This research examines associations between dietary patterns, demographics, and mental health outcomes using cross-sectional data from the 2021-2023 National Health and Nutrition Examination Survey. Simple ordinal logistic regression models and tests of group mean differences reveal an association between higher adherence to a plant-based diet and higher odds of reporting less severe depression, less frequent anxiety, and better cognitive function. Principal component analysis and more complex ordinal logistic regression models reveal that greater adherence to a plant-based diet, coupled with a lower intake of vitamin B12 and added B12, is linked to increased odds of experiencing more severe depression, suggesting that an inadequately supplemented plant-based diet may negatively impact mental well-being. Unexpectedly, higher intakes of B and E vitamins are also linked to worse mental health outcomes, suggesting possible reverse causality. Future studies should explore these mechanisms longitudinally to better understand causality, identify unmeasured confounders, and inform targeted interventions.