Depression is a common mental health condition affecting approximately 280 million people globally (World Health Organuzation (WHO), 2021a). Depression causes significant impairment in daily life and is one of the major contributors to the number of years living with disability worldwide (WHO, 2021a). Depression can also lead to suicide, which accounts for 700,000 deaths by suicide every year (WHO, 2021a). Consequently, depression is a priority condition requiring appropriate interventions and effective prevention (WHO, 2021a). Antidepressant medications (e.g., selective serotonin reuptake inhibitors and tricyclic antidepressants) have been prescribed as one of the first line of treatments for controlling the symptoms of severe depression. However, antidepressants are not recommended for treating mild depression or for use in adolescents (WHO, 2021a). While there are alternative treatments, such as cognitive behavioral therapy and interpersonal psychotherapy, it is better to prevent the development of depression in the first place (Frydenberg et al., 2004).

Programs that promote healthy lifestyles, such as increasing physical activity and healthy eating, provide a promising direction for the prevention and treatment of depression (Knapen et al., 2015; Melnyk et al., 2015), especially given the relationship between depression and physical health problems, such as, obesity (Milaneschi et al., 2019), and diabetes (Moulton et al., 2015). Healthy dietary patterns have also been used as management of depressive symptoms because diet may influence brain function and mental health via mechanisms involving inflammation, oxidative stress, epigenetics, tryptophan–kynurenine metabolism, the HPA axis, neurogenesis and BDNF (Marx et al., 2021). A recent review also presented evidence for the association between gut microbiome imbalance and brain dysfunction, which could contribute to the development of depression (Peirce and Alviña, 2019). This has led to a large interest in the effect of gut health on mental health (Bambury et al., 2018; Dinan et al., 2013; Sarkar et al., 2016). For example, preclinical and clinical studies have provided evidence for the bidirectional interaction between the brain and gut microbiota, as well as the role of dysbiosis (impaired microbiota composition), leaky gut, and germ-free conditions in neurodevelopment and neuroinflammation (Evrensel and Ceylan, 2017).

Based on these and similar findings, adopting a diet to improve gut health could potentially improve mental health, such as mitigating depressive symptoms (Mueller et al., 2020; Palepu and Dandekar, 2022). Diets are generally categorized based on the manipulation of macronutrient content, the restriction of specific foods or food groups, and the manipulation of timing, frequency, and quantity of food consumption (Freire, 2020). A dietary pattern with consumption of fermented food and high consumption of dietary fiber is generally considered a gut-health promoting diet (Stiemsma et al., 2020; Zmora et al., 2019). Fermented food can improve gut health via three main pathways: 1) providing biogenics (metabolites from microbial metabolism), such as gamma-aminobutyric acid (GABA) and vitamins (Chang et al., 2022), 2) inoculating the gastrointestinal tract with beneficial microorganisms (probiotics), and 3) providing prebiotics to promote activities, survival, and growth of beneficial microorganisms in the gastrointestinal tract (Aslam et al., 2020). Prebiotics are dietary substrates that are selectively utilized by host microorganisms conferring a health benefit and cannot be degraded by host enzymes (Gibson et al., 2017). Dietary fibers (e.g., resistant starch) are also considered to be prebiotics and can contribute to changes in activities and growth of beneficial microorganisms in the gastrointestinal tract (Mudgil and Barak, 2013). Therefore, the prebiotic component enhances the beneficial effects of fermented food by improving survival and colonization of the probiotics in the gastrointestinal tract.

The benefit of a diet rich in probiotics and prebiotics goes beyond better physical health. Recent research has shown the beneficial effect of prebiotics on mental health through growing specific commensal bacteria with psychophysiological effects (Sarkar et al., 2016). In addition, Dinan et al. (2013) demonstrated that a class of probiotics yields a positive influence on mental health and neuropsychiatric functions through the endocrine, metabolic, and immune systems when ingested in adequate amounts (Evrensel and Ceylan, 2017). For example, depression-like behaviors have been found to decrease after administration of Lactobacillus plantarum and Bifidobacterium infantis (Desbonnet et al., 2010; Liu et al., 2015; Liu et al., 2016). Although most past research has been performed using animal studies, there is preliminary evidence showing that regular consumption of probiotics and prebiotics exerts beneficial effect on mental health in humans. For instance, compared with controls, participants using a probiotic combination of Lactobacillus helveticus and Bifidobacterium longum exhibited a reduction in depression (Messaoudi et al., 2011).

Given the findings described above, it is important to study the effect of diet on depression to address the preventative and therapeutic potential of diet for depression. The purpose of this study was to use a representative sample of the United States to examine the association between a gut-health promoting diet (i.e., consumption of fermented food and high consumption of dietary fiber) and low level of depressive symptoms. Given that higher Body Mass Index (BMI) is an established risk factor for depression across the lifespan (Schaakxs et al., 2017) and that a previous experimental study has demonstrated the association between administration of probiotics and reduction in depression (Messaoudi et al., 2011), this study also explored the potential mediating role of improved physical health (indicated by Body Mass Index, BMI) in the association between a gut-health promoting diet and low level of depressive symptoms. Taken together, this study examined the mediating roles of both self-reported BMI and BMI measured by trained health technicians because subjective physical health and objective physical health could relate to mental health differently (Cho et al., 2011).