It’s much easier, unfortunately, to tell you what not to do. But here at The Upshot, we don’t avoid the hard questions. So I’m going to put myself on the line. Below are the general rules I live by. They’re the ones I share with patients, with friends and with family. They’re the ones I support as a pediatrician and a health services researcher. But I acknowledge up front that they may apply only to healthy people without metabolic disorders (me, for instance, as far as I know).