A First-Principles Theory of the Human Body: Phases of Life

This study examines whether age-related structure becomes visible along the integral body-density measure k = m/(w²h), calculated from body mass, waist circumference, and height. Using unweighted NHANES 2017–2023 data, distributions along the k axis were compared across five-year age bins. Three sections emerged: 0–19, 20–54, and 55+. Mean k positions declined in all three, with bin-mean slopes of approximately −0.283, −0.243, and −0.132 units per year, respectively. The direction of drift remained constant, forming a piecewise organization of approximately linear sections separated by transition regions where the rate of decline changed markedly. Near the boundaries, local behaviour became less regular, with sharper KDE-peak excursions and oscillatory fluctuations qualitatively reminiscent of flickering. Mean and median remained smoother, indicating that the instability was concentrated in local distribution shape rather than disrupting the overall downward trajectory. BMI showed a different, non-monotonic age pattern and stronger effects of asymmetry and right tails. The results show that a scalar constructed from mass and measured body geometry can reveal ordered phase structure and transition dynamics that were not built into the formula. This work is part of the research series A First-Principles Theory of the Human Body. Series repository: Open Science Framework, DOI: https://doi.org/10.17605/OSF.IO/ENAUQ.