Beyond BMI: Mass-Flux Endophenotypes for the Genetic Dissection of Macronutrient Partitioning

The genetic architecture of obesity is highly polygenic. Recent genomic analyses have demonstrated that the heritability of Body Mass Index (BMI) is largely recovered when the full spectrum of common and rare variants is considered. Therefore, the challenge for the field is no longer a "missing heritability" gap. The challenge is a lack of mechanistic resolution: mapping genetic variants to specific physiological pathways. This paper proposes that supplementing BMI with mass-flux endophenotypes can achieve this resolution. I introduce three endophenotypes grounded in the Mass Balance Model (MBM). The primary endophenotypes are 24-hour urinary nitrogen excretion (nitrogen mass balance) and the respiratory quotient (substrate oxidation). The mass-clearance parameter k is introduced as an exploratory, biophysical measure of clearance efficiency. Reinterpreting canonical obesity genes (FTO, MC4R, leptin) through this framework reveals that they likely govern specific aspects of macronutrient partitioning. I acknowledge the dominant functional genomics at the FTO locus (IRX3/IRX5 enhancers) and the primary role of leptin in human appetite regulation (NMI). I outline a targeted research program using metabolic chamber studies, integrated with biobank data, to test these endophenotypes.