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All, As discussed in this thread, research suggests that the gut microbiome can have a dramatic impact on physical, and even mental, health. But the relationship between the gut and health remains pretty murky, and research in the area is still in its infancy. Today everyone's favorite nutrition pundit, Dr. Greger had what I think even his skeptics will agree was a helpful video outlining one mechanistic account of how gut bacteria impact health via their influence on systemic inflammation, which itself has been implicated in most of the diseases of aging. In the video, he suggests that our body has a 'love/hate' relationship with the bacteria in our gut. On the one hand, some bacteria are quite helpful, turning what would otherwise be indigestible food (i.e. fiber) into useful metabolites, like short chain fatty acids that our body can burn as fuel. On the other hand, some bacteria like cholera or e. Coli are quite detrimental to our health, and can sometimes be fatal. So how does our immune system, which is tasked with coping with all these bacteria, handle the job? Specifically, how does it distinguish between the good bacteria which it should ignore and the bad bacteria which it should combat by triggering an inflammatory response? Dr. Greger points to research [see his citations at the bottom of this post] suggesting that the immune system uses the presence of a high level of the short chain fatty acid butyrate as the signal to distinguish between a gut populated with mostly 'good' vs. mostly 'bad' bacteria. More specifically, during our evolutionary heritage, when our ancestors were all eating a very high fiber (> 100g) diet, a healthy gut population would have generated a lot of butyrate, signally 'all clear' to the immune system, which would 'stand down' as a result. But when the gut became overgrown with 'bad' bacteria (which don't produce butyrate), the immune system would notice this lack of butyrate and swing into action, triggering a (systemic) inflammatory response to combat the bad bacteria. The problem is that today, people are eating a crappy, low-fiber, toxin-loaded Western diet, and as a result, even if a person has mostly 'good' bacteria in their gut, the bacteria don't have enough of their food (i.e. fiber) to produce much butyrate. The immune system interprets this lack of butyrate as a sign that the gut is infested with bad bacteria, and so triggers a persistent, systemic inflammatory response in order to fight the (non-existent) threat from the (non-existent) bad bacteria. This permanent inflammatory state in turn leads to all kinds of chronic disease outcomes, from cardiovascular disease, to inflammatory bowel disease, to neurodegenerative diseases like Alzheimer's. That's where Dr. Greger leaves the story, at least in this video. So which types of bacteria (as reported by uBiome) are the 'good', butyrate-producing guys that will signal our immune system that 'all is well'? According to : Eighty percent of the butyrate-producing isolates [from a sample of human gut bacteria] fell within the XIVa cluster of gram-positive bacteria The common gram-positive bacteria reported at the highest level of the uBiome reports is the phylum "firmicutes". From the firmicutes wikipedia entry: The Firmicutes (Latin: firmus, strong, and cutis, skin, referring to the cell wall) are a phylum of bacteria, most of which have Gram-positive cell wall structure. In contrast, the other common high-level phylum of bacteria reported by uBiome are the gram-negative, non-butyrate-producing Bacteroides. From the microbiome wiki entry for Bacteriodes: Bacteroides are gram-negative, non-spore-forming, anaerobic, and rod-shaped bacteria. So overall, to first approximation, it appears preferable to have an abundance of firmicutes and a relative dearth of bacteroides on one's ubiome report of gut bacteria, at least from the perspective of avoiding the ill effects of systemic inflammation by maintaining a high level of butyrate. But it is undoubtedly not quite this simple. In fact I started down a rabbit hole of reading about gut bacteria that I can't entirely make heads or tails of, and that reinforced my belief that researchers a long way from understanding the impact of gut bacteria on human health - see Note 1 below for one such complication. If anyone has a different, better understanding of all of this, and wants to challenge Dr. Greger's account as an oversimplification, I'd love to hear about it! --Dean --------- Note 1: Perhaps paradoxically, vegetarians have been found to have relatively more non-butyrate producing bacteroides in their guts than omnivores, and the resulting relative dearth of energy-harvesting, butyrate-producing firmicutes in vegetarians has been used to explain the leanness of vegetarians compared to omnivores . In other words, the obesogenic gut microbiome profile appears to be a higher ratio of firmicutes to bacteroides, since firmicutes are able to extract more calories from food by turning fiber into the short chain fatty acid butyrate which the body can metabolize for energy. So while firmicutes may be helpful for signalling the immune system that 'all is well' via butyrate production, the resulting abundance of butyrate produced by the firmicutes may increase one's tendency to gain weight by extracting more calories from food. But if this is true, why do firmicute-lacking vegetarians have lower levels of inflammation, and generally better health, than omnivores? Perhaps your average vegetarian doesn't actually eat that much fiber, so they aren't feeding their firmicutes sufficiently... As I said, it is complicated... -----------  Appl Environ Microbiol. 