Mismatch between physician supply and inpatient utilization in Japan: evidence for structural health inequity

Background: Equitable allocation of the health workforce is essential for achieving health equity, yet structural mismatches between physician supply and healthcare utilization may persist even in universal health systems. Japan provides near-universal access to care, but whether physician distribution aligns with regional inpatient utilization remains unclear. This study tested the hypothesis that physician supply is structurally misaligned with inpatient utilization across prefectures, reflecting patterned inequity rather than random variation. Methods: We conducted a nationwide ecological analysis using publicly available 2021 prefectural data from the Ministry of Health, Labour and Welfare and the Statistics Bureau of Japan. Physician supply and inpatient utilization were standardized per 100,000 population. Associations were examined using Spearman’s rank correlation and stratified analyses by population size. A quadrant classification framework was applied to identify structural mismatch profiles. Results: Substantial inter-prefectural variation was observed in both physician supply and inpatient utilization. The overall correlation between physician density and inpatient utilization was weak (ρ = 0.032), indicating lack of proportional alignment at the national level. Stratified analyses revealed systematic divergence in highly populated regions (ρ = − 0.694, p = 0.012), suggesting patterned inequity. Quadrant classification demonstrated persistent clusters of high-utilization/low-supply and low-utilization/high-supply regions consistent with structural mismatch. Conclusions: Physician supply in Japan is not uniformly aligned with inpatient utilization, revealing structural inequities within a universal coverage system. The quadrant framework provides a scalable tool for identifying regional mismatch profiles and may support equity-oriented workforce planning. Addressing these structural imbalances is essential for advancing health equity in aging societies. Trial registration Not applicable.