Decoupled but reconcilable: a nutrition-feasible global reallocation lowers dietary inflammation and environmental footprints simultaneously in every country

Background: Pro-inflammatory and high-environmental-impact diets both threaten population and planetary health, but whether the two objectives align or conflict across countries is unresolved. We tested whether a supply-based dietary inflammatory index (sDII) is coupled to greenhouse-gas (GHG), land and freshwater footprints, and whether a nutrition-feasible reallocation can lower both simultaneously. Methods: From FAO Food Balance Sheets we built sDII (12 inflammatory-weighted components; construct validity r=0.9999) and five per-capita footprints using three independent life-cycle inventories for 182 national food-supply series. For each country, constrained optimisation reallocated 13 food-group supplies under isoenergetic, protein-preserving and food-group-bound constraints, minimising sDII and GHG jointly (Pareto frontier). Health burden was estimated via pooled relative-risk meta-analysis and 2023 World Bank population data. Results: sDII was only weakly associated with GHG (Spearman rho=0.14), land (rho=0.13) and freshwater (rho=0.28) in 2023. The balanced-Pareto reallocation lowered both sDII and GHG in 182/182 series (100% synergy): population-weighted delta sDII=-0.235, GHG -37.5%, land -49.3%, water -17.2%, i.e. 4.28 Gt CO2e/yr avoided. The associated reduction in metabolic-syndrome burden was directionally consistent but modest (~1.1% of the prevalent pool, ~2.84 million cases). Results were robust to three life-cycle inventories and three feasibility-bound regimes. Conclusions: Anti-inflammatory and low-carbon goals are decoupled rather than conflicting, and an isoenergetic, protein-preserving reallocation reconciles them in every country. Environmental gains are large and robust; health gains are directionally consistent but modest--triangulation, not a causal claim.