2000 Apr;66(4):1654-61. Phylogenetic relationships of butyrate-producing bacteria from the human gut. Barcenilla A(1), Pryde SE, Martin JC, Duncan SH, Stewart CS, Henderson C, Flint HJ. Author information: (1)Rowett Research Institute, Bucksburn, Aberdeen AB21 9SB, United Kingdom. Butyrate is a preferred energy source for colonic epithelial cells and is thought to play an important role in maintaining colonic health in humans. In order to investigate the diversity and stability of butyrate-producing organisms of the colonic flora, anaerobic butyrate-producing bacteria were isolated from freshly voided human fecal samples from three healthy individuals: an infant, an adult omnivore, and an adult vegetarian. A second isolation was performed on the same three individuals 1 year later. Of a total of 313 bacterial isolates, 74 produced more than 2 mM butyrate in vitro. Butyrate-producing isolates were grouped by 16S ribosomal DNA (rDNA) PCR-restriction fragment length polymorphism analysis. The results indicate very little overlap between the predominant ribotypes of the three subjects; furthermore, the flora of each individual changed significantly between the two isolations. Complete sequences of 16S rDNAs were determined for 24 representative strains and subjected to phylogenetic analysis. Eighty percent of the butyrate-producing isolates fell within the XIVa cluster of gram-positive bacteria as defined by M. D. Collins et al. (Int. J. Syst. Bacteriol. 44:812-826, 1994) and A. Willems et al. (Int. J. Syst. Bacteriol. 46:195-199, 1996), with the most abundant group (10 of 24 or 42%) clustering with Eubacterium rectale, Eubacterium ramulus, and Roseburia cecicola. Fifty percent of the butyrate-producing isolates were net acetate consumers during growth, suggesting that they employ the butyryl coenzyme A-acetyl coenzyme A transferase pathway for butyrate production. In contrast, only 1% of the 239 non-butyrate-producing isolates consumed acetate. PMID: 10742256 ------------  Ann Nutr Metab. 2009;54(4):253-7. doi: 10.1159/000229505. Epub 2009 Jul 27. Characterization of bacteria, clostridia and Bacteroides in faeces of vegetarians using qPCR and PCR-DGGE fingerprinting. Liszt K(1), Zwielehner J, Handschur M, Hippe B, Thaler R, Haslberger AG. Author information: (1)Department of Nutritional Sciences, University of Vienna, Vienna, Austria. BACKGROUND/AIMS: This study aimed to investigate the quantitative and qualitative changes of bacteria, Bacteroides, Bifidobacterium and Clostridium cluster IV in faecal microbiota associated with a vegetarian diet. METHODS: Bacterial abundances were measured in faecal samples of 15 vegetarians and 14 omnivores using quantitative PCR. Diversity was assessed with PCR-DGGE fingerprinting, principal component analysis (PCA) and Shannon diversity index. RESULTS: Vegetarians had a 12% higher abundance of bacterial DNA than omnivores, a tendency for less Clostridium cluster IV (31.86 +/- 17.00%; 36.64 +/- 14.22%) and higher abundance of Bacteroides (23.93 +/- 10.35%; 21.26 +/- 8.05%), which were not significant due to high interindividual variations. PCA suggested a grouping of bacteria and members of Clostridium cluster IV. Two bands appeared significantly more frequently in omnivores than in vegetarians (p < 0.005 and p < 0.022). One was identified as Faecalibacterium sp. and the other was 97.9% similar to the uncultured gut bacteriumDQ793301. CONCLUSIONS: A vegetarian diet affects the intestinal microbiota, especially by decreasing the amount and changing the diversity of Clostridium cluster IV. It remains to be determined how these shifts might affect the host metabolism and disease risks. Copyright 2009 S. Karger AG, Basel. PMID: 19641302 Dr Greger Video References: C J North, C S Venter, J C Jerling. The effects of dietary fibre on C-reactive protein, an inflammation marker predicting cardiovascular disease. Eur J Clin Nutr. 2009 Aug;63(8):921-33. J R Goldsmith, R B Sartor. The role of diet on intestinal microbiota metabolism: downstream impacts on host immune function and health, and therapeutic implications. J Gastroenterol. 2014 May;49(5):785-98. S M Kuo. The interplay between fiber and the intestinal microbiome in the inflammatory response. Adv Nutr. 2013 Jan 1;4(1):16-28. J M Harig, K H Soergel, R A Komorowski, C M Wood. Treatment of diversion colitis with short-chain-fatty acid irrigation. N Engl J Med. 1989 Jan 5;320(1):23-8. D M Saulnier, S Kolida, G R Gibson. Microbiology of the human intestinal tract and approaches for its dietary modulation. Curr Pharm Des. 2009;15(13):1403-14. J Tan, C McKenzie, M Potamitis, A N Thorburn, C R Mackay, L Macia. The role of short-chain fatty acids in health and disease. Adv Immunol. 2014;121:91-119. P V Chang, L Hao, S Offermanns, R Medzhitov. The microbial metabolite butyrate regulates intestinal macrophage function via histone deacetylase inhibition. Proc Natl Acad Sci U S A. 2014 Feb 11;111(6):2247-52. R Peltonen, J Kjeldsen-Kragh, M Haugen, J Tuominen, P Toivanen, O Førre, E Eerola. Changes of faecal flora in rheumatoid arthritis during fasting and one-year vegetarian diet. Br J Rheumatol.1994 Jul;33(7):638-43.