(CNN) -- There's a lot said about how to lose weight. As it turns out, a lot of what's said may not be true.
To sort fact from fiction, a group of doctors and nutritionists researched the medical evidence behind common claims and presented their findings Wednesday in the New England Journal of Medicine.
Beyond academia, however, the doctors and nutritionists also have deep ties to industry, receiving grant support and consulting fees from food, drug, and diet companies, raising questions about how wide a net of inquiry the authors were willing to cast.
Still, here are what the researchers say are the seven myths about obesity:
1. Weight loss is just "calories in" vs. "calories out"
"Predictions suggesting that large changes in weight will accumulate indefinitely in response to small sustained lifestyle modifications rely on the half-century-old 3,500 calorie rule, which equates a weight alteration of 2.2 lb to a 3,500 calories cumulative deficit or increment," write the study authors.
The 3,500-calorie rule "predicts that a person who increases daily energy expenditure by 100 calories by walking 1 mile per day" will lose 50 pounds over five years, the authors say. But the true weight loss is only about 10 pounds if calorie intake doesn't increase, "because changes in mass ... alter the energy requirements of the body."
"This is a myth, strictly speaking, but the smaller amount of weight loss achieved with small changes is clinically significant and should not be discounted," says Dr. Melina Jampolis, CNN diet and fitness expert.
2. Set realistic weight-loss goals
The thinking here is that people who aim too high might be setting themselves up for disappointment.
"Although this is a reasonable hypothesis, empirical data indicate no consistent negative association between ambitious goals and program completion or weight loss," write the study authors.
"Indeed, several studies have shown that more ambitious goals are sometimes associated with better weight-loss outcomes."
3. Big, fast weight loss won't stick
Going on a very restrictive diet led to faster weight loss, the study authors found, and dieters did not necessarily gain that weight back, either.
"There was no significant difference between the very-low-energy diets and low-energy diets with respect to weight loss at the end of long-term follow-up," write the authors.
4. You won't lose the weight unless you're really ready
"Readiness does not predict the magnitude of weight loss or treatment adherence among persons who sign up for behavioral programs or who undergo obesity surgery," write the study authors.
"The explanation may be simple - people voluntarily choosing to enter weight-loss programs are, by definition, at least minimally ready to engage in the behaviors required to lose weight."
5. Kids are losing weight in physical education class
"Physical education, as typically provided, has not been shown to reduce or prevent obesity," write the study authors.
"That doesn't take away the fact that physical activity has been consistently associated with decreasing childhood obesity," says Krista Casazza, the lead study author.
"But the way physical education is currently given in the schools is the issue. Oftentimes, it's just kids going outside or being in a physical education class. It has to be an actual, purposeful event."
6. Breast-feeding reduces child obesity
"Although existing data indicate that breast-feeding does not have important anti-obesity effects in children, it has other important potential benefits for the infant and mother and should therefore be encouraged," write the study authors.
7. Sex is a good workout
Well, depending on how you do it.
The researchers cite evidence that sex takes about as much exertion per minute as going for a walk, but lasts on average about six minutes. That adds up to about 21 calories, which isn't such a good workout, but may be good for stress relief.
"Does it make any difference if you do calorie labeling? Does it make any difference if you cap soda size? Does it make any difference if you remove food advertising to children from television?" says Marion Nestle, professor of nutrition at New York University who is not associated with the study.
"Those are the really important things that people are looking at to change the environment of food choice to help people eat more healthfully, and I don't see any of that in here."