By studying fecal samples from 144 vegetarians and 105 vegans, researchers from the University Hospital in Tubingen, Germany report that the fiber and carbohydrate content of the diets led to a lower pH in stools, and such acidic conditions do not support the growth of bacteria like E. coli and Enterobacteriacea.
“The degradation of dietary fibers by exoenzymes mainly leads to greater amount of short-chain fatty acids such as acetate, propionate and butyrate that create a slightly acidic milieu with values between pH 5.5 and 6.5,” explained researchers led by
“This effect may have been amplified by germs that grow because of the large amount of fibers. These pH ranges do not support bacteria such as E. coli and Enterobacteriacea in their growth as they prefer pH ranges greater than 6.5.”
Gut microbial profiles are increasingly of interest as we begin to understand how gut microbial profiles are linked to specific conditions. Indeed, other scientists have already reported that microbial populations in the gut are different between obese and lean people, and that when the obese people lost weight their microflora reverted back to that observed in a lean person, suggesting that obesity may have a microbial component.
Also, late in 2011 Dr Jeffrey Gordon and his team at the Washington University in St Louis reported that probiotic bacteria consumed in yogurt may influence carbohydrate metabolism.
More benefits for women?
Writing in the European Journal of Clinical Nutrition, the researchers note that their most relevant finding is the differential effects of the vegan diet on the pH of the stool: Studies have shown that the pH of stools in omnivorous women is higher than in men, despite both genders consuming similar amounts of fiber.
However, when men and women adhere to a strict vegan diet rich in fibers for a long time both then differences in stool pH are no longer observed.
“This indicates that females profit more from maintaining a strict vegan diet than do men,” wrote the researchers.
Commenting independently on the study’s findings, Professor Glenn Gibson from the University of Reading in the UK said that the data only tells “part of the story” because of the cultural (conventional) microbiology approach employed by the researchers.
As such, the non-culturable diversity will be missed, he said. “Also, it is difficult to be absolutely confident of colony identities on the plates. They used API which is a biochemical procedure, but gene sequencing is preferable.
“Having said that, you would expect the flora a vegetarians/vegans to differ from meat eaters: Meat eaters consume more protein and this leads to less desirable metabolites.
“A high carbohydrate diet (non absorbable CHO, not sugar) should stimulate a more +ve microbiota - e.g. bifidobacteria and lactobacilli,” added Prof Gibson.
The study’s findings were based on data from vegetarians and vegans and compared with people consuming an “ordinary omnivorous diet”.
As noted by Prof Gibson, the researchers used a conventional culture method to assess bacterial counts in the stools of the study participants.
Results showed no differences in total bacterial counts between the groups. However, vegans had considerably lower counts of Bacteroides spp., Bifidobacterium spp., E. coli and Enterobacteriaceae spp., compared with controls, whereas there were no differences in counts of bacteria such as Lactobacillus spp., Enterococcus spp., and Clostridium spp.
Data from vegetarians ranked between vegans and controls, added the researchers.
“Higher consumption of animal protein is one possible explanation for higher stool pH values in subjects on an omnivorous diet, as proteolytic putrefactive bacteria are able to increase stool pH by producing alkaline metabolites,” explained the scientists.
“This speculation is strengthened upon closer examination of the mean pH values. The vegetarians’ mean pH value of 6.6 is between that of vegans and of omnivores (6.3 and 6.9, respectively).”
Source: European Journal of Clinical Nutrition
2012, Vol. 66, Pages 53-60; doi:10.1038/ejcn.2011.141
"A vegan or vegetarian diet substantially alters the human colonic faecal microbiota"
Authors: J Zimmer, B Lange, et al.