Adherence to the planetary health diet index and urinary phytoestrogen excretion: a nationally representative cross-sectional study of US adults.

Background: The Planetary Health Diet Index (PHDI) was developed to quantify adherence to dietary patterns that support both human health and environmental sustainability. However, whether higher PHDI adherence is accompanied by measurable differences in diet-derived biochemical exposure remains unclear. We examined the association between PHDI adherence and urinary phytoestrogen excretion among U.S. adults. Methods: We analyzed cross-sectional data from 5,416 adults participating in the National Health and Nutrition Examination Survey (NHANES) 2003-2010. Dietary intake was assessed using two 24-h dietary recalls, and PHDI scores were calculated according to the EAT-Lancet-based scoring framework. Urinary phytoestrogens, including lignans, isoflavones, and equol, were creatinine-standardized and natural log-transformed. Survey-weighted multivariable linear regression models between PHDI quintiles and urinary phytoestrogen concentrations, using the lowest quintile as the reference. Complementary analyses modeled PHDI continuously per 1-SD higher score. Component-level analyses were conducted with false discovery rate (FDR) correction. Results: In fully adjusted quintile-based analyses, participants in the highest PHDI quintile had higher urinary concentrations of total phytoestrogens, total isoflavones, and total lignans than those in the lowest quintile, with positive graded trends across quintiles for all three composite measures. In complementary continuous models, each 1-SD higher PHDI was associated with 23.4% higher total phytoestrogens, 17.4% higher total isoflavones, and 24.6% higher total lignans. Similar associations were observed for daidzein, genistein, O-desmethylangolensin, enterodiol, and enterolactone. In contrast, equol-a microbiota-dependent metabolite-showed no association with PHDI. Component-level analyses indicated that soybean/soy foods were most strongly associated with isoflavones, and whole grains with lignans, consistent with their established dietary sources. Conclusion: Higher PHDI adherence was associated with higher urinary concentrations of selected plant-derived phytoestrogens among U.S. adults. These findings support biological coherence between planetary health-oriented dietary patterns and systemic exposure to plant-derived bioactive compounds. However, urinary phytoestrogens should be interpreted as complementary population-level exposure indicators of plant-forward dietary patterns rather than validated or specific biomarkers of PHDI adherence.