Physics of the Human Body: Cohorts Matter

Population distributions may differ between demographic groups in location, scale, and shape. This study tests whether these differences persist after affine standardization of BMI and the integral body-density measure k = m/(w²h) in an unweighted NHANES 2017–2023 working sample. Cohort distributions defined by sex, age, and NHANES race and ethnicity categories were transformed to u = (x − μ)/σ, represented by Gaussian-kernel density estimates, and compared using pairwise Jensen–Shannon divergence. In the primary nine-cohort comparison, k met both criteria for shape invariance, with mean JS = 0.0050 and max JS = 0.0143. BMI had mean JS = 0.0076 and max JS = 0.0228, exceeding the specified max JS threshold. In participants aged 20 years and older, twelve standardized k profiles were nearly superimposed, with mean JS = 0.0022 and max JS = 0.0074. Cohort differences remained in location and scale and were represented in the (μₖ, σₖ) plane. The results show that sex, age, and race and ethnicity cohorts can remain distinct in their original k distributions while retaining a common standardized form. The same definition of k is therefore retained across the compared groups without cohort-specific modification of the measure.