Diet Quality, Food Processing, and Nutrient Adequacy in Vegan, Vegetarian, and Omnivorous Dietary Patterns: A Critical Narrative Review

Vegan and vegetarian dietary patterns have moved from minority dietary choices to central elements of contemporary nutritional guidance, supported by observational cohorts, short-term randomised trials, and umbrella-level evidence (i.e., syntheses of multiple systematic reviews) reporting favourable associations with body weight, blood lipids, blood pressure, glycaemic control, and inflammatory markers. Recent direct comparisons against healthy omnivorous comparators—including the OMNIVEG controlled crossover transitioning from a traditional to a vegan Mediterranean diet and the Landry identical-twin trial—indicate that well-formulated plant-based patterns can produce additional short-term improvements in selected cardiometabolic markers even when the comparator is not the modern Western diet. Interpretation of the broader evidence base is nonetheless constrained by comparator quality, residual confounding, exposure misclassification, reliance on intermediate biomarkers, and limited follow-up. This critical narrative review integrates clinical, methodological, nutritional, and physiological evidence to compare vegan, vegetarian, and omnivorous dietary patterns, and argues that the dominant explanatory axis of long-term outcomes is dietary pattern quality—minimally processed plant-rich composition, nutrient adequacy, low ultra-processed food intake, and adherence—rather than the binary inclusion or exclusion of animal-source foods. Well-planned vegan and vegetarian patterns are valid dietary expressions that require systematic attention to vitamin B12, iodine, iron, zinc, calcium, vitamin D, EPA/DHA, and protein quality. Public health messaging should prioritise food quality, processing level, nutrient adequacy, adherence, and outcome hierarchy over binary dietary identities.