Intermediate role of gut microbiota in vitamin B nutrition and its influences on human health

1Vitamin B is a group of water-soluble essential nutrients that serve as precursors of essential cofactors in numerous metabolic pathways. Vitamin B deficiency would cause a series of diseases including cognitive dysfunction, neuropathy, cardiovascular disease (CVD), and osteoporosis. Humans cannot synthesize these vitamins de novo, so vitamin B is mainly supplied through diet and, to a lesser extent, synthesized by gut microbiota. With the assistance of specific transporters, dietary vitamin B is primarily absorbed in the small intestine while the vitamin B synthesized by gut microbiota is absorbed in the colon, where microbes are densely populated.

2As the most dominant member of the gut microbiota, gut bacteria could interfere with the utilization of vitamin B either directly or indirectly. Gut bacteria that are capable of synthesizing and supplying an excessive amount of vitamin B for the host, as well as other intestinal microbes are defined as vitamin B-producers. On the other hand, bacteria that can’t produce vitamin B but require vitamin B to maintain normal physiological functions are called vitamin B-consumers. The balance between vitamin B-producers and -consumers determines the role of gut microbiota, whether as suppliers or competitors, to the host. Gut microbiota could indirectly affect vitamin B utilization by interfering with the nutrient absorption process, which is composed of the release of vitamin B from food and its transport across the intestinal epithelial layer. Several physiological factors jointly determine the absorption process of vitamin B: digestive enzymes, gut motility, acidity of gastrointestinal tract, the transporters, and the bound proteins. These factors are largely affected by disease progress of inflammatory bowel diseases (IBD), which could be triggered by invasion of the adherent pathogenic microbiota. In contrast, probiotics could prevent or alleviate IBD, resulting in a normalized physiological feature of the gut. Hence, gut microbiota’s bidirectional effects on gut might influence the absorption process of vitamin B.

3Moreover, vitamin B supplementation could change the profiles of gut microbiota, including the diversity, abundance, and function. Considering the crucial role of gut microbiota on human health, disruptions of gut microbiota are associated with multiple disease progressions, such as neurological disorders, CVD, obesity, metabolic diseases, and non-alcoholic liver disease. Since many bacterial functions originate from their metabolites, a variation in the production of metabolites caused by vitamin B could modulate host health. For now, short-chain fatty acids (SCFAs) are the most well-studied small-molecule metabolites of gut microbiota linking vitamin B nutrition to the maintenance of the host’s intestinal homeostasis and benefits of extra-intestinal organs. Other metabolites, however, due to limited published knowledge concerning vitamin B’s influences on them, would not be discussed in the current review.

4The potential interaction between vitamin B and gut microbiota has raised the interest to study the relationship between vitamin B and gut microbiota. Recently, excellent review papers have been published to reveal the role of vitamin B on gut microbiota, how gut microbiota affect the absorption of vitamin B remains unclear. To establish the relationship between vitamin B and gut microbiota, it is required to understand not only the role of gut microbiota on vitamin B absorption, but also the influence of gut microbiota on vitamin B absorption. The purpose of this review is to summarize the role gut microbiota on vitamin B absorption and the indirectly beneficial role of vitamin B on human health via gut microbiota.

5Vitamin B1, also known as thiamine monochloride, thiamine chloride, and aneurine, is a thermal unstable water-soluble essential vitamin. Thiamine is essential for all organisms to metabolize carbohydrates and branched-chain amino acids through glycolysis and the tricarboxylic acid cycle. Dietary vitamin B1 occurs to be thiamin pyrophosphate (TPP), which is catalyzed by thiamine pyrophosphate kinase in the presence of ATP to produce thiamine. Vitamin B1 performs as a cofactor of transketolase in the pentose phosphate pathway and pyruvate dehydrogenase and α-ketoglutarate dehydrogenase in the mitochondria. The World Health Organization/Food and Agriculture Organization recommends a daily intake of 1.1–1.2 mg of vitamin B1 for adults. Vitamin B1 deficiency can induce drowsiness, beriberi, polyneuritis, and Wernicke–Korsakoff syndrome (WKS). Thiamine administration reduces the progression of neurological deficits caused by WKS. Nutritional intervention for the treatment of hepatic encephalopathy includes adequate, but not excessive, vitamin B1 supplementation.

6Dietary vitamin B1 is primarily absorbed in the small intestine, while those vitamin B1 produced by intestinal microbes are mainly absorbed in the large intestine (Figure 1; ). Human beings are not able to store free thiamine, and only small amounts of phosphorylated thiamine exist in cells. Therefore, a continuous supply of dietary vitamin B1 is required. Most dietary thiamine is present in phosphorylated forms, which are subsequently hydrolyzed by intestinal alkaline phosphatase to free thiamine. Studies have suggested that the proximal small intestine is the primary site for the absorption of free thiamine. Thiamine absorption relies on both unsaturated passive diffusion and saturated active transport. In the case of an oral dose below 5 mg, free thiamine is mainly absorbed by the intestinal epithelium through thiamine transporters (THTR-1 and THTR-2, encoded by genes SLC19A2 and SLC19A3). Similarly, microbiota-produced vitamin B1 is absorbed either identically with to the dietary origin or directly by the colon through the TPP transporter (encoding gene SLC44A4).

7Bacteroidetes and Fusobacteria are the most common two bacterial phyla that are able to synthesize TPP. Some intestinal microbiota can produce vitamin B1, such as Bacteroides fragilis, Prevotella, Fusobacterium varium, Actinobacteria, and Clostridium. About half number of enzymes presented in these microbiotas take part in the de novo synthesis of thiamine. However, there also remains gut microbiota in which vitamin B1 synthesis pathway is absent and external source is required for their growth, such as Ruminococcaceae. For these bacteria, competing with the host for vitamin B1 is a potential approach for survival, especially in the small intestine.

