Trends in diet quality and associated comprehensive environmental impacts in the United States, 2001 to 2018: a serial cross-sectional study

BACKGROUND: Diet quality is pivotal to both human health and environmental sustainability; however, the comprehensive environmental impacts and socioeconomic disparities associated with longitudinal shifts in diet quality in the United States remain incompletely characterized. OBJECTIVES: To evaluate longitudinal trends in diet quality and associated comprehensive environmental impacts, and examine their interrelationships among United States adults over an 18-y period. METHODS: This serial cross-sectional study analyzed data from 25,678 United States adults aged ≥20 y across the 9 consecutives NHANES cycles (2001-2018). We assessed 6 validated diet quality indices and 4 core environmental indicators, with analyses performed via Mann-Kendall tests, multivariable linear regression, and Shapley value variance decomposition. RESULTS: From 2001 to 2018, United States diet quality improved significantly across most health-focused indices (P < 0.001), with the largest gain observed for the Dietary Approaches to Stop Hypertension score (standardized Sen's slope = 1.22 per 2-y cycle). Diet-related greenhouse gas emissions (Sen's slope = -0.36 per 2-y cycle, P < 0.001), cumulative energy demand (Sen's slope = -1.32 per cycle, P < 0.001), and land use (Sen's slope = -0.30 per cycle, P < 0.001) declined significantly (all P < 0.001), whereas water use increased markedly (+51.64 L/cycle, P < 0.001). The Planetary Health Diet Index showed the strongest inverse environmental associations, was the only index linked to reduced water use, and explained 38.2% [95% confidence interval: 31.2%, 39.0%] of greenhouse gas emission variance and 35.8% (95% confidence interval: 30.6%, 39.0%) of land-use variance. Improvements were concentrated among higher-education and higher-income groups. CONCLUSIONS: Over 18 y, United States diet quality evolved toward healthier and more environmentally sustainable trajectories, demonstrating clear health-environment synergies. Nevertheless, rising water use and uneven progress across socioeconomic groups highlight critical tradeoffs. The Planetary Health Diet Index effectively captures these comprehensive environmental benefits. Achieving truly sustainable food systems requires integrating comprehensive environmental metrics into dietary guidelines and enacting targeted policies to ensure equitable improvements in diet quality.