In recent years, the perils of body mass index, or BMI, have become a hobbyhorse for professionals in several fields of medicine and research. For decades, doctors have used BMI to help diagnose and treat obesity, diabetes, and other chronic conditions, even as evidence has accumulated that the metric is a poor proxy for excess fat. BMI factors in height and weight but not actual body composition; many people with high BMIs are the picture of health, and many with “healthy” BMIs are at serious risk of metabolic disease. The case against BMI is strong enough that many in medicine would like to be free of it. Gripes have been raised, too, about medical guidance that relies on race. Although race can track with some factors that influence health, such as lifestyle and socioeconomic status, its relationship to genetic differences is tenuous: Designations such as “Black” and “Asian” cover so many people, with such varied backgrounds, that they’re essentially meaningless as biological categories. When doctors have used race to assess well-being, they’ve missed diagnoses and …