Maintaining a strict vegan or vegetarian diet changes the colonic microflora

Dietary changes affect the colonic bacterial flora from day to day, but the effects of a strict vegan or vegetarian diet have rarely been investigated in healthy subjects.

Methods: We collected stool samples from 260 individuals (age: 53.6±15.1 years) attending the 38th International Congress of the World Vegetarian Union (Dresden, Germany August 2008) together with questionnaire data on their eating habits (vegan, vegetarian), health status, and sociographic informations. Stool samples were analyzed by conventional microbiological culturing technique (SymbioPharm, Herborn, Germany) for total colony forming units (CFU) (per g stool) as well as relative abundance of Bifidobacteria, Bacteroides spp., Escherichia coli, Enterococcus spp., and Lactobacillus spp. and other commensal bacteria. Data were compared to a random sample of 260 individuals drawn from a sample of 35;000 adults published previously (Enck et al., Z Gastroenterol 2009), and analysed by ANOVA and Chi-Square-Test, as appropriate.

Results: Subjects on a vegan (n=108) or vegetarian diet (n=144) had lower (p=.08) total CFU (3.11×10E11 and 2.44×10E11) than had control subjects (3.74×10E11). Especially bacteroides (p=0.001), bifidobacteria (p<0.001) and E.coli counts (p=0.094) were lower with a vegan and vegetarian diet compared to controls (1.66×10E9, 2.01×10E9, and 3.88×10E9, resp. for bacteroides; 6.32×10E8, 4.91×10E8 and 1.44×10E9, resp. for bifidobacteria; 1.25×10E8, 1.82×10E8, and 4.20×10E8 for E.coli, resp.). The relative proportion (abundance) of bifidobacteria remained low with a vegan diet (5%), while abundance of enterococcus was high (5.7%), compared to controls (15% for bifidobacteria and 1.3% for enterococcus). With a vegetarian diet, the abundance of enterococcus and lactobacillae was significantly higher (3.2% and 2.4%, resp.) than in the controls (1.2% and 0.43%, resp.). Stool pH was significantly lower with a vegan (pH 6.29±0.76) and with a vegetarian diet (pH 6.65±0.77) than in controls (pH 6.84±0.65).

Conclusion: Both a strict vegan and a vegetarian diet significantly alter the colonic flora in terms of total bacterial mass as well as in relative proportion of individual bacterial strains. This may be due to an altered intestinal milieu, e.g. the pH with such diets.