The Subject Is Part of the Dose: Body Composition Transforms Nominal Protein Prescription into Effective Protein Exposure

Background: Protein recommendations have been repeatedly updated in magnitude, including recent U.S. guidancelisting 1.2-1.6 g/kg body weight/day, but the dominant unit remains grams per kilogram of total body weight. Thisconvention treats body weight as a neutral scaling denominator, although total body weight aggregatescompartments with different biological roles. This paper proposes a measurement principle: nominal protein intake isnot necessarily equivalent to effective exposure, because the subject's body-composition state transforms the input. Methods: A proof-of-principle analysis used NHANES 2015-2018 DXA-derived body-composition data in adults aged20-59. A nominal protein dose P expressed per kilogram of total body weight was re-expressed as P x total bodyweight / lean tissue. Additional worked examples re-expressed the Morton et al. 1.62 g/kg/day proteinsupplementation break-point under different lean-tissue fractions and modeled a representative GLP-1 weight-losstrajectory. Results: At a fixed 1.2 g/kg total-body-weight prescription, effective lean-tissue-referenced exposure increasedprogressively with adiposity, reaching approximately 1.25x in men and 1.26x in women with class III obesity relativeto normal-BMI reference groups. Re-expressing the widely cited 1.62 g/kg/day resistance-training proteinsupplementation break-point produced a range from 2.03 to 3.18 g/kg lean tissue under plausible body-compositionassumptions. In a representative GLP-1 weight-loss trajectory, current-body-weight dosing created a 9.5-12.2 g/daygap versus preserving baseline lean-tissue exposure. Because the amplification factor is independent of the chosenprotein target, higher protein targets increase the absolute mismatch in grams/day. Conclusions: The same nominal protein prescription can be the same rule and a different exposure. The practicalrequest is not belief but recalculation: grams per kilogram of which compartment, in which subject, for which effect?