Fact or fiction? Sex burns a lot of calories. Snacking or skipping breakfast is bad. School gym classes make a big difference in kids' weight.All are myths or at least presumptions that may not be true, say researchers who reviewed the science behind some widely held obesity beliefs and found it lacking.Their report in today's New England Journal of Medicine says dogma and fallacies are detracting from real solutions to the nation's weight problem."The evidence is what matters," and many feel-good ideas repeated by well-meaning health experts just don't have it, said the lead author, David Allison, a biostatistician at the University of Alabama at Birmingham.Sex, for instance. Not that people do it to try to lose weight, but claims that it burns 100 to 300 calories are common, Allison said. Yet the only study that scientifically measured the energy output found that sex lasted six minutes on average -- "disappointing, isn't it?" -- and burned a mere 21 calories, about as much as walking, he said.