Association Between the Korean Healthy Eating Index (KHEI) and Healthcare Costs Among Adults: The Korea National Health and Nutrition Examination Survey (KNHANES) 2016 and 2021

Background/Objectives: This cross-sectional study examined the association between diet quality, measured by the Korean Healthy Eating Index (KHEI), and medical expenditures among Korean adults. Methods: We used data from the Korea National Health and Nutrition Examination Survey (2016–2021). Adults aged ≥20 years with complete data on diet, sociodemographics, and healthcare use were included. Medical costs were estimated from self-reported service use and converted to USD. KHEI scores were categorized into quartiles. Multivariable linear regression was used to assess the association between KHEI quartiles and log-transformed costs. Subgroup analyses were conducted by age (<57 vs. ≥57 years), and sensitivity analyses treated KHEI as a continuous variable. A two-part model addressed skewed, zero-inflated cost data. Results: Compared to Q1, participants in Q4 had significantly lower inpatient (β = −0.080; 95% CI: −0.139 to −0.020) and total costs (β = −0.086; 95% CI: −0.144 to −0.027). In the younger group, Q4 was associated with lower total costs (β = −0.115; 95% CI: −0.198 to −0.031). Higher continuous KHEI scores were also linked to lower costs. In the two-part model, Q4 participants had a higher probability of incurring any cost but lower conditional costs (Q3 vs. Q1: β = −0.173; Q4 vs. Q1: β = −0.160; both p < 0.05). Conclusions: Higher diet quality was associated with reduced healthcare costs in Korean adults, especially among younger individuals. Promoting healthy eating may help lower economic burdens in aging societies.