Planetary Health Diet-Compatible Food Substitutions in Obesity: Direct Evidence, Gut–Organ Mechanisms, and Translational Priorities

Background: The Planetary Health Diet (PHD) is a plant-rich reference pattern rather than a validated obesity treatment. Its clinical value is difficult to interpret when direct PHD studies are combined with evidence from related dietary patterns, isolated substrates, and experimental models. Methods: This narrative review searched PubMed/MEDLINE, Web of Science, Scopus, and Google Scholar through September 2026. Evidence was organized as direct PHD evidence, evidence from comparable dietary patterns or components, and predominantly experimental evidence. Human interventions and prospective cohorts were prioritized for clinical conclusions, while mechanistic studies were retained only when they clarified a PHD-compatible substitution or a testable gut–organ pathway. Results: Direct PHD evidence is mainly observational and supports associations with lower obesity risk, central adiposity, diabetes incidence, or metabolic dysfunction-associated steatotic liver disease; randomized studies more often test an energy-restricted program or a related plant-rich pattern. Component trials support effects on liver fat, appetite, barrier biology, and microbial function, but do not establish that a complete PHD produces the same effects. Short-chain fatty acids, bile acids, tryptophan metabolites, barrier signals, endocrine pathways, and redox/mitochondrial stress responses are plausible mediators with substantial tissue and person specificity; processing and food-safety exposures remain contextual modifiers rather than established PHD mechanisms. Conclusions: PHD-compatible food substitutions can be used as a clinically adaptable framework when energy intake, protein and micronutrient adequacy, cultural feasibility, affordability, and tolerability are addressed. Weight, waist, glycemia, hepatic risk, and muscle function should be evaluated separately. The complete PHD–microbiome–organ causal sequence and the added value of microbiome-guided personalization remain unproven.