Have nothing to do with them , Paul writes. He isn't talking about myths, or Mother Goose. He is talking about the kinds of fables that you are likely to take seriously - like what you do is more important than what you believe. Or the idea that bodily discipline is salvific. Or the same with cigarettes.
Some people think physical fitness is a moral issue.
There are even those who equate overweight with sin and thin with self-control and decency and morality. God tells us that bodily discipline is only of little profit. That means that it is only marginally useful. It can serve you, but it cannot save you. What we do is not the central issue.