**4. Link between diet, microbiota and health**

The majority of microorganisms in the human intestine belong to the phyla Firmicutes (which includes Clostridium, Enterococcus, Lactobacillus and Ruminococcus) and Bacteroidetes (which includes Bacteroides and Prevotella in proportions determined in part by diet) [27].

Vegan samples had a significantly lower number of microbes compared to omnivores for four bacterial taxa: Bacteroides, Bifidobacterium, *E. coli*. coli and Enterobacteriaceae. Interestingly, the vegetarian sample also showed significantly reduced bacteria and bifidobacteria. It is important to note that vegans and vegetarians did not differ significantly from each other in these four taxa, nor did they differ in Enterobacter, Enterococcus, Clostridium, Klebsiella, or Lactobacillus, compared to each other or to omnivorous specimens. The vegan diet contains more carbohydrates and fiber than omnivores, and as such, vegan samples significantly reduced stool pH than control groups. The lower pH is strongly correlated with the reduced number of *E. coli* and Enterobacteriaceae, species that are not tolerant of acidic environments. Microbiota and pH of vegetarian stools fall on continuity between vegans and omnivores. These results suggest that the composition of the human gut changes with diet along the continuum, with the vegan diet differing most from omnivores, but not necessarily and significantly from those of other vegetarians. It is possible that the disproportionately high prevalence of this beneficial bacterium in the vegan gut is attributable to a high fiber diet. The role of dietary fiber needs to be examined in greater depth, beyond its mechanical effect of increasing stool bulk and speeding transit time. Dietary fiber also influences the intestinal environment by inhibiting pathogen adhesion, altering bacterial fermentation patterns and short chain fatty acid concentrations, modifying microbiota community profiles, and lowering stool pH [28]. A vegan diet promotes an intestinal microbiota that directly reduces the risk of metabolic diseases. Studies have noticed a link between a vegan diet and protection against autoimmune diseases. For example, an analysis of the Adventist cohort found that a vegan diet, but not a vegetarian one, was associated with a lower risk of hypothyroidism [29]. Four fecal hydrolytic enzymes, associated with toxic and inflammatory products, decreased

during the vegan diet. However, these changes in fecal urease, hololglycine hydrolases, β-glucuronidase and β-glucosidase disappeared within two weeks of starting a normal diet. The authors attribute this reduction in fecal enzymes not only to bacterial activity during the dietary change, but also to the high fiber content in the vegan diet, which can affect fecal weight, transit time, and bacterial metabolism. More detailed research has focused on the vegan diet and the "extreme" raw vegan diet (live food movement) as a promising treatment for rheumatoid arthritis (RA). This possibility that a vegan diet may cause a rapid change in bowel profile was supported by studies in patients with rheumatoid arthritis in which a one-month transition to a vegan diet was sufficient to significantly alter fecal microflora, as determined by stool sample gas–liquid chromatographic profiles of bacterial cellular fatty acids [30]. Thus a patient's personal taste and cultural traditions may need to dictate whether a vegan diet is the ideal choice for medical nutrition therapy [31].