8The gut microbiota may have impact on gastrointestinal functions, therefore affecting the absorption of vitamin B1. Clostridium botulinum isolated from the feces of botulism infant can produce thiaminase I to degrade vitamin B1. According to in vitro study on Caco-2 cells, thiamin uptake was significantly reduced by Enterotoxigenic Escherichia coli infection, which reduces the expression levels of THTR-1 and THTR-2. Transcellular H+/OH– gradient has been proved to be the driver of THTR-1 and THTR-2 dependent thiamine transport. In addition, vitamin B1 is easy to be oxidized and lose activity in neutral and especially alkaline environment. Lactic acid bacteria can produce acids such as Bifidobacterium, Lactobacillus, Enterococcus, and Streptococcus in the intestinal tract. The growth of these microbiota is then deduced to improve the absorption of thiamine via their regulation of intestinal pH. In the study of Ibrahim Elmadfa et al., the probiotic yogurt microbiota including Lactobacillus casei GG could reduce vitamin B1 bioavailability in humans. In contrast, yogurt containing Lactobacillus. exhibited an oppositive effect on plasma vitamin B1 levels in healthy adult humans, and this observation might be attributable to the complex interactions among vitamin B1, gut microbiota, and host. Thus, further studies are required to reveal these complex interactions.

9Vitamin B1 is essential to the growth of microorganisms, therefore changing the form of gut microbiota. In the absence of thiamine, the population of thiamine acquisition mutant strain was rapidly decreased in vitro. An initial population of 90% had fallen to 0.1% by day 5. Another study found that the Eubacterium rectale A1-86, Roseburia intestinalis M50/1 were unable to grow in the absence of thiamine even though they have the genes encoding thiamin synthesis. By contrast, the growth level was improved when supplied with thiamine. With the increase of exogenous thiamine, the population of thiamine acquisition mutant was increased in vitro. The optimal growth was achieved in human intestinal thiamine-nutrient-deficient bacteria, such as Subdoligranulum variabile DSM 15176, E. rectale A1-86, R. intestinalis M50/1, and Roseburia inulinivorans A2-194, when thiamine concentrations were at or above 5 ng/mL. The relative abundance is correlated with some intestinal bacteria that require peripheral intake of vitamin B1. A positive correlation was reported between vitamin B1 and the relative abundance of Ruminococcaceae (Firmicutes phylum), which lacks the vitamin B1 synthesis pathway and requires a supply of vitamin B1 from the host diet. Supplying thiamine to cows’ diet could increase the abundance of cellulolytic microbiota such as Bacteroides, Ruminococcus 1, Pyramidobacter, Succinivibrio, and Ruminobacter, and enhance ruminal acetic acid production. Their increase enhanced the fiber degradation and ruminal acetate production, thus improving ruminal function.

10Vitamin B1 has an important influence on SCFAs production by gut microbiota. Faecalibacterium, the most abundant of the rumen cocci family, expresses pyruvate iron oxidoreductase to convert pyruvate to acetyl coenzyme in the butyrate production pathway. During this process, vitamin B1 is required as a coenzyme, emphasizing its importance in butyrate production.

11As the precursor of flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), vitamin B2 (Riboflavin) plays a crucial part in multiple biological redox reactions, energy metabolism, antioxidant, and anti-inflammatory, synthesis, and activation of vitamin B6 and vitamin B9. Considering the protective role of vitamin B2 in various medical conditions, vitamin B2 deficiency is related to multi-system dysfunction. Lack of vitamin B2 can alter cellular metabolic homeostasis, which leads to night blindness, cataracts, anemia, migraines, and dermatological symptoms. Early diagnosis coupled with riboflavin supplementation can heal up ariboflavinosis in an early stage. In addition, a previous study showed that vitamin B2 intake was inversely associated with colorectal cancer and its adequate supply can effectively lower blood pressure in hypertensive patients identified with the MTHFR 677TT genotype.

12Dairy products, green vegetables, fruits, eggs, and meat are excellent sources of vitamin B2. The absorption of vitamin B2 depends on the riboflavin hydrolyzed from FMN and FAD by alkaline phosphatases and FMN/FAD pyrophosphatases (Figure 1;, ). Enterocytes actively involves in the absorption of vitamin B2 through a carrier-mediated process, contributing up to 30 mg of riboflavin at each meal. Following cellular uptake, riboflavin may generate FMN by ATP-dependent phosphorylation, and most FMN is further converted to FAD. Vitamin B2 can be absorbed through the basolateral membrane of the enterocytes in the form of free riboflavin or FMN, and then released into the portal blood toward the liver.

13De novo synthesis pathway of vitamin B2 was found in nearly all genomes of Bacteroidetes, Fusobacteria, and Proteobacteria. It is estimated that 65% of bacterial genomes (a total number of 256 genomes), can produce vitamin B2 according to a systematic genome assessment of B-vitamin biosynthesis. Interestingly, Clostridium acetobutylicum, Eremothecium ashbyii, Ashbya gossypii, Bacillus subtilis, and many other strains have been industrialized to produce vitamin B2. Vitamin B2 produced by gut microbiota is mainly adsorbed in the colon, serving as an additional source of daily vitamin B2 uptake other than dietary supply. Over-expression of riboflavin biosynthesis genes in Lactococcus lactis could switch its role from a consumer into a producer of vitamin B2. Compared with the native strain, administration of L. lactis modified to produce riboflavin in riboflavin-deficiency rats could improve their riboflavin status, suggesting its importance on maintaining the state of vitamin B2.

