Countless different metrics are used to assess metabolic health. Body mass index (BMI) – the mass of the body divided by height squared – is among the simplest and most commonly used, but while BMI may provide a fair indication of overall trends in metabolic health across a given population, it falls short for assessing the metabolic health of any given individual. I often joke that at the individual level, it tells me little more about one of my patients’ health than their eye color! The reason for this failure is that BMI does not take into account how mass is distributed between fat mass and lean mass, which have largely opposing effects on metabolic health. In other words, BMI provides no information about body composition, a set of variables with far more relevance to health and mortality than body weight or BMI alone.
The importance of body composition for healthspan and lifespan cannot be overstated, which is why it is a subject that repeatedly comes up in our content (e.g., see AMA 17, AMA 40, and AMA 44). So given its importance, how do we know where we stand with respect to body composition? What are the best ways of assessing body composition, and what are their respective limitations? Which metrics in particular are most predictive of healthspan and lifespan, and what are the ideal values we should be aiming for within each? We will devote the rest of this piece to exploring these key questions in depth.