The relatively new field of nutritional psychology and nutritional psychiatry has a growing number of research supporting the connection between vegetable/fruit consumption and mental health. The results of the single-nutrient studies, apart from those of omega-3 fatty acids, have not been able to produce unambiguous results so far. In contrast, complex research designs focusing on the relationships between overall dietary quality and mental disorders have revealed promising results. The consumption of the vegetables-fruits, whole grains, legumes and oil seeds could be one of the key factors in this respect since the dietary fibre found primarily in plant foods feeds billions of bacteria in our guts. The evidence for the role of gut bacteria in modulating brain health and behavior is extensive – the microbiome-gut-brain axis (MGBA) consists of bidirectional communication between the central and the enteric nervous system, linking emotional and cognitive centres of the brain with peripheral intestinal functions. Therefore this paper argues that psychologists are strongly advised to consider the dietary habits of the client, even if this is not in the focus of their symptoms.