Discussion
mood and behavior
Given that the gut microbiota has emerged as a key conduit for brain processes, mood and behavior, developing microbiota-targeted approaches that can benefit mental health and brain function is an exciting opportunity, not only for improving the mental health of populations but also in the quest of developing new therapeutic approaches for patients who might be resistant to traditional therapies. Although the more traditional psychobiotic candidates, such as probiotic and prebiotic supplementation, are promising approaches, using whole dietary approaches offers a number of advantages. This includes the benefits associated with nutritional needs met primarily through whole diet, a more accessible way due to the necessity to consume a food daily, costs of supplemental products as well as the potential synergistic effects in the microbiota that could be elicited by complex diets. While some trials have shown promise is using a microbiota-targeted approach [Most notably we observed a reduction in perceived stress in the cohort undergoing the dietary intervention, although there was no difference between the groups post-intervention. Importantly, reduction in perceived stress was dose dependent, meaning that higher adherence to the study diet resulted in stronger decreases in the PSS score. Strong evidence between stress and health behaviors, including healthy diet quality, has been presented in cross-sectional studies [Contrary to our hypothesis, we only observed subtle changes in microbial composition and function. While this lack in microbial changes could partly be attributed to other study factors, such as sample size or short duration of the study (which might not have allowed enough time for new taxa to emerge and microbial remodeling to be elicited), these observations are in line with earlier studies suggesting difficulty in observing global microbial changes in response to dietary interventions, such as increased dietary fiber intake [Different mechanisms have been proposed to be underlying the microbiota-gut-brain communication, including the immune system, microbial metabolites or the hypothalamus-pituitary adrenal (HPA) axis. Although causational relationships cannot be established from human cohorts, it is important to investigate potential biological changes that could be associated with mental health benefits observed. Such results could then inform future animal studies to investigate mechanistic pathways. Here, it appears that neither the immune system nor the HPA axis were underlying the stress-relieving effect of the psychobiotic diet. While the main components of the psychobiotic diet, namely dietary fiber and fermented foods, have been associated with anti-inflammatory effects [Another potential pathway of the microbiota-brain communication are microbial metabolites. Here, intriguingly, urine metabolomics revealed that urine metabolites were altered in the tryptophan metabolism. Tryptophan metabolism has been shown to be closely regulated by the microbiota and can serve as important bioactive messengers in the microbiota-brain communication [Although, to the best of our knowledge, this study is one of the first investigating a psychobiotic diet approach to improve mental well-being, we acknowledge its limitations in interpreting the findings. Firstly, the sample size, restricted due to the COVID-19 pandemic, the University closure and related national restrictions, limits the generalization of the study findings. Secondly, the short duration of the study could have limited the biological and microbial changes observed and the possibility of more pronounced changes during longer interventions needs to be considered. Thirdly, the intervention included a nutrition education without assessment of behavioral change and while food intake was assessed with comprehensive diet record, these methods are susceptible to measurement error and bias in estimating food intake. Despite the observed increase in dietary fiber intake, dietary adherence revealed a variation in how well participants were able to follow the intervention. Furthermore, while a best effort was put forth to assure blinding of the participants to the study diets, participants could have guessed their group allocation due to the nature of the nutrition education, which may have affected their responses. Lastly, the intervention included a cohort of healthy individuals, potentially limiting some of the changes induced by the diet, specifically the biological changes, as a change in an already healthy individual would be minimal. Future studies in clinical cohorts are warranted to investigate potential therapeutic use of the study diet.