"Up Day Down Day Deadly Day," by Ellen Larson, in Murder Most Edible, edited by Verona Rose, Rita Owen, and Shawn Reilly Simmons, 2019.

This is Larson's second appearance here.

I have written here before about didactic mysteries, tales which teach you about some subject as you enjoy the story.  This is a good example.

The narrator is the police chief of a small town in New York.  He has joined a group called the Slim Janes, not for professional reasons, but to watch his diet.  Oops!  Don't call it a diet.  They call it a Way of Eating, or WOE. 

And he is learning so much about WOEs that his head is swimming, but then he is called away on a case.  Becca, one of the groups leaders, is hospitalized after a bad reaction to food.  Allergy?  Poison?  Shoddy vegan supplements?

To get to the bottom of it all the chief has to learn a lot about how different diets work.  It's clever, informative, and best of all, the solution really does depend on what he learns.