14Gut microbiota may change the physiological or pathological conditions of the gastrointestinal tract, and then influence the absorption of vitamin B2. The decreased gastric emptying rate has been reported to improve the bioavailability of vitamin B2. Lactobacillus reuteri and Lactobacillus gasseri OLL2716were able to slow down the gastric emptying rate. The phenomenon indicated that these species of bacterial have a potential influence on vitamin B2 absorption. On the other hand, several bacteria could intervene the gastric disease progression. For example, a lower plasma level of vitamin B2 was observed in gastric cancer patients with the presence of Helicobater pylori than in patients without infections. Meanwhile, lactic acid bacteria, as complementary treatments for intestinal inflammation, have been proved to be effective in the suppression or prevention of Helicobater pylori infection. These results indicate that gut microbiota may affect the absorption of vitamin B2 by the way of changing rate of gastric emptying.

15The growth of anaerobic bacteria Blautia coccoides, R. intestinalis and Enterococcus faecalis was stimulated by vitamin B2 in vitro. Replenishment of vitamin B2 has been reported to alter the composition and β diversity of gut microbiota in mice. Colon-targeted vitamin B2 supplementation for 3 weeks improved the α diversity of gut microbiota in healthy volunteers but not in Crohn’s disease patients, suggesting that effects of vitamin B2 on gut microbiota are different between groups of people with different physiology situation microbiota. In clinical interventions, vitamin B2 supplementation for 2 weeks increased the relative abundance of F. prausnitzii in healthy individuals. Moreover, treatment of vitamin B2 and vitamin C significantly reduced the number of Proteobacteria, and showed a trend to increase the number of Firmicutes and decrease the number of Bacteroidetes. Another study also reported that the relative abundance of Streptococcus was negatively correlated with dietary intake of vitamin B2. Several groups of Streptococcus are thought to be correlated with strep infection, such as Streptococcus pyogenes and Streptococcus pneumoniae. Consequently, sufficient daily intake of vitamin B2 might protect the host from strep infections. In volunteers with migraine, daily supplementation of vitamin B2 increased the abundance of anaerobic bacteria Roseburia, and F. prausnitzii, while decreased the abundance of E. coli.

16Vitamin B2 forms part of the electron transfer flavoprotein complex of butyryl-CoA dehydrogenase. Consequently, butyrate production is directly affected by vitamin B2. Concentration of SCFAs produced by human gut isolates rely upon vitamin B2 in in vitro condition. Furthermore, cecal SCFAs content of vitamin B2 depletion-repletion mice model was remarkably increased by the repletion of vitamin B2. Faecalibacterium prausnitzii is an effective producer of SCFA, especially butyrate, and a 2-week vitamin B2 supplementation in healthy individuals could increase the abundance of Facealibacterium. prausnitzii, which is an effective producer of SCFAs, especially butyrate. A randomized trial showed that butyrate production was significantly increased in groups treated with 50 and 100 mg/d of vitamin B2.

17Nicotinamide (NAM) and nicotinic acid (NA) are collectively referred to as vitamin B3. The functional cofactors derived from vitamin B3 are nicotinamide adenine dinucleotide and nicotinamide adenine dinucleotide phosphate, providing reducing equivalents for cellular biochemistry and energy metabolism. Vitamin B3 is considered as a potent antioxidant capable of protecting the brain’s cellular membranes and is deemed to be effective in neurodegenerative diseases. Aging-related nicotinamide adenine dinucleotide deficiency in the retina is considered as a possible reason for glaucoma in the elderly, and oral administration of vitamin B3 could alleviate the symptoms. The most typical disease caused by vitamin B3 deficiency is pellagra, which is characterized by inflammation of mucous membranes, skin lesions, diarrhea, and dementia. Besides, NA is widely used to treat lipid disorders.

18Vitamin B3 can be obtained from both endogenous and exogenous sources. The endogenous source is produced from tryptophan, and the exogenous source is dietary NA and NAM (Figure 1). NAM mainly exists in animal products, while NA mainly exists in plant-based foods such as beans. With human-derived intestinal epithelial Caco-2 cells and purified isolated brush-border membrane vesicles, a study on the mechanism of NA uptake in humans suggested the existence of an acidic pH-dependent and carrier-mediated mechanism in the physiological range of niacin concentrations. Based on the environmental acidity, NA is absorbed in the stomach and the upper part of small intestine by proton co-transporters and an anion antiporter. Besides, it has also been reported that vitamin B3 could be absorbed by passive diffusion at higher concentrations.

19Several gut microbial species possess vitamin B3 biosynthesis pathways and can synthesize vitamin B3 from tryptophan. Bacteria with vitamin B3 biosynthesis pathway include B. fragilis, Prevotella copri, Ruminocccus lactaris, Clostridium difficile, Bifidobacterium infantis, H. pylori, and F. varium. Nabokina et al. reported that NA uptake by Caco-2 cells increased when the extracellular pH reduced from 8.0 to 5.0. Considering that the absorption of vitamin B3 is pH-dependent, we can deduce that lactic acid bacteria, such as Bifidobacterium, Lactobacillus, Enterococcus, and Streptococcus, can change the intestinal acidity and influence the absorption of NA.

20For now, the role of vitamin B3 on gut microbiota remains unclear. Fangmann et al. found a reduced α diversity and a decreased abundance of Bacteroidetes in the microbiome of obese subjects under a low dietary NA intake. For this reason, delayed-release microcapsules targeting the ileocolonic region were developed to deliver vitamin B3 to the microbiome, avoiding the adverse side effects of NA. Such vitamin B3 containing microcapsules significantly increased the abundance of Bacteroidetes in human gut. Another study has also shown that NA plays an essential role in maintaining normal intestinal homeostasis, reducing intestinal inflammation, and regulating the production level of intestinal antimicrobial peptides. The supplementation of NA was also reported to increase the production of acetate while reducing the ratios of propionate/acetate and butyrate/acetate in the colonic contents of piglets, suggesting a beneficial effect of NA on SCFAs production.

