**2.4. Vitamin synthesis and the microbiome**

production [29] and three pathways for propionate production have been identified in bacteria; noteworthy peptide and amino acid can be used to form propionate and butyrate from some species of *Bacteroidetes* and *Firmicutes*. The main sources of propionate are aspartate, alanine, threonine and methionine, instead glutamate, lysine, histidine, cysteine, serine and methionine for butyrate production [30]. Sequencing targeted gene instead of 16S rRNA genes indicates that most bacteria had the capability to produce exclusively propionate or butyrate but not both. Conversely, bacteria change their fermentation depending on growth conditions and produce different SCFAs. *Roseburia inulinivorans* produces butyrate normally, but it can change its gene expression pattern in the presence of fucose producing propionate and propanol [31]. *Ruminococcus obeum* produces acetate, formate, and lactate on glucose growth and also produces propionate in the presence of fucose. *Bacteroides thetaiotaomicron* in the presence of fucose also increases fucosylated glycan to be used in absence of nutrients; it has been described that it is also important in early colonization of the infant gut [32]. By decreasing the carbohydrate content of the diet significantly reduced both fecal butyrate concentrations and numbers of the *Roseburia/E. rectale* group [33]; wheat bran has >70% arabinoxylan oligosaccharides (AXOS) that increase the SCFA content [34]. Unfortunately, the increased SCFA content causes reduced transit time and thus a decreased colonic absorption of SCFA. Excluding those vegetables rich in short fermentable carbohydrates such as oligosaccharides, monosaccharides, and polyol (FODMAP diet) reduces bacterial fermentation, showing a decrease in the total numbers of bacteria, and the fecal concentration of different SCFAs is similar to the control diet [35].

HIM generates hydrogen, carbon dioxide, and methane, all of them odorless gases; odoriferous

and volatile amines. There are many bacteria that do not produce gas [36] such as lactobacilli and bifidobacteria, so they can be used as probiotic able to reduce the gas content in colon. Gases can be excreted by flatus (several liters per day in a healthy human) [37]. Hydrogen is produced by *Bacteroides* and *Clostridium* [38] and produces a high energy yield, and it can be used by other bacteria from the gut to produce lactate, succinate, and ethanol and sulfatereducing bacteria (SRB), where *Desulfovibrio* is the principal [39]. In the methanogenesis CO2

both use hydrogen [38]. Carbon dioxide is between 5 and 50% of the total flatus volume, and it is produced by acidification of bicarbonate in the upper gastrointestinal tract, and bacterial metabolism [40], *C. sporogenes*, *C. butyricum*, and *C. perfringens*, produces hydrogen and CO2

HIM has an important role converting protein metabolism, enzymes, mucin in short peptides,

and species of *Bacteroides* and *Propionibacterium* as the predominant proteolytic characteristics in fecal samples [41]. A preference for amino acid fermentation at higher ranges of colonic pH and a reduction in quantity when fermentable carbohydrate was available are observed [42]. The proximal colon was predominantly saccharolytic by nature; whereas protein fermentation increased distally, the fermentation was associated with the presence of phenol, indole, ammonia, and branched-chain fatty acids [21]. Aromatic amino acids phenylalanine, tyrosine,

, and in the acetogenesis dioxide and hydrogen are converted into acetate

S), Clostridia, Streptococci, Staphylococci, *Bacillus*

, hydrogen sulfide, indole, skatole,

.

**2.2. Gas production and the microbiome**

is converted to CH4

fatty acids and gases (H2

**2.3. Proteins**

14 Diabetes Food Plan

gases constitute less than 1% of total flatus and include NH<sup>3</sup>

, NH4

, CO2

and H2

Gut microbiome can synthesize certain vitamins, such as vitamin K, biotin, cobalamin, folates, nicotinic acid, pantothenic acid, pyridoxine, riboflavin, and thiamine of B group [44]. For instance, subjects having low vitamin K diet showed an important decrease in plasma prothrombin when treated with broad-spectrum antibiotic [45]. Explored genomes in gut showed presence of eight vitamin B synthesis pathways. The most represented were riboflavin [46] and niacin with 162 genomes. *Bacteroidetes* is the phylum with larger B predicted vitamin generators. In the same line, bacteria can complement the biosynthesis of vitamins. In sum, GM can contribute with 25% of total dietary vitamin intake [47].

#### **2.5. Bile acids and the microbiome**

Gut microbiota can modify the structure of bile acid in the colon, because bile acids have antimicrobial activity causing membranes and DNA damage [48]. Deoxycholic acid produced by microorganisms is tenfold greater than cholic acid producing a feedback to control bacteria population [49]. Bile salt hydrolase enzyme has been recognized in *Bacteroides*, *Bifidobacterium*, *Clostridium*, *Lactobacillus*, and *Listeria* [50]. It can deconjugate bile acids reducing its toxicity [51]. Microbial dehydroxylation by *Clostridium and Eubacterium* transforms chenodeoxycholic acid and cholic acid into lithocholic acid and deoxycholic acid, it can produce a cytotoxic effect on enterocytes, and it can be associated with colon cancer. On the other hand, bile acids are also a ligand for nuclear receptor farnesoid X receptor (FXR) and plasma membranebound GPR TGR [52] that regulates their synthesis and affects the lipid and glucose metabolism [53]. Bacteria deconjugate bile acids reducing the efficacy of lipid emulsification showing a downstream effect in metabolic processes.
