Vitamin B, Role of Gut Microbiota and Gut Health

Abstract

The human gastrointestinal system is constantly exposed to pathogenic microorganisms and beneficial compounds, such as food components and commensal bacteria. Vitamin B are a class of water-soluble organic compounds obtained through diet, supplementation, and gut microbiota synthesis. B vitamins are absorbed for host metabolism in the small intestine, whereas microbes produce and absorb B vitamins in the large intestine. The authors have accumulated evidence from various studies that each B vitamin plays an essential role in gastrointestinal health and has a reciprocal relationship with the gut microbiota. Previous studies have also proven that microbial imbalance in the gut lead to competition for the utilization of B vitamins between the host and microbes, affecting the gut microbial composition, gut health, and host metabolism. This review aims to explain further the types of B vitamins in human digestion, the mechanism of B vitamin synthesis, and the role of B vitamins in the composition of the gut microbiota and the health of the gastrointestinal tract. Thus, it can help practitioners to consider administering B vitamins to maintain the patient’s gut health.

1. Introduction

The human gastrointestinal system is continuously exposed to toxic compounds, such as pathogenic microorganisms, and beneficial compounds, such as food components and commensal bacteria. Therefore, the immunity of the gastrointestinal system must be balanced between an active and suppressive immune response. Vitamins are micronutrients essential for normal human metabolism because they have various physiological effects, one of which is immunity. So, vitamin deficiency causes an increased risk of infectious diseases, allergies, and inflammation that can damage the gastrointestinal system. Over the past decade, a large number of studies have investigated the role of vitamins in various gastrointestinal diseases, including their potential in the prevention or treatment of various malignancies, inflammatory diseases, and hepatobiliary disorders [1, 2].

One of the vitamins that have a role in the gastrointestinal system is vitamin B, a group of water-soluble organic compounds with various functional roles, including cofactors in many enzymatic reactions, cellular energy-producing reactions, neurotransmitter synthesis, cell signaling, nucleic acid biosynthesis, and immune function. The intestine is an important organ, especially for storing and absorbing food. The gut also has the densest microbiota in the body, consisting primarily of four major phyla, that is, Bacteroidetes, Firmicutes, Proteobacteria, and Actinobacteria. The microbiota will maintain a symbiotic relationship with the host and protect against harmful pathogens. Some gut microbiota also produces limited amounts of B vitamins, and some require B vitamins for growth. Much evidence has shown that a healthy gut is associated with a healthy microbiota ecology, and B vitamins play an essential role in modifying the gut microbiota [3].

Although B vitamins from food are mainly absorbed in the small intestine, excess B vitamins cannot be absorbed in the small intestine and are transferred to the more distal intestine. Thus, the biosynthesis of B vitamins is mostly supplied by the distal gut microbiota. B vitamins in the distal intestine are dominant in performing important physiological functions in the body, including acting as nutrients for the host and microbiota, regulating immune cell activity, mediating drug efficacy, suppressing colonization of pathogenic bacteria, and modulating colitis. Therefore, the deficiency of B vitamins will certainly damage normal cell metabolism and trigger the development of several pathogenic microorganisms in the intestines [3, 4].

2. Types of B Vitamins in human digestion

Vitamins are essential micronutrients that all living cells need to carry out biochemical reactions. Vitamins are classified into fat-soluble and water-soluble. The fat-soluble vitamins are vitamins A, D, E, and K, while the water-soluble vitamins include vitamin C, biotin (vitamin H or B7), and a series of B vitamins—thiamin (B1); riboflavin (B2); niacin (B3); pantothenic acid (B5); pyridoxine, pyridoxal, pyridoxamine (B6); folic acid (B9), and cobalamin (B12) [5]. Fat-soluble vitamins act as essential elements of cell membranes, and the excess is stored by cells. Water-soluble vitamins function as coenzymes for specific chemical biochemical reactions, and the excess is excreted in the urine [6].

2.1 Vitamin B1

Vitamin B1 (thiamin) is a cofactor for several enzymes, including pyruvate dehydrogenase and a-ketoglutarate dehydrogenase, which are involved in the tricarboxylic acid (TCA) cycle. The thiamin molecule consists of a pyrimidine ring (4-amino-2-methylpyrimidine) and a thiazolium (4-methyl-5-(2-hydroxyethyl)-thiazolium), linked by a methylene bridge between the C3 carbon atom of the pyrimidine ring and the N3 nitrogen atom of the thiazolium ring. Vitamin B1 is found in high concentrations as thiamin pyrophosphate (TPP) [2, 7]. Thiamine strengthens the immune system, degrades glucose, helps nerve communication, and maintains processes in cells and tissues [8].