21Vitamin B5, also known as pantothenic acid, is available in a variety of plants and animal products and unprocessed grains. Food-derived vitamin B5 is converted to pantethine and then to Acetyl CoA and acyl carrier protein. These two compounds are critical in burning fats and carbohydrates in energy metabolism. Acetyl CoA is also required to produce adrenal hormones including Cortisol (steroid hormone). Hence, vitamin B5 deficiency is associated with impaired adrenal cortical function. Patients with rheumatoid arthritis were observed with lower blood pantothenic acid and the severity of arthritis is negatively correlated to the level of vitamin B5. The most common symptoms associated with vitamin B5 deficiency were headache, fatigue, and a sensation of weakness. Health volunteers fed a semisynthetic, pantothenic acid-free diet for 9 weeks developed subclinical signs of fatigue ad listlessness without clinical symptoms. Low serum vitamin B5 level also associates with an increased incidence of hypertension.

22The intestinal tract is exposed to two sources of vitamin B5: diet- and bacteria-origin. Dietary vitamin B5 exists mainly in the form of coenzyme A, which is hydrolyzed to pantetheine by alkaline phosphatase and then quickly converted into the absorbable forms of pantothenic acid by pantetheinase in the intestinal lumen (Figure 1; ). At low luminal concentrations, free pantothenic acid is actively transported via the sodium-dependent multivitamin transporter (SMVT, SLC5A6). At higher concentrations, passive diffusion of vitamin B5 occurs, and no significant difference in the transport rate in different segments of the intestine. It is indicated that absorption of the bacterially synthesized vitamin B5 in the large intestine also involves the SMVT system, though direct evidence is lacking.

23Vitamin B5-producing bacteria and vitamin B5-consuming bacteria coexist in the intestinal tract. Vitamin B5-producing bacteria include E. coli and Salmonella typhimurium. Escherichia coli can utilize aspartate and intermediate metabolites of valine biosynthesis as substrate. Salmonella typhimurium produces pantothenate from alpha-ketoisovalerate using acetohydroxy acid synthase isozyme I and dihydroxy acid dehydratase enzymes. Other bacteria, such as Lactobacillus helveticus, Streptococcus, and E. faecalis, members of the vitamin B5-non-producing Firmicutes phylum, require vitamin B5 for their growth in vitro, which might compete vitamin B5 with the host.

24For now, the regulatory effect of gut microbiota on vitamin B5 absorption is still unclear. However, extensive evidence supports that utilization of bacterially synthesized vitamin B5 depends on the gut microbiota, especially in vitamin B5 deficiency. Chemicals containing sulfonamide functional group, including succinylsulfathiazole and sulfathiazole, can change the composition and function of the gut microbiota. It was reported that mice consuming the vitamin B5-deficient diet and receiving succinylsulfathiazole exhibit signs of pantothenic acid deficiency. A similar phenomenon has also been observed in rats. Moreover, the addition of succinylsulfathiazole to a vitamin B5-normal diet deteriorated the deficiency of pantothenic acid, resulting in signs of pantothenic acid deficiency characterized by achromotrichia and porphyrin-caked whiskers appearing in 3 weeks.

25At present, little is known about the role of dietary vitamin B5 supplementation on gut microbiota. Enhanced vitamin B5 intake appears to increase the relative abundance of Prevotella and Actinobacteria and to decrease the abundance of Bacteroides in lactating women. Non-linear effects of vitamin B5 on the diversity and abundance of intestinal microbiota were observed. In fish, a diet supplemented with 26.0 mg/kg of vitamin B5 increased the diversity and abundance of intestinal microbiota compared with other levels of vitamin B5 supplementation. The maximum portion of Proteobacteria, Firmicutes, and Mycoplasma appeared in the 20.0 mg/kg, 26.0 mg/kg, and the no-vitamin B5-supplement group, respectively. The minimum portion of Proteobacteria, Firmicutes, and Tenericutes appeared in the 37.0, 20.0, and 26.0 mg/kg groups. Among the investigated bacterial species, Mycoplasma is suggested to be correlated with infectious disease pathogenesis. It can be inferred from the experimental results that vitamin B5-supplement could inhibit the number of Mycoplasma thus increasing the anti-infection abilities. An in vitro study of L. helveticus demonstrated that a vitamin B5-deficient medium greatly inhibited the synthesis of fatty acid and protein, and this observation is likely to be explained by the downregulated expression of genes associated with fatty acid synthesis and biotin metabolism. Together, these results indicate that deficiency of vitamin B5 might change the growth profile and the biological function of intestinal microbiota.

26Vitamin B6 is a group of compounds with a defined structure, including pyridoxine (PN), pyridoxal, and pyridoxamine, and all of them have 5-position phosphoryl derivatives. All these compounds cannot be synthesized by the human body and are mainly provided by diet. Animal products mostly contain pyridoxal 5’-phosphate (PLP) and pyridoxamine 5’-phosphate, while pyridoxine 5’-phosphate (PNP) is the dominant form in plant products. The recommended daily intake amount is 1.3∼1.7 mg for adults. Vitamin B6 is a cofactor in many metabolic reactions that include amino acid metabolism, biosynthesis, and degradation of sphingolipid and carbohydrate metabolism. Moreover, PLP is the primary active form among all forms of these above derivatives. It acts as a cofactor for many enzymes, such as tryptophan synthase, O-acetylserine sulfhydrylase, 5-Aminolevulinate synthase, serine hydroxymethyltransferase, and branched-chain amino acid aminotransferase. Vitamin B6 deficiency may cause neuromuscular irritability, peripheral neuropathy, dermatitis, stomatitis, cheilosis, depression of the immune system, and sideroblastic anemia. Moreover, vitamin B6 has also shown its importance in maintaining a regular function of cognition and keeping the elderly away from CVD. Additionally, supplementation of vitamin B6 could protect against reactive oxygen free radicals.

