The Unit Commensurability Problem in the Nutri-Score Algorithm: a Construct Validity Analysis
DOI:
https://doi.org/10.52609/jmlph.v6i3.313Keywords:
Construct Validity, Dose-response Relationship, Food Labelling, Nutrient Profiling, Nutri-ScoreAbstract
Background: The Nutri-Score labelling system, based on the Food Standards Agency modified Nutrient Profiling System (FSAm-NPS) algorithm, assigns penalty points to four food components — energy, total sugars, saturated fatty acids (SFAs), and sodium — calculated per 100 g. Prospective cohort studies have validated the FSAm-NPS dietary index as a mortality predictor.
Aims: To examine whether the per-100-g reference unit of the FSAm-NPS algorithm is commensurable with the epidemiologically relevant exposure unit in dose-response nutrition research, and to evaluate implications for construct validity.
Methods: Analytical review of the FSAm-NPS scoring architecture, Messick’s construct validity framework, and the exposure units reported in dose-response meta-analyses for the four penalised components.
Results: Dose-response evidence for all four FSAm-NPS components is expressed in daily intake units (mg/day or g/day), not per 100 g of food. Converting these estimates to per-100-g units requires the assumption of a fixed portion size, rendering component weights arbitrary from the perspective of individual health risk. Prospective cohort validations do not resolve this problem: they validate a composite dietary quality index, not individual component weights.
Conclusion: The FSAm-NPS algorithm contains an unresolved construct validity gap arising from unit incommensurability. Future iterations should either derive component weights from portion-corrected dose-response functions or explicitly declare these weights as normative conventions subject to periodic revision.
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