Many widely held beliefs about weight loss don't stand up to scientific scrutiny, say doctors who want to set the public health record straight on myths like the calorie-burning benefits of sex or the value of eating breakfast.

In Wednesday's online issue of the New England Journal of Medicine, U.S. doctors tackle seven obesity-related myths commonly found in the media and material from government agencies as well as six presumptions thought to be true despite a lack of convincing evidence.

The myths were:

Small sustained changes in energy intake or expenditure will produce large, long-term weight changes. Setting realistic goals in obesity treatment is important, because otherwise patients will become frustrated and lose less weight. Large, rapid weight loss is associated with poorer long-term weight outcomes than is slow, gradual weight loss. Assessing the stage of change of diet readiness is important in helping patients who seek weight-loss treatment. Physical-education classes in their current format play an important role in preventing or reducing childhood obesity. Breastfeeding is protective against obesity. A bout of sexual activity burns 100 to 300 calories for each person involved.

The myths were based on sources such as national health guidelines and studies from the 1960s of very-low-calorie diets. In his paper, David Allison of the University of Alabama at Birmingham and his co-authors discuss more recent experimental studies that debunk each myth.

"I always say to my patients, the first victory is to stop gaining weight," Tom Ransom, a Halifax endocrinologist and obesity specialist said of the goal-setting myth. "The second is to lose some weight, and the third victory is to tolerate the plateau, because it will happen."

Randomized control trials do not support that snacking contributes to weight gain but the type and size of snacks matter. (Peter Dejong/Associated Press)

Ransom told CBC News the paper is a great topic of discussion on what works and doesn't.

Stefanie Senior, a registered dietitian in Toronto, agreed the paper offers important information for health professionals and the general public who she said may tend to believe something once they hear it without researching contradictory advice.

On the streets of downtown Toronto, one woman said burning calories during sex "depends who's doing all the work."

Senior rejected the calorie-burning benefits of jumping in the sack.

"At the end of the day, it only burns less than 50 calories," she told CBC News.

The researchers also tackled six presumptions including the purported benefits of regularly eating breakfast, eating fruits and vegetables, snacking and yo-yo dieting, also called weight cycling.

Breakfast no guarantee

"If you're snacking on carrot sticks or Doritos, there's a big difference," Ransom said of snacking. Whether people add calories from snacks without reducing those consumed in meals matters, too.

The same line of thinking applies to eating breakfast. If someone eats a well balanced meal in the morning and tapers their food over the day, it tends to help with managing hunger and appetite but it's not a guarantee that they'll consume less, Senior said.

Both experts stressed what works for weight loss varies between individuals. A host of factors like other medical conditions, sleep and medications all need to be considered when assessing and planning weight loss.

Diana Petramala of Toronto said she tried fad diets that just decreased her energy.

"It's all dedication," she said of how she lost about 30 pounds over three years through "clean eating" and staying active.

The study was funded by the U.S. National Institutes of Health. Several authors have received grants payments from multinational food and pharmaceutical companies.

"It raises questions about what the purpose of this paper is" and whether it's aimed at promoting drugs, meal replacement products and bariatric surgery as solutions, said Marion Nestle, a New York University professor of nutrition and food studies.

"The big issues in weight loss are how you change the food environment in order for people to make healthy choices," such as limits on soda sizes and marketing junk food to children, she said.