The intestinal epithelium absorbs free thiamine through thiamine transporters, that is, THTR-1 and THTR-2, which are transported to the blood for distribution throughout the body. Free thiamine is converted back to TPP and used for energy metabolism in the TCA cycle. The mechanism of thiamine absorption from food and microbiota is relatively similar. However, TPP produced by the gut microbiota is not converted to free thiamin because alkaline phosphatase is not secreted in the large intestine. Thus, TPP from the microbiota is absorbed directly by the large intestine. The TPP transporter is widely expressed in the apical membrane of the colon. Absorbed TPP enters the mitochondria via MTPP-1 and is used as a cofactor for ATP formation. This suggests that TPP microbiota is vital for energy generation in the large intestine, by a mechanism that differs between vitamin B1 from the diet and from the microbiota [2].

2.2 Vitamin B2

Vitamin B2 (riboflavin) and its active forms (flavin adenine dinucleotide FAD] and flavin mononucleotide FMN]) are cofactors for enzymatic reactions in the TCA cycle and electron transport mediators of fatty acid oxidation (β-oxidation). Riboflavin is also involved in the metabolism of folate, vitamin B12, and vitamin B6, so it helps maintain the integrity of the mucosa, skin, eyes, and nervous system. Riboflavin is essential in the early development of the brain and postnatal gastrointestinal tract, as it can modulate some metabolic activities such as DNA repair, iron absorption and distribution, inflammation, and immune responses [2, 9].

The process of riboflavin absorption in the small intestine and colon is specific and mediated by the transporters RFVT-1, RFVT-2, and RFVT-3. All three are products of the SLC52A1, SLC52A2, and SLC52A3 genes expressed in the human gut, respectively, with RFVT-3 expression being the dominant one [9]. Exogenous vitamin B2 in the form of FAD or FMN is converted into free riboflavin by FAD pyrophosphatase and FMN phosphatase in the small intestine [2].

2.3 Vitamin B3

Niacin and niacinamide, known as nicotinic acid (NA) and nicotinamide (Nam), are different forms of vitamin B3. Vitamin B3 is a biosynthetic precursor for nicotinamide adenine dinucleotide (NAD+) and nicotinamide adenine dinucleotide phosphate (NADP+), which are coenzymes in respiratory oxidation processes, the Krebs cycle, the formation and inhibition of reactive oxygen species (ROS), post-translational protein and regulatory protein modification, and the formation of a second messenger. Vitamin B3 also functions for DNA proliferation. Thus, vitamin B3 is the center of homeostasis and cellular growth [2, 8, 10].

In contrast to other B vitamins, vitamin B3 can be produced by mammals via the endogenous enzymatic pathway from tryptophan and stored in the liver. However, vitamin B3 must also be obtained from food [2]. Vitamin B3 deficiency is endemic in some areas of the world where malnutrition is common. In more developed countries, vitamin B3 deficiency is caused by poor food choices, adverse drug reactions, alcoholism, and infectious or autoimmune diseases [6, 10].

2.4 Vitamin B5

Vitamin B5 (pantothenic acid) is a coenzyme A (CoA) precursor, an important cofactor for the TCA cycle and fatty acid oxidation. CoA has a role in various human biochemical reactions, such as cell growth, intermediate metabolism, and neurotransmitter synthesis. The structure of CoA functions as an activating carbonyl group and as an acyl group carrier to help facilitate these reactions. Like vitamins B1 and B2, vitamin B5 is also involved in controlling host immunity through energy production by immune cells [2, 11].

From dietary sources, vitamin B5 is found in high concentrations as CoA or phosphopantethein. CoA and phosphopantetheine are then converted to free pantothenic acid by endogenous enzymes such as phosphatase and pantetheinase in the small intestine. Whereas, from the microbiota, Vitamin B5 is produced in the form of free pantothenic acid, which is directly absorbed in the large intestine, converted to CoA, and distributed in the same way as vitamin B5 from food [2].

