FFMI-B: Original Formulation, Physiological Rationale, and Calculation Method

The FFMI-B, or Barrena-adjusted Fat-Free Mass Index, is a methodological proposal intended to contextualize the interpretation of the conventional Fat-Free Mass Index according to adiposity relative to skeletal muscle mass. The proposed formula is FFMI-B = FFMI / [1 + max(0, G:SMM − U)], where G:SMM represents the fat mass to skeletal muscle mass ratio and U is a predefined phenotype-specific threshold. This document describes the original formulation, calculation hierarchy, proposed thresholds, physiological rationale, and methodological limitations of the index. FFMI-B should be considered an exploratory methodological proposal and not a standalone diagnostic criterion.