27The phosphorylated form of vitamin B6 in the diet is hydrolyzed by pyridoxal phosphatase and then absorbed in the intestinal lumen (Figure 1; ). Many studies demonstrate that, under normal circumstances, the concentration of vitamin B6 in food is not saturable and vitamin B6 enters intestinal cells by passive diffusion. However, a study with Caco-2 cells has recently challenged this concept. It gives clear evidence for a specialized, carrier-mediated system for the uptake of PN. Another study finds that the unique carrier-mediated mechanism for PN uptake functional exists in mammalian colonocytes. And these results indicate that the uptake of vitamin B6 could be affected by several extracellular and intracellular factors. As an instance, Said et al. demonstrated a pH-dependent and amiloride-sensitive system involving PN uptake in intestinal epithelial cells.

28For humans, there are two primary sources of vitamin B6, one is from the diet, and the other is produced by normal gut microbiota. For example, B. fragilis and P. copri (Bacteroidetes), Bifidobacterium longum and Collinsella aerofaciens (Actinobacteria) and H. pylori (Proteobacteria), and these species of bacteria can produce vitamin B6. Furthermore, many studies have shown that there are two biosynthetic pathways, the deoxyxylulose 5-phosphate (DPX)-dependent pathway for PLP and the DPX-independent pathways for PNP, and different gut microflora decide on different pathways to choose.

29Under normal circumstances, dietary and bacterial sources can offer enough vitamin B6 for the human body to absorb, and deficiency of vitamin B6 is a rare phenomenon. However, some researchers reported that drugs, alcohol, and smoking, deoxyxylulose 5-phosphate (DPX). Furthermore, these factors above could change gastrointestinal motility, which might bring about a narrowed absorption window and a reduced bioavailability for vitamin B6. Moreover, such intestinal environmental factors could change gut microbiota composition. Ferrer et al. found that gut microbiota in the lean gut seems to get more involved with vitamin B6 biosynthesis and offer more vitamin B6 for absorption. At the same time, the hydrolysis of vitamin B6 is a significant procedure for absorbing vitamin B6 and is closely related to the pH of the intestinal lumen. Since some species of lactic acid bacteria, including Bifidobacterium, Lactobacillus, Enterococcus, and Streptococcus, can produce acid to lower the pH, we may infer that variation in the proportion of these specific bacterial can influence vitamin B6. Meanwhile, alkaline phosphatase is the essential enzyme during this process. It is also able to affect the growth of gut microbiota. The intervention between alkaline phosphatase and gut microbiota is complicated, and it is essential to figure out its influence on vitamin B6 absorption. Unfortunately, there is not much investigation about it.

30In the intestine, vitamin B6 serves as essential nutrients for the gut microbiota. Some species of microbiota lack the ability to biosynthesize vitamin B6, such as most genera within the Firmicutes phylum (Veillonella, Ruminococcus, Faecalibacterium, and Lactobacillus spp.). Hence, they acquire exogenous vitamin B6 from the intestinal tract to maintain their life activities. The proportion of food compositing vitamin B6 can influence the profile of the gut microbiota. For example, a larger amount of vitamin B6 absorbed from food was associated with greater richness and evenness of the gut microbiota. Mayengbam et al. investigate the composition of gut microbiota and their metabolites in a rat model with vitamin B6 deficiency. They reported that arginine biosynthesis was impaired in this rat model, and the vitamin B6 metabolism was affected. Arginine not only performs as a substrate for protein synthesis but also as a precursor for a variety of molecules linked to cell function, such as nitric oxide. Insufficient vitamin B6 might interferes the host’s de novo protein synthesis and related cell functions. Vitamin B6 produced by microbiota is not enough for gut microbiota, and the composition of gut microbiota is changed for this reason.

31Vitamin B7, also known as biotin, acts as a cofactor for multiple carboxylases that are associated with fatty acid, glucose, and amino acid metabolism. Vitamin B7 is exclusively synthesized by plants and microbiota such as bacteria and yeast, so vitamin B7 synthesized by microbiota in human large intestine make an important contribution to daily supplements for human in addition to food. The adequate daily requirement of vitamin B7 is 35 μg for infants and 150–300 μg for adults. And it is relatively non-toxic even at doses of greater than 60 mg/day for several months. Vitamin B7 participates in normal immune function, maintaining the integrity of the intestinal mucosa or homeostasis. It also plays an important role in maintaining skin health and anti-inflammation via inhibiting NF-κB activation. Therefore, severe vitamin B7 deficiency results in skin abnormalities, neurological disturbances, and growth retardation. Symptoms of vitamin B7 deficiency contain inflammation, loss of appetite, glossitis, dandruff dermatitis, and hair removal. Hence, therapeutical vitamin B7 supplementation can improve hair loss and prevent not only seborrheic hair loss, but also juvenile gray hair in the pathological case of biotin deficiency. Although neuroleptic effect of vitamin B7 has not been proven, it does demonstrate a beneficial effect on treatment of depression and insomnia.

32Humans are exposed to two sources of vitamin B7, including dietary sources and bacterial source in the large intestine. Dietary vitamin B7 exists in either free form or protein-bound form. Ingested protein-bound forms of vitamin B7 are firstly broken down by gastrointestinal proteases and peptidases to biocytin (biotinyl-L-lysine) and biotin-oligopeptides (Figure 1). These products are further processed in the intestinal lumen to release free biotin before absorption. Free biotin from the gastrointestinal tract is rapidly absorbed. Absorption of free biotin by the proximal intestine is mediated by SMVT, which also transports vitamin B5 and antioxidant lipoates. In the intestine, SMVT is exclusively expressed on the apical membrane of polarized intestinal absorptive cells and thus SMVT system is the only biotin uptake system in the mammalian gut.

