The Gut Microbiome and Diet in Psychiatry: Focus on Depression

Sarah Dash; Gerard Clarke; Michael Berk; Felice N. Jacka

Disclosures

Curr Opin Psychiatry. 2015;28(1):1-6. 

In This Article

Microbiota and Diet

The gut microbiota is central in metabolism, breaking down dietary components of foods to fuel energy generation.[74] Metabolism of dietary components supports energy production, signalling and homeostatic functions. A previous study[75] postulated that modern modifications to diet and subsequent changes to the gut microbiota are responsible for the increasing rate of inflammatory disease such as cardiovascular disease, rheumatoid arthritis and depression. The Mediterranean diet, a gold standard healthy eating model, can have a beneficial effect on the host microbiota, and in turn, on host health and wellness.[76]

Carbohydrate consumption, particularly of dietary fibre, is an important determinant of microbial composition and results in short chain fatty acid (SFCA) production, promoting a shift towards different types of 'beneficial' bacteria and inhibiting the proliferation of 'bad' bacteria.[77] Dietary fibre provides substrates for bacterial fermentation that results in the SCFAs acetate, propionate and butyrate; these are mediators of the colonic inflammatory response.[78] A recent trial placed 10 healthy volunteers on either a 'plant-based' or 'animal-based' diet, showing rapid (5-day) change in the function profile of the gut.[79] Poor quality or 'western' diets, particularly low in nondigestible fibre, lessen microbial diversity and support fewer antipathogenic bacteria.[20] A recent review suggests that microbiota may even affect eating behaviour by eliciting cravings for foods that fuel bacterial fitness, but not necessarily host health, and that increased microbial diversity may limit bacterial control over dietary choices.[80]

Microbial exposure and long-term, habitual diets are shown to be one of the strongest influences on gut microbial composition, determining an individual 'enterotype'.[81] Data from studies examining the effect of dietary modification on microbial composition are beginning to emerge, with evidence that the consumption of complex carbohydrates, plant-based foods/fruits and vegetables[81,82] and fermented food[83] influence microbial composition, synthesis of anti-inflammatory SCFAs and host health. Conversely, high-fat diets trigger microbial dysbiosis, intestinal permeability and inflammation,[73] with behavioural disruptions that are independent of obesity.[44]

In a landmark study, De Filippo et al.[20] saw significant differences in gut microbial composition between African children consuming a 'traditional' diet compared with European children eating a 'Western'-style diet. In this study, African children on a plant-based diet had greater microbial diversity and had anti-inflammatory bacteria that were functionally absent in the European children consuming a Western-style diet. In support of this finding, Wu et al.[81] found that diets higher in fat and lower in fibre were associated with more Bacteroidetes and Actinobacteria in healthy volunteers. Conversely, a healthy diet, high in fibre and low in fat, was characterized by the phyla Firmicutes and Proteobacteria. These bacterial ratios are important in determining pro and anti-inflammatory balance in the gut. The authors then conducted a controlled feeding study with 10 of the healthy individuals, who followed either a high-fat/low-fibre or a low-fat/high-fibre diet for 10 days. Phenotypal microbiota composition was noted to change quite quickly; however, microbial enterotype remained constant over the 10 days.[81] This suggests that longer-term dietary change may be required to make lasting, significant changes to gut health. Concordantly, reviewers Power et al.[84] reported that levels of dominant phyla of bacteria in the gut are diet-dependant and may be specifically related to the type of nondigestible carbohydrate consumed. However, relationships between diet and bacteria might be dependent on individual factors and people may have different responses to dietary change. More data are needed to understand the complex ways in which dietary patterns influence gut microbiota composition and activity, with the ultimate objective aimed at achieving precise changes in the gut microbiota composition that can lead to improved mental health.[85]

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