Which Diets make the Gut Happiest?

By Katherine Bethell

It is widely accepted that diet has a strong correlation with health and disease.  A healthy diet which is considered high in fibre, reduces the risk of inflammatory diseases such as asthma whilst also lowering the likelihood of cardiovascular and respiratory diseases (Thorburn A et al., 2014).

Diet has a large impact on the risk of disease because of its effect on gut homeostasis (Thorburn A et al., 2014). Humans rely on healthy commensal bacteria in the gut to digest fibre and produce certain vitamins. A major metabolite produced by the gut microbiota are short chain fatty acids (SCFAs) which play a role in disease prevention by improving gut epithelial integrity and having an anti-inflammatory role, they are vital in reducing the risk of inflammatory bowel disease. Individuals with low gut bacterial diversity have an increased risk of disease as digestion is not efficient and health-promoting metabolites, such as SCFAs, are not produced- hence it is beneficial to have a species rich, high diversity gut microbiome. The gut microbiome is influenced by diet and the nutritional content of food, hence why having a healthy diet is important for reducing the risk of disease and keeping your gut happy!

A Western diet is typically characterised by high intake of animal proteins, saturated fats  with lower consumption of fruit and vegetables (Zinocker M and Lindseth I, 2018). This is defined as an “obesogenic” diet and is strongly associated with obesity and related metabolic diseases (Garcia-Mantrana et al., 2018). A large proportion of energy is obtained from acellular nutrients which are easily broken down by human digestive enzymes and do not require bacterial digestion in the gut. This reduces the gut bacterial diversity as they are no longer needed to obtain nutrients from food consumed in a western diet (Zinocker M and Lindseth I, 2018). Several studies have shown reduced bacterial diversity in adult populations who consume high levels of processed foods compared to those with a fibre-rich diet. Whilst in mice studies it was shown that if a western diet, high in fat and simple carbohydrates, was consumed over several generations the progressive loss of bacterial diversity is irreversible; despite reintroducing carbohydrates into the diet which required microbiota aided digestion the bacterial diversity did not increase. This is particularly concerning if a similar trend is observed in humans over the next several decades as a permanent reduction in bacterial diversity due to diet will undoubtedly cause an exponential increase in the number of people suffering from metabolic diseases (Sonnenburg E et al., 2016). 

Diets associated with the Mediterranean, which have a high consumption of fish, fruit and vegetables may reduce the risk of cardiovascular diseases (Thorburn A et al., 2014). These food groups contain complex carbohydrates that are difficult to digest by the human body alone. Hence the body relies on bacterial fermentation in the gut which will also produce large quantities of SCFAs as a by-product (Garcia-Mantrana et al., 2018). As mentioned earlier SCFAs, such as butyrate and acetate, promote health and reduce the risk of disease by having an anti-inflammatory role and promoting epithelial barrier integrity which will prevent pathogen invasion and disease (Kobayashi M et al., 2017). As the body requires bacterial fermentation to digest the complex carbohydrates the gut microbiota composition in people consuming a “Mediterranean” diet is highly diverse with large proportions of health-promoting commensal bacteria (Cani P and Van Hul M, 2020).

As the majority of vegan and vegetarian diets are made up of fruit and vegetables, a high diversity gut microbiota is required to digest the plant cells. Plant cells have nutrients encased in a fibre rich cell wall which require fibre-degrading bacteria to hydrolyse the wall and release the nutrients ((Zinocker M and Lindseth I, 2018).  Studies have shown that plant-based diets increase bacterial diversity, for example, vegans have increased numbers of Bifidobacterium and Lactobacillus species in their gut (Wu G et al., 2016); these microbiota have anti-inflammatory effects and can protect against cardiovascular diseases (Tomova A et al., 2019).

The human gut contains trillions of microorganisms which are responsible for aiding digesting and producing metabolites which reduce the risk of disease and promote health (Leeming E et al., 2019). However, the type and number of bacteria which reside in the gut is strongly influenced by diet. By consuming a diet high in simple sugars and saturated fats, typically associated with Western countries, the human body can digest these molecules itself and does not require bacteria to aid digestion, hence reducing the bacterial diversity found in the gut. A low bacterial diversity can increase the risk of diseases such as inflammatory bowel disease and cardiovascular disease. Therefore, to remain healthy and reduce the risk of disease it is now widely advised to consume a diet rich in fibre to ensure high bacterial diversity in the gut!


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Zinocker M, Lindseth I. 2018. The Western Diet-Microbiome-Host Interaction and Its Role in Metabolic Disease. Nutrients. 10(3). Available from DOI: 10.3390/nu10030365 

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Cani P, Van Hul M. 2020. Mediterranean Diet, Gut Microbiota and Health: When Age and Calories do not add up. Gut. 69(7). Available from: DOI: 10.1136/gutjnl-2020-320781

Wu G, Compher C, Chen E, Smith S, Shah R, Bittinger K et al. 2016. Comparative Metabolomics in Vegans and Omnivores reveal constraints on diet-dependent gut microbiota metabolite production. Gut. 65(1). Available from: DOI: 10.1136/gutjnl-2014-308209

Tomova A, Bukovsky I, Rembert E, Yonas W, Alwarith J, Barnard N et al. 2019. The Effects of Vegetarian and Vegan Diets on Gut Microbiota. Frontiers in Nutrition. 6(47). Available from: DOI: 10.3389/fnut.2019.00047

Leeming E, Johnson A, Spector T, Roy C. 2019. Effect of Diet on the Gut Microbiota: Rethinking Intervention Duration. Nutrients. 11(12). Available from: DOI: 10.3390/nu11122862

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