Adherence to the planetary health diet index and risk of glycemic progression or regression in prediabetic individuals: a longitudinal study

Whether adherence to the Planetary Health Diet Index (PHDI)—a dietary pattern jointly promoting human and environmental health—influences glycemic trajectories among individuals with prediabetes remains unknown. This study examined associations between PHDI adherence and odds of regression to normoglycemia and progression to type 2 diabetes mellitus (T2DM) in prediabetic adults. We conducted a prospective cohort analysis among 4,143 prediabetic adults (age range 19–70 years, 46.2% men, mean age 43.3 ± 10.1 years) enrolled in the Tehran MMRT study between 2017 and 2018 and followed for 5 years. PHDI scores were derived from validated food-frequency questionnaires. Multinomial logistic regression estimated odds ratios (ORs) for glycemic regression and progression across PHDI quartiles, adjusting for demographic, lifestyle, metabolic, and dietary covariates. Component‑level and subgroup analyses, as well as restricted cubic spline modeling, were conducted. Over follow‑up, 1,288 participants regressed to normoglycemia; 775 progressed to T2DM. In fully adjusted multinomial logistic regression models, the highest versus lowest PHDI quartile exhibited 28% greater odds of regression (OR 1.28; 95% CI: 1.04–1.58; P‑trend = 0.003). Each standard‑deviation increase in PHDI conferred a 10% higher likelihood of regression (OR 1.10; 95% CI: 1.02–1.18). This association was significantly stronger among men than women (P‑interaction < 0.001). Higher intakes of legumes, vegetables, dairy, vegetable oils, and poultry were independently associated with regression, whereas red meat intake was inversely associated. This relationship was attenuated after HOMA‑IR adjustment, suggesting partial mediation by insulin sensitivity. No independent association was observed with diabetes progression after full adjustment (OR 1.04; 95% CI: 0.82–1.31; P‑trend = 0.840). Greater adherence to the PHDI was prospectively associated with higher odds of returning to normoglycemia, but not with reduced diabetes progression, in prediabetic adults. Insulin sensitivity may partly mediate this benefit. These findings underscore the importance of considering glycemic directionality when evaluating sustainable dietary interventions.