2.5 Vitamin B6

Vitamin B6 has the basic structure of 2-methylpyridine, 3-hydroxypyridine, and 5-hydroxymethyl pyridine. Vitamin B6 consists of various forms, that is, pyridoxal (aldehyde, eCHO), pyridoxine (alcohol, eCH2OH), and pyridoxamine (amine, eCH2NH2). These forms of vitamin B6 are precursors of the coenzymes pyridoxal phosphate (PLP) and pyridoxamine phosphate (PMP), which are involved in various cellular metabolic processes, including amino acid, lipid, and carbohydrate metabolism. Vitamin B6 also plays a role in nucleotide synthesis, neurotransmitter metabolism, and heme synthesis. So, this vitamin affects almost all aspects of metabolic function and cellular homeostasis. Vitamin B6 deficiency causes inflammation such as allergies and rheumatoid arthritis, as well as nerve dysfunction [2, 12].

Dietary vitamin B6 is available in the form of PLP or PMP, which is then converted into free vitamin B6 by the endogenous enzyme pyridoxal phosphatase and subsequently absorbed by the small intestine. Absorption of B6 from food sources occurs primarily in the small intestine jejunum, with the absorption rate varying according to the B6 species present. Meanwhile, microbial-synthesized vitamin B6 in the form of PLP is converted into free vitamin B6 in the large intestine, then absorbed through passive transport, transported to the blood, and distributed throughout the body [2, 12, 13].

2.6 Vitamin B7

Biotin (vitamin B7 or vitamin H) is a B-complex vitamin that acts as an essential coenzyme for five carboxylases: pyruvate carboxylase, 3-methylcrotonyl-CoA carboxylase (MCC), propionyl-CoA carboxylase (PCC), and acetyl-CoA carboxylase 1 and 2. This carboxylase helps several chemical processes in cells, including gluconeogenesis, amino acid metabolism, and fatty acid synthesis. Acetyl-CoA carboxylase 1 is found in the cytoplasm and catalyzes the binding of bicarbonate to acetyl-CoA during the synthesis of fatty acids, while another carboxylase is found in mitochondria. PCC catalyzes a critical step in the metabolism of propionyl-CoA, which is derived from the catabolism of odd-chain fatty acids and several other nutrients. Meanwhile, MCC catalyzes the metabolism of the amino acid leucine. Pyruvate carboxylase catalyzes the conversion of pyruvate into oxaloacetate, which is a key step in gluconeogenesis [14, 15].

The enzyme holocarboxylase synthetase (HLCS) catalyzes the binding of biotin to all five carboxylases, thus playing an essential role in biotin-dependent metabolic pathways. In addition, HLCS also functions in gene regulation at the chromatin level. Meanwhile, the biotinidase enzyme in the small intestine catalyzes the release of biotin from the breakdown product of carboxylase, thus playing an important role in biotin recycling. Free biotin is then absorbed via the biotin transporter SMVT [2, 15].

2.7 Vitamin B9

Vitamin B9 (folate), in its active form as tetrahydrofolate, is a cofactor in several metabolic reactions, including the synthesis of DNA and amino acids. A folic acid is a synthetic form of folate found in supplements. Bacteria use folate to synthesize the nucleic acids that make up their DNA. In an animal model of endometrial carcinoma study, folate was found to be the most important B-complex vitamin for nucleic acid synthesis, amino acid conversion, and antioxidant properties for eliminating free radicals [16]. In addition to DNA synthesis, folate also functions as a cofactor in homocysteine methylation and reduces the risk of neural tube defects [8]. Folate supplementation studies have also demonstrated a role in preventing other diseases, such as neurological disorders and cognitive and psychiatric disorders, and protection against degeneration in ulcerative colitis [17].

Vitamin B9 in food is available in monoglutamate and polyglutamate folate. In the intestinal epithelium, the folate transporter PCFT deconjugates polyglutamate folate to monoglutamate folate, which is then absorbed in the small intestine. Before being transported to the blood, monoglutamate is converted to tetrahydrofolate (THF), an active form and cofactor. Intestinal bacteria produce vitamin B9 as THF from GTP, erythrose 4-phosphate, and phosphoenolpyruvate. Bacterial THF is absorbed directly in the large intestine via PCFT and circulated throughout the body by the blood [2]. Folic acid is converted by the body into DHFR (DiHydro-Folate Reductase), which commensal and pathogenic bacteria can use to form nucleic acids, thus being the basis of their survival and reproduction cycles [2, 17].