33It has long been recognized that the normal microbiota of large intestine can synthesize large amounts of biotin. Vitamin B7-producing microbiota includes B. fragilis, F. varium, and Campylobacter coli. Meanwhile, vitamin B7-consuming bacteria must gain vitamin B7 from the environment to maintain microbial functions and these bacteria lack the vitamin B7 biosynthetic pathways. For instance, Lactobacillus possesses genes involved in obtaining biotin from environments.

34Human body lacks the capacity to produce vitamin B7, for which vitamin B7 is mainly supplied by the jejuna and to a lesser extent, from the distal gut. However, absence of gut microbiota may negatively affect circulating vitamin B7 levels. In rodent model, enhanced vitamin B7 transport was observed in decreased intestinal pH. Since lactic acid bacteria such as Bifidobacterium, Lactobacillus, Enterococcus, and Streptococcus can produce lactic acid and lower the local acidity in the intestinal lumen, it is suggested that supplementation of lactic acid bacteria might increase the absorption of vitamin B7. Rat intestinal infection of Salmonella enterica serotype Salmonella typhi results in a significant reduction in intestinal vitamin B7 intake. Obese mice induced by a high-fat diet demonstrate a changed gut microbiota profile, resulting in fewer microbes expressing genes for vitamin B7 synthesis. As a result, a reduction in vitamin B7 synthesis and lowered plasma vitamin B7 levels in obese mice were observed, indicating the significant importance of intestinal microbiota in maintaining vitamin B7 levels in obesity.

35The composition of gut microbiota may be influenced by vitamin B7. Vitamin B7-consuming bacteria with free biotin transporter, including Prevotella, Bifidobacteria, Ruminococcus, and Lactobacillus, require vitamin B7 to maintain their microbiological functions. Hence, vitamin B7 deficiency might interfere with the abundance of the above bacteria. For instance, deprivation of vitamin B7 has been reported to cause intestinal dysregulation and overgrowth of Lactobacillus murine.

36Vitamin B9 (folate) is a micronutrient for the synthesis and functional regulation of many biomacromolecules in humans. In fortified foods, supplements, and pharmaceuticals, vitamin B9 occurs in the synthetic form of folic acid. As a critical cofactor in one-carbon metabolism, vitamin B9 could transfer carbon units in methylation reaction, DNA and RNA biosynthesis, and amino acid metabolism. Megaloblastic anemia is one of the most common symptoms of vitamin B9 deficiency. The main reason for this disease is the inhibition of the maturation of erythropoietic precursors. Lack of vitamin B9 also correlates with neural tube defects. Furthermore, vitamin B9 inadequacy is associated with the pathogenesis of several chronic diseases including CVD, cancers (colorectal, prostate, and breast cancer), and Alzheimer’s disease.

37Natural folate and folic acid have similar absorption processes. In food, vitamin B9 usually occurs as folate polyglutamate, which is hydrolyzed to the monoglutamate form by glutamate carboxypeptidase II before the absorption in the brush border of the proximal part of the jejunum (Figure 1; ). Monoglutamate folate can be transported by the proton-coupled folate transporter (PCFT) across the apical membrane of enterocytes. After vitamin B9 is metabolized to 5-methyl-tetrahydrofolate in the enterocytes by dihydrofolate reductase, it can be transported by multidrug resistance-associated protein (MRP) into the portal vein. Vitamin B9 could experience enterohepatic circulation, which means it can be discharged into the bile and then reabsorbed in the intestine. Bacteria synthesized folate might be absorbed in the colon because the colon has abundant PCFTs for vitamin B9 absorption. Moreover, the enzymes that favor the absorption of vitamin B9, such as folate hydrolase, γ-glutamyl hydrolase, and folate hydrolase 2, are also highly expressed in the colon.

38The gut microbiota also plays a valuable role in producing and consuming vitamin B9. According to an evaluation of human gastrointestinal bacterial genomes, 13.3% of the bacteria possess the ability of vitamin B9 de novo synthesis, and 39% could produce vitamin B9 with extra para-aminobenzoic acid provided from other bacteria or food. It is also reported that 26% of Actinobacteria, 71% of Proteobacteria, 79% of Fusobacteria, and 15% of Firmicutes in the human gut microbes have the potential to de novo synthesize vitamin B9. A systematic genome evaluation of the vitamin B family suggested human intestinal microorganism is capable of producing 37% of daily required vitamin B9 in non-pregnant adults. Folate-producing strains have been extensively screened to fortify vitamin B9 content. Liu et al., have isolated Latilactobcillus sakei LZ217, a good producer of vitamin B9, from raw milk. Zhang et al. screened high vitamin B9-producing strains from 12 lactic acid bacteria and then obtained its variant, Lactobacillus plantarum GSLP-7 V after stressing with drugs. Based on a vitamin B9 deficient rat model induced by a vitamin B9-free diet, they further proved GSLP-7 V and its fermented yogurt could restore serum vitamin B9 and homocysteine (Hcy) to normal levels. A case that is closer to clinical application is L. reuteri ATCC PTA 6475. It has been proved to be safe for humans and could produce vitamin B9 with additional para-aminobenzoic acid. Collectively, the microbiome has beneficial potential in treating vitamin B9 deficiency. Another study pointed out that 86% of the 512 investigated bacterial reference genomes required vitamin B9 or its intermediates from food or other microbiota.

