and meal replacements, all of which are made by companies with financial ties to the authors. " She noted that the researchers who signed the paper also provided a long list of companies they have been paid to work with. 

The first myth the group attacks, one promoted by government agencies, diet books and the web, is that doing a little something every day will add up to pounds lost over the years. It doesn’t take into account laws of physics and biology, the researchers argue.

It's the idea that if someone burns even 100 extra calories a day, he or she will lose a pound every 35 days. This is what the researchers call the 3,500 calorie myth – that burning 3,500 calories burns off a pound for everyone, every time. Over five years that person should lose 50 pounds, but studies have shown the true weight loss over five years is 10 pounds.

“What it doesn’t take into account is that as I lose weight, I get smaller and it takes less energy to push my mass through space," Allison says. In other words, the body will compensate.

That doesn’t mean that exercise won’t help people lose weight, or that people shouldn’t try to exercise. Exercise even without weight loss improves health, Allison said. But he said people need to demand good evidence before changing policies or dispensing advice.

The team also identified six common presumptions which, while they haven’t been proven wrong, also haven’t actually been proven to be true, either.

These include the idea that eating breakfast helps people lose weight, that children develop eating and exercise habits for life at an early age, that eating more fruits and vegetables alone will somehow displace more calorie-laden food in the diet and help people lose weight, that built-up environments help make people obese, that snacking causes weight gain, and that yo-yo dieting can kill you.

The breakfast idea especially annoys Allison. "We couldn’t include all the wonderful quotations that people are saying about breakfast,” he said. “This is a very, very prevalent belief.”

But study after study only shows an association – that people who skip breakfast weigh more. The idea is that eating early in the day somehow boosts metabolism -- another study suggesting this was published Tuesday.

No one has done what scientists consider the gold standard -- randomly assigning people to either eat breakfast or skip it over a controlled period of time, and keeping everything else equal, Allison says. That would show whether it’s the act of eating breakfast, rather than something about people who choose to eat breakfast or to skip it, that makes the difference.

This one’s an example of why false beliefs are so common, Allison says. “If you say something often enough, if people hear it often enough, they believe it,” he said.

“Why do we spend all this time and money arguing about obesity, passing laws, instead of doing the simple randomized trial? We at UAB have decided we are going to do the trial. We are just going to find the answer.”

So what does work? Drugs do, to an extent, says Allison, a biostatician. “For people who are very obese, pharmaceuticals work a little bit,” he said. So does surgery to make the stomach smaller. The companies that make the devices used for the surgery, and the surgical centers, are doing the randomized, controlled clinical trials that can prove whether something works, Allison says. “Clearly, these are things we should be investing in,” he said.

Other researchers said the article points out some useful facts. "As with any serious problem, the best public policy solutions should be driven by facts. The article in the NEJM provides a significant service by pointing out the myths, from presumption and facts regarding obesity as a means to guide better public policy decisions," said Dr. Frank Greenway of the Pennington Center, who wasn’t involved in the research.

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