2.8 Vitamin B12

Vitamin B12 (cobalamin) is composed of a corrin ring, with a cobalt center, and has upper and lower ligands as coordinates. The active forms of this vitamin are methylcobalamin and adenosylcobalamin, which catalyze the synthesis of methionine. Food vitamin B12 is decomposed from protein into free vitamin B12 by pepsin in the stomach. Free vitamin B12 is then absorbed by small intestinal epithelial cells via intrinsic factor (IF), a gastric glycoprotein. In epithelial cells, the IF-vitamin B12 complex is decomposed into free vitamin B12 by lysosomes and then released into the blood, becoming an active form and distributed throughout the body. Cobalamin and cobamide contain cobalt, but cobamide has a lesser ligand consisting of 5, 6-dimethylbgensiidazole (DMB). Mechanically, the lower DMB ligand is essential for binding Vitamin B12 to intrinsic factor (IF); then, it can be recognized by cubilin and megalin, which facilitate endocytosis in intestinal epithelial cells. Bacterial Vitamin B12 is synthesized from precorrin-2 to produce adenosylcobalamin, which is absorbed directly by the large intestine and distributed throughout the body [2, 18].

Vitamin B12 functions for nucleotide synthesis, branched-chain amino acid regulation, long-chain fatty acid metabolism, and cellular development. Vitamin B12 is also a vital cofactor in cytoplasmic methionine synthase and in mitochondrial methylmalonyl-CoA mutase, leading to homocysteine methylation to methionine and the conversion of methylmalonyl-CoA to succinyl-CoA. Furthermore, Vitamin B12 is a cofactor that other gut microbiota use to regulate the breakdown of short-chain fatty acids such as butyrate, propionate, and acetic acid. Vitamin B12 has also been shown to maintain healthy nerve cells and help red blood cell synthesis. This vitamin plays a role to immune homeostasis, utilization of microbial metabolites, and cellular metabolism, making it an essential factor in immunity against pathogens [2].

3. Synthesis of Vitamin B and role of microbiota

Humans cannot synthesize vitamins except for vitamin D. So, other vitamins must be obtained exogenously from food or the gut microbiota. Commensal bacteria found in the gut, such as Lactobacillus and Bifidobacterium, can synthesize de novo vitamins in the human body. Members of the gut microbiota can synthesize vitamin K and most of the water-soluble B vitamins, such as vitamins B12, B9, B6, B2, and B1 [6]. Each of the B vitamins has a different synthesis mechanism, and the microbiota synthesize most.

3.1 Vitamin B1

Various gut microbiota, mainly in the large intestine, produce vitamin B1 as free thiamin and TPP. Bacteroides fragilis and Prevotella copri (phylum Bacteroidetes); Clostridium difficile, some Lactobacillus spp., Ruminococcus lactaris (Firmicutes); Bifidobacterium sp. (Actinobacteria); and Fusobacterium varium are microbiota that can produce vitamin B1 through thiazole and pyrimidine synthesis pathways [2]. It is estimated that synthesizing thiamin by gut microbes supplies about 2.3% of the daily requirement of human vitamin B1. The enzymes involved in the thiamin biosynthetic pathway are dominantly found in enterotype 2, which is one of the gut microbiota clusters rich in Prevotella sp. [4]. In the intestine, there is Faecalibacterium spp. (Firmicutes), which lacks the vitamin B1 synthesis pathway and requires vitamin B1 for growth. Therefore, these bacteria must obtain vitamin B1 from other bacteria or from the host’s diet via thiamin transporters in the mucosa. This indicates that there is competition for vitamin B1 requirements between the host and specific microbiota [5].

