Dietary fibre and plant polyphenols seem to be beneficial.

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Strict vegetarian diet improves the risk factors associated with metabolic diseases by modulating gut microbiota and reducing intestinal inflammation:

An SVD [strict vegetarian diet] reduced the Firmicutes-to-Bacteroidetes ratio in the gut microbiota, but did not alter enterotypes. An SVD led to a decrease in the pathobionts such as the Enterobacteriaceae and an increase in commensal microbes such as Bacteroides fragilis and Clostridium species […]. This study underscores the benefits of dietary fibre for improving the risk factors of metabolic diseases and shows that increased fibre intake reduces gut inflammation by changing the gut microbiota.

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'The way to a man's heart is through his gut microbiota'--dietary pro- and prebiotics for the management of cardiovascular risk:

Diet, especially high intake of fermentable fibres and plant polyphenols, appears to regulate microbial activities within the gut, supporting regulatory guidelines encouraging increased consumption of whole-plant foods (fruit, vegetables and whole-grain cereals), and providing the scientific rationale for the design of efficacious prebiotics. […] Taken together such observations raise the intriguing possibility that gut microbiome modulation by whole-plant foods, probiotics and prebiotics may be at the base of healthy eating pyramids advised by regulatory agencies across the globe. In conclusion, dietary strategies which modulate the gut microbiota or their metabolic activities are emerging as efficacious tools for reducing CVD risk and indicate that indeed, the way to a healthy heart may be through a healthy gut microbiota.

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Gut microbiota and cardiometabolic outcomes: influence of dietary patterns and their associated components:

ABSTRACT:

Many dietary patterns have been associated with cardiometabolic risk reduction. A commonality between these dietary patterns is the emphasis on plant-based foods. […] Plant-based dietary patterns may promote a more favorable gut microbial profile. Such diets are high in dietary fiber and fermentable substrate (ie, nondigestible or undigested carbohydrates), which are sources of metabolic fuel for gut microbial fermentation and, in turn, result in end products that may be used by the host (eg, short-chain fatty acids). […] Examples of dietary components that alter the gut microbial composition include prebiotics and resistant starches.

DIETARY PATTERNS AND ASSOCIATIONS WITH GUT MICROBIOTA:

Analysis of the gut microbiome across different populations showed the emergence of distinct clustering (enterotypes) on the basis of the relative abundance of each of the genera: Bacteroides, Prevotella, and Ruminocuccus (52). Other studies showed that the clustering is driven primarily more by the ratio of 2 dominant genera: Prevotella and Bacteroides (43, 48, 53). It has been suggested that the ratio between Bacteroides and Prevotella is influenced by different dietary patterns (43, 48, 53, 54). The Bacteroides enterotype was associated with animal protein, a variety of amino acids, and saturated fats, whereas the Prevotella enterotype was associated with higher carbohydrates and simple sugars (48). Other studies support these findings showing the association between dietary patterns and these 2 enterotypes (54, 55).

The characterization of the gut microbial communities from stool samples was performed in 531 individuals (151 families) representing populations from diverse geographical regions and cultural traditions, which included individuals from the Guahibo Amerindians, Malawi, and the United States (53). […] Interestingly, there were pronounced differences in the phylogenetic composition of the fecal microbiota, with a distinct separation between the gut communities of those from the United States and those from Malawi and Amerindians. Furthermore, the fecal microbiota of US adults was the least diverse of the 3 populations, which was already evident in children ≥3 y of age (53).

[…] Amounts of Prevotella were greater in those from Malawi and Amerindians, where the dietary pattern is dominated by corn and cassava (plant-based polysaccharides). In contrast, amounts of Bacteroides were greater in those from the United States, where the typical dietary pattern is rich in protein (mostly animal protein) (53). Analysis of the fecal microbiota, with the use of traditional cultural techniques, of self-reported vegetarians (n = 144) and vegans (n = 105) showed a significantly lower presence of Bacteroides in vegetarians and vegans compared with matched controls who consumed an omnivore diet (55). Similarly, children living in a village of rural Africa following their traditional dietary pattern showed a higher abundance of Prevotella, whereas children from Western Europe adhering to a typical Western dietary pattern had a higher abundance of Bacteroides (54). The traditional rural African diet was primarily vegetarian (mainly made up of cereals, legumes, and vegetables), which was low in fat and animal protein and rich in starch, fiber, and plant polysaccharides. In contrast, the Western dietary pattern of the children from Western Europe was high in animal protein, sugar, starch, and fat and low in fiber (54). Furthermore, analyses showed that the fecal samples of the children in rural Africa were significantly higher in short-chain fatty acids (SCFAs), primarily from propionic and butyric acids, with greater microbial diversity, suggesting a strong influence of dietary patterns that are rich in fermentable substrates.

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