Transport-capacity thresholds for urban nutrition security under extreme food-supply disruptions: evidence from Shanghai.

Urban nutrition security is increasingly vulnerable to extreme weather, public health emergencies, and interruptions to interregional food logistics, especially in import-dependent megacities. Guided by the Greater Food Approach, a Chinese policy framework that extends food security from grain sufficiency to diversified food sources, nutrition, and resilience, this study develops a scenario-based, nutrition-constrained assessment of Shanghai's emergency food-security capacity in 2023, focusing on a minimum energy- and macronutrient-balanced emergency standard rather than comprehensive nutritional adequacy. We combine a baseline net-inflow model, a static minimum transport-capacity ratio model, a nutrition-constrained emergency ration optimization model, and a dynamic response-threshold framework to identify food-category bottlenecks under observed consumption, healthy-diet targets, and a minimum energy- and macronutrient-balanced emergency standard. Shanghai's food system is structurally dependent on extra-local inflows: edible vegetable oil, beef and mutton, poultry, eggs, milk, aquatic products, and fruit all show high external dependence, whereas vegetables retain stronger local buffering capacity. Under normal-consumption targets, the principal bottlenecks are meat subcategories, with the system requiring about 70% of normal external transport capacity to maintain observed supply. Under the healthy-diet target, bottlenecks shift to fruit and milk, and fruit reaches a model-derived planning reference value of 1.33, suggesting that guideline-consistent fruit supply would require transport and supply support beyond the current normal benchmark under the specified assumptions. Under the benchmark callable-reserve assumption, local production and estimated reserves are sufficient in the model to satisfy the minimum energy- and macronutrient-balanced emergency standard for 15 days, while the model-derived bottleneck transport reference values rise to 0.34, 0.62, and 0.84 for 21-, 30-, and 45-day disruptions, respectively. These findings suggest that, under explicit scenario assumptions, emergency food resilience in Shanghai and comparable import-dependent megacities may depend not only on reserve size, but also on bottleneck food-category configuration, nutrient-equivalent substitution, reserve-release accessibility, and the minimum availability of external logistics corridors.