Associations of the Planetary Health Diet Index with diet quality, environmental footprints, and sustainable food choices in adults attending a diet clinic

Background Sustainable dietary patterns are increasingly recognized as essential for promoting both human and planetary health. Although the Planetary Health Diet was developed to integrate human health and environmental sustainability; empirical studies jointly evaluating PHDI adherence, estimated environmental footprints, and sustainable food-choice behaviors within the same population remain limited. Methods This cross-sectional study evaluated adherence to the Planetary Health Diet and its relationships with diet quality, environmental footprints, and sustainable food choices among 300 adults aged 18–64 years attending a diet clinic. Dietary intake was assessed using a 24-h dietary recall. The Planetary Health Diet Index (PHDI), Healthy Eating Index-2020 (HEI-2020), carbon and water footprints, and Sustainable Food Choice Questionnaire (SUS-FCQ) scores were evaluated. Results A moderate positive correlation was found between PHDI and HEI-2020 scores ( r = 0.48, p < 0.001). Carbon (kg CO2 eq/d) and water footprints (L/d) were strongly correlated ( r = 0.96, p < 0.001). Weak inverse correlations were observed between PHDI and both environmental footprint indicators ( r = −0.13, p = 0.030). These associations remained weak but significant after multivariable adjustment for carbon ( β = −0.105, p = 0.020) and water footprints ( β = −0.102, p = 0.017), with similar findings in energy-standardized sensitivity analyses. Although environmental footprint values tended to decrease across PHDI quartiles, these differences were not statistically significant ( p > 0.05). Higher SUS-FCQ scores were observed among women, individuals with higher educational attainment, and students and civil servants ( p < 0.05). Conclusion Higher PHDI scores were associated with improved diet quality and showed modest inverse associations with estimated environmental footprint indicators. Given that both indices were derived from the same dietary recall and include overlapping dietary components, this association should be interpreted cautiously. Sustainable food choice behaviors varied according to sociodemographic and lifestyle characteristics. These findings support integrated public health nutrition strategies that address both nutritional quality and environmental sustainability.