Invariance of Human Storage Triglyceride Density Across Anatomical Depots and Age: Reconciling Classical Densitometry with Structural Lipid Physics

For over seven decades, body composition analysis and radiological segmentation models have anchored fat mass calculations to an extracted total lipid density of 0.9007 g/cm^3 at core body temperature (37C). However, this classical constant conflates neutral storage lipids with dense structural membrane lipids. Here, we evaluate the liquid-phase physical chemistry of pure storage triacylglycerols (triglycerides; TRI) across diverse anatomical depots (visceral, subcutaneous, gluteal, intramuscular, hepatic, and bone marrow) and age cohorts (infants to the elderly). By applying harmonic specific volume mixing equations to empirical gas-chromatography acyl chain distributions, we demonstrate that the weighted density of pure human storage triglycerides (rho_TRI) is an immutable physical constant: 0.8843 +/- 0.0012 g/cm^3 at 37C. Furthermore, we show that non-triglyceride structural lipids (rho_nonTRI; unesterified cholesterol, phospholipids, sphingolipids) exhibit a similarly constant density of 1.0189 +/- 0.0035 g/cm^3. We prove that the classic 0.9007 g/cm^3 anchor arises naturally when pure storage triglycerides (87%) and structural membrane lipids (13%) are extracted together as a composite pool. Finally, we resolve a historical discrepancy in classical two-compartment body composition models, showing that while Siri (1956) anchored his equation to total extracted lipids (0.900 g/cm^3), Brozek et al. (1963) operationalized an effective lipid compartment density (0.883 g/cm^3) that converges directly on pure storage triglyceride density.