39A vitamin B9-supplement diet slightly increased gut bacterial community richness according to the abundance-based coverage estimator, compared to a vitamin B9-deficient diet in the high-fat-diet-induced obesity mouse model. The relative abundance of Actinobacteria was significantly increased, while it is the opposite for Clostridia. However, Wang et al. proved additional folic acid didn’t make significant differences in the indices of diversity in the cecum, but increased relative abundance of Lactobacillus salivarius, L. reuteri, and Lactobacillus mucosae. Vitamin B9 deficiency can influence bacterial diversity. Compared to a micronutrient-sufficient diet in gnotobiotic mice, the vitamin B9 deficiency diet increased β diversity after 21-day treatment. But a 14-day full diet treatment did not change this trend. Another study based on human gut microbiota assessed fecal microbiota composition. The fecal microbiota community has lower α and β diversity when healthy volunteers with less vitamin B9 diet. And the fecal microbiota community has a higher potential to produce vitamin B9 in vitro experiments. It’s suggested that vitamin B9 deficiency probably decreases the richness of human gut microbiota.

40Vitamin B9 also can influence the amount of SCFAs in the gastrointestinal tract. In the study of Wang et al., there were more acetic acid and valeric acid in the cecum and colon of weaned piglets fed with vitamin B9 supplementation. Liu et al. proved the vitamin B9-produced probiotics, L. sakei LZ217, could increase SCFAs content, especially for propionic acid and butyric acid in the fecal slurry cultures.

41Vitamin B12 (cobalamin) is a member of the corrinoids that is required by methionine synthase and methylmalonyl-CoA mutase. Methionine synthase is pivotal in catalyzing the conversion of Hcy to methionine. The subsequently adenosylated of methionine would generate S-adenosylmethionine to supply methyl groups for biological methylation modifications of proteins and nucleic acid. Methylmalonyl-CoA mutase is involved in mitochondrial metabolism. A daily intake of 4 μg is adequate to maintain the normal biological functions of vitamin B12, which could be satisfied by dietary supplementation with 5–30 μg. Vitamin B12 deficiency is correlated with several pathological progressions due to its important role in methylation and catabolism. Without sufficient vitamin B12 to convert total homocysteine (tHcy) to methionine, circulating level of accumulated tHcy might increase the risk of CVD. Moreover, vitamin B12 deficiency is responsible for cognitive impairment and neurological disorders, which might result from the accumulation of tHcy and methylmalonic acid. In addition, deficiency of vitamin B12 has also been reported to perform a positive association with osteoporosis, macular degeneration, and frailty.

42For human, the major source of vitamin B12 is animal products while intestinal microbiota synthesis might also contribute to a minor fraction. Vitamin B12 could be absorbed by both passive diffusion and receptor mediated endocytosis in the intestine (Figure 1). Passive diffusion is negligible in physiological doses (100–1000 μg) supplied by food or supplementation. The absorption of vitamin B12 by receptor mediated endocytosis is a multistep process. In the upper gastrointestinal tract, vitamin B12 is released from the protein carriers with the assistance of gastric acid and pepsin and then binds to haptocorrin under the acidic condition. After the degradation of haptocorrin by pancreatic proteases, the released vitamin B12 binds to intrinsic factors in duodenum. The generated vitamin B12-intrinsic factor complex is then endocytosed by mucosal cells in the distal ileum with the help of receptor cubilin, transmembrane protein amnionless, and megalin/LRP2. After entering the mucosal cells, vitamin B12-intrinsic factor complex is dissociated from cubilin in the early endosomal compartment. In the lysosome, intrinsic factor is degraded and released vitamin B12 enters the cytoplasm via LMBD1. The exit of free vitamin B12 from enterocyte might depend on MRP1. With enterohepatic circulation, the secreted vitamin B12 in duodenum would bind to intrinsic factor and then reabsorbed into the circulation.

43Intestinal microbiotas are either producers or consumers of vitamin B12. Moreover, the intestinal absorption of vitamin B12 could be in turn influenced by intestinal microbiota. Several bacteria have been reported to be vitamin B12 producers, such as L. reuteri, and Enterococcus faecium. It is supposed that vitamin B12-producing bacteria supplementation could improve vitamin B12 utilization in the gastrointestinal tract. Such an assumption has been proved in mice fed with vitamin B12 deficient diets. The supplementation of L. reuteri CRL1098, a vitamin B12-producing strain, prevented the signs of vitamin B12 deficiency, suggesting the therapeutical effect of intestinal bacteria in vitamin B12 deficiency. However, these beneficial effects might be limited if the bacteria are colonized in the colon, where lack of necessary transporters. In order to develop a probiotic treatment for vitamin B12 deficiency, the position of bacterial colonization should be considered. Around 80% of microbiota in the gastrointestinal tract are considered as consumers of vitamin B12. Hence, the overgrowth of bacteria might compete with the exogenous vitamin B12 with their host and then reduce the bioavailability. In small intestinal bacterial overgrowth, consumption of vitamin B12 by the increased anaerobesis was considered a major reason for vitamin B12 deficient symptoms. Reducing the abundance of vitamin B12-consuming bacteria is of benefit to vitamin B12 deficiency. For instance, daily probiotic treatment of Lactobacillus performed a beneficial effect on both bacterial overgrowth and vitamin B12 absorption, suggesting the probiotic treatment might improve the vitamin B12 deficiency via inhibiting the vitamin B12-consuming bacteria overgrowth.

44Besides production or consumption of vitamin B12, intestinal microbiota might indirectly change the bioavailability of vitamin B12 via exerting influences on the absorption-related physiological factors. Gastrointestinal diseases associated with reduced acid secretion or enzyme content might interfere with the release of vitamin B12 from food or the translation of vitamin B12 to intrinsic factors. Reduced vitamin B12 absorption is also observed in IBD, which is characterized by abnormal gut permeability. As a probiotic, Lacidofil treatment significantly improved the gastric acid secretion in H. pylori-infected mongolian gerbils, which conduced to the release of vitamin B12 from food. Some gut bacteria have also presented the remission effect on IBD, which might improve the absorption of vitamin B12 via normalizing the gut permeability. Besides, excessive competition between gut microbiota and the host might interfere with the bioavailability of vitamin B12. For instance, Bacteroides thetaiotaomicron expresses an essential surface-exposed lipoprotein for vitamin B12 transport named BtuG. The higher binding affinity of BtuG could remove vitamin B12 from intrinsic factors and reduce vitamin B12 absorption.

