Trends in the Quality and Processing of Plant-Based Diets in U.S. Youth: A Serial Cross-Sectional Analysis of NHANES, 1999-2020.

BACKGROUND: Although U.S. youth generally underconsume healthy and overconsume unhealthy plant-based foods, national patterns of plant-based diet quality and relationships with ultra-processed food (UPF) intake are not well established. OBJECTIVE: The aim of this study was to examine temporal trends in plant-based diet quality among U.S. youth, associations with UPF intake, and differences across sociodemographic groups. METHODS: We conducted a serial cross-sectional analysis of 24-hour dietary recalls from 32,456 participants 2-19 years in the National Health and Nutrition Examination Survey (NHANES), 1999-2000 through 2017-2020. Plant-based diet quality was evaluated using the overall (oPDI), healthy (hPDI), and unhealthy (uPDI) Plant-Based Diet Indices. UPF intake was estimated as the percentage of total energy from Nova 4 foods. Survey-weighted regression analyses assessed temporal trends, relationships between PDIs and UPF, and differences by sociodemographic groups. RESULTS: Overall plant-based dietary intake increased over time (46.3 to 48.0 points), accompanied by increases in healthy plant-based dietary intake (46.7 to 49.4 points) and decreases in unhealthy plant-based dietary intake (51.8 to 47.8 points) (all p-trend <0.001). Improvements were not uniform across sociodemographic groups. Overall plant-based dietary intake was higher among youth from households with higher-income, education, and food security. Healthy plant-based dietary intake was highest among females and youth from households with higher income, education, and food security. Conversely, non-Hispanic Black youth had lower healthy and higher unhealthy plant-based dietary intake, with unhealthy intake also higher among older youth and males. Healthy plant-based dietary intake was associated with lower UPF intake and unhealthy intake with higher UPF intake, with both associations strengthening over time. CONCLUSIONS: Plant-based diet quality scores modestly improved among youth, but sociodemographic differences persisted. Healthy and unhealthy plant-based dietary patterns were strongly associated with UPF intake, underscoring the importance of considering diet quality, food processing, and structural determinants of diets and health.