AS A FOOD writer, I inhabit a world where eating with abandon is the golden rule. I’m not complaining. And it’s easy to understand the particular disdain my food-media colleagues reserve for all the fad diets that come buzzing our way. Why would any gastronome want to plant land mines around meals?

At a food writers’ conference in Brooklyn last spring, talk turned to keto, yet another take on the decades-old quest to banish carbs. The eye rolls were deafening. A ketogenic diet typically caps carbohydrates at less than...