45Vitamin B12 serves as a critical cofactor of diverse enzymes in human gut microbes for nucleotide synthesis, amino acid metabolism, carbon and nitrogen metabolism, and secondary metabolite synthesis. The biosynthesis of vitamin B12 involves about 30 enzyme-mediated steps and only a small fraction of bacteria could produce this vitamin. Most gut bacteria utilize vitamin B12 that escapes the absorption in the ileum and reaches the large intestine. The competition of vitamin B12 in gut microbiota might influence their growth, colonization, and metabolic processes.

46In vitro study of colonic model suggested that vitamin B12 supplementation may increase α diversity, but the results depend on the form and dose of cobalamin administered. In another in vitro study, α diversity is reduced after methylcobalamin supplementation but not in cyanocobalamin treatment group. In mice, no significant difference in α diversity had been observed after vitamin B12 treatment, even under different doses. A study suggested that cyanocobalamin supplementation had an increase in the α diversity and exerted a significant difference in β diversity at the genus level. However, some studies were unable to support this conclusion. In humans, vitamin B12 intake could promote the increase of α diversity in adults but not in infants or children. However, the association between vitamin B12 intake and β diversity was only observed in infants at 6 months of age and in older veterans rather than in other observed groups, including infants at the age of 4 or 5 months, lactating women, and children aged between 2 and 9 years.

47In the colonic model, cobalamin supplementation would increase the relative abundance of Firmicutes and Bacteroidetes and is opposite to Proteobacteria and Pseudomonas. After 7 days of methylcobalamin supplementation, an increased proportion of Acinetobacter and declined a fraction of Bacteroides, Enterobacteriaceae, and Ruminococcaceae had been observed in another colonic model-based study. In the studies of murine, the associations between vitamin B12 supplementation and the relative abundance of bacteria have been also reported. The supplementation of vitamin B12 has elevated the fraction of Firmicutes and reduced the proportion of Bacteroidetes. Compared to methylcobalamin, cyanocobalamin treatment resulted in higher levels of Bacteroidetes and Proteobacteria and lower levels of Firmicutes in mice. In humans, vitamin B12 intake might increase the proportion of Proteobacteria and Verrucomicrobia and reduce the abundance of Bacteroidetes. However, some clinical studies also suggested that vitamin B12 intake had no influence on bacterial abundance. These controversial results might be due to the different study designs and participants.

48In vitro study suggested that the addition of cobalamins increases the generation of SCFAs, especially butyrate and propionic acid. Another in vitro study indicated that low-dose cyanocobalamin-enriched spinach could increase the generation of butyrate and acetate. In mice, a reduction of SCFAs has been observed in dietary vitamin B12 restriction. However, the effect of oral vitamin B12 on cecal SCFA was absent in mice with dextran sodium sulfate induced colitis.

49In this review, we summarized current knowledge about the interaction between gut microbiota and vitamin B nutrition (Figure 2) to infer the consequence of probiotics supplementation, which might be helpful to optimize the treatment of probiotics. Vitamin B perform as essential micronutrients for human. The absorption process of multiple dietary vitamin B requires the assistance of many transporters, which generally occurs in the small intestine (Table 1). At the same time, gut microbiota not only act as producers and/or consumers to modify the supplementation of vitamin B in the gut (Table 2) but also affect the absorption of vitamin B by altering the physiological or pathological factors of the gastrointestinal tract (Table 3). As cofactors of multiple enzymes, supplement with vitamin B can change the diversity, abundance, and functions of gut microbiota (Table 4). Understanding the interactions between vitamin B and gut microbiota can help us prevent vitamin B deficiency and, more importantly, allow us to recognize beneficial potential of probiotics on human healthy.

50In addition to supplements, the probiotic product has recently been approved by FDA as the therapeutical approach for disease treatment, which has significantly extended the application of probiotics. Considering their ability for vitamin B production and intestinal function modification, probiotics might be a potential therapeutical approach for vitamin B deficiency. Although conventional vitamin B supplements have been applied, they are insufficient to treat vitamin B deficiency patients of malabsorption. In this population, improved intestinal absorption rather than vitamin B supplementation would be a more efficient approach to improve vitamin B nutrition. The combination of probiotics regulating intestinal function and vitamin B producers/supplements could further increase the absorption of vitamin B in the gastrointestinal tract. Despite the beneficial effect on human health, risks are remaining in probiotics treatment. The overgrowth of Lactobacillus murinus after vancomycin treatment might deplete vitamin B7 available, suggesting the potential risk of nutrition competition after probiotics treatment. For the purpose of reducing the risk of nutrition competition, vitamin B should be supplied along with probiotics. Another consideration of vitamin B supplementations in probiotics treatment is the beneficial effect of vitamin B on the growth and function of gut microbiota. But types and dosages of vitamin B should be optimized as supplements of probiotics treatment. Overall, probiotics treatment could be a potential treatment for vitamin B deficiency, and vitamin B supplement might either reduce the risk of probiotics-induced vitamin B deficiency or improve the efficacy of vitamin B. Nonetheless, these assumptions are still requiring further scientific and clinical studies to optimize the combination of vitamin B and probiotics treatment.

51HH and JK designed the manuscript. ZW, LS, JWZ, SL, YXZ, RW, YCZ, JHZ, ZZ, JD, and LC performed the literature review. ZW, JHZ, and ZZ drafted the manuscript. ZW, HH, KH, and JK revised the manuscript. All authors contributed to the article and approved the submitted version.