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BACKGROUND: The potential for diet to prevent and treat mental health conditions is an exciting area of investigation; however, the impact of different protein sources on mental health outcomes is unclear. OBJECTIVE: To evaluate the association between dietary protein intake and psychological distress, in people aged >50 years of age, living in Greece. METHODS: A combined data set of older people living in the Athens metropolitan area and 20 Greek islands, from the ATTICA (n = 1,128) and MEDIS (n = 2,221) population-based cross-sectional studies was developed. Anthropometric, clinical and socio-demographic and lifestyle characteristics, and protein consumption (total, animal, plant) consumed based on validated FFQ, were derived through standard procedures and questionnaires. "Psychological distress" (PDS) was assessed as a combined variable representing symptoms of depression and anxiety using Item Response Theory methodology and fitting a Graded Response Model. RESULTS: Animal protein, but not plant protein intake, was associated with higher PDS following adjustment for age, sex, education level, Mediterranean diet adherence and physical activity (b±SE: 0.399±0.090, p = 0.003). Following analysis by Mediterranean diet adherence level, among low adherers, animal protein intake was positively associated with PDS (b±SE: 1.119±0.174, p = 0.003), and no associations were observed in moderate or high adherence groupsin regards to plant protein intake and PDS. CONCLUSIONS: Animal protein intake is associated with PDS, suggesting a bi-directional relationship, which may be influenced by Mediterranean diet adherence. [ABSTRACT FROM AUTHOR]