3.2 Vitamin B2

Not much different from vitamin B1, the synthesis of vitamin B2 is also widely played by the microbiota. Riboflavin from food, plus riboflavin produced by commensal bacteria, causes excess riboflavin in the distal intestine. In addition to the common lactic acid bacteria producing riboflavin in the gut, genomic analysis of 256 species of human gut microbiota has found 56% of the microbiota possess a cluster of genes for de novo riboflavin biosynthesis [4]. B. fragilis, P. copri, C. difficile, Lactobacillus plantarum, Lactobacillusfermentum, and R. lactaris have important factors for the synthesis of vitamin B2, so these bacteria are important sources of vitamin B2. Bacillus subtilis and Escherichia coli can also carry out riboflavin biosynthesis. Riboflavin biosynthesis requires guanosine 5′-triphosphate (GTP) and ribulose 5-phosphate as precursors. The first step of the branch of the GTP-dependent biosynthetic pathway is encoded by ribA, which catalyzes the 3,4-dihydroxy-2-butanone 4-phosphate configuration of ribulose 5-phosphate [5]. In clinical trials, it was found that daily consumption of 200 g of yogurt for 2 weeks can contribute to total vitamin B2 in the body, which is reflected in an increase in plasma-free riboflavin levels. This clinical trial also showed that most of the Lactobacilli strains consume riboflavin, thereby reducing its bioavailability in fermented products. Tempe, a traditional Indonesian fermented soybean meal, has been shown to increase the concentration of B vitamins such as riboflavin due to its microbial biosynthesis. The last article also reported that bacterial isolation from tempeh, which was shown to belong to Streptococcus and Enterococcus, significantly increased the concentration of riboflavin in this fermented product [19].

3.3 Vitamin B3

Unlike other B vitamins, vitamin B3 can be synthesized by the body from the amino acid tryptophan through endogenous enzymatic pathways and then stored in the liver. Niacin is a group of nicotinamide and nicotinic acid. These two metabolites are precursors for nicotinamide adenine dinucleotide (NAD), so nicotinamide and nicotinic acid can also be produced by recycling NAD in cells. An organism is considered a niacin producer when it contains the de novo synthesis pathway of NAD. These organisms include B. fragilis, P. copri, R. lactaris, C. difficile, Bifidobacterium infantis, Helicobacter pylori, and F. varium [2, 20]. Colonocytes have mechanisms that mediate niacin uptake transporters. Supplementation of microcapsules containing niacin will release its contents in the ileocolonic area, which increases the serum niacin concentration according to the dose consumed. Intake of 900 to 3000 mg niacin microcapsules will significantly increase the Bacteroidetes population [4].

3.4 Vitamin B5

For the synthesis of vitamin B5, bacteria synthesize from 2-dihydropantoate and beta alanine via the de novo synthesis pathway. Bacteria that have a biosynthetic pathway for vitamin B5 include B. fragilis, P. copri, some Ruminococcus spp., Salmonella enterica, and H. pylori. Some bacteria synthesize free pantothenic acid, which is directly absorbed in the large intestine to be converted into Co, and distributed in the same way as vitamin B5 from food. Various bacteria, including E. coli, Salmonella typhimurium, and Corynebacterium glutamicum, can synthesize vitamin B5. Some of these bacteria use aspartate and intermediate metabolites of valine biosynthesis to produce vitamin B5. In contrast, most of the Fusobacterium spp., Bifidobacterium spp., some strains of C. difficile, Faecalibacterium spp., and Lactobacillus spp. do not have this pathway, and some express pantothenic acid transporters to utilize vitamin B5 as an energy source. This indicates that these bacteria compete with the host for vitamin B5 [2, 3, 6, 21].

3.5 Vitamin B6

There are various forms of vitamin B6 in the body, that is, pyridoxine, pyridoxal, and pyridoxamine. Vitamin B6 in food is converted into free vitamin B6 by endogenous enzymes, such as pyridoxal phosphatase, before being absorbed by the small intestine. Microbiota with a biosynthetic pathway for vitamin B6 includes B. fragilis, P. copri, Bifidobacterium longum, Collinsella aerofaciens, and H. pylori. In contrast, most of the Firmicutes genera (Veillonella, Ruminococcus, Faecalibacterium, and Lactobacillus spp.) lack the vitamin B6 biosynthetic pathway [2].

3.6 Vitamin B7

As much as 40% of the human gut microbiota can synthesize de novo vitamin B7. The microbiota produce this vitamin as free biotin, which is synthesized from malonyl CoA or pimellate via pimeloyl CoA. Microbiota with a biosynthetic pathway for vitamin B7 include B. fragilis, P. copri, F. varium, and Campylobacter coli. The production of vitamin B7 affects others microbiota; for example, B. longum, which produces pimelate, a precursor of vitamin B7 that certainly increases the production of vitamin B7 by other gut microbiota. Biotin absorption in the small and large intestines occurs via a transporter-mediated process encoded by the SLC5A6 gene. Lipopolysaccharides inhibit colonic biotin uptake by impairing the expression of these membrane transporters. Biotin utilization also occurs between the host and bacteria, like Lactobacillus murinus, which consumes and reduces the biotin available in the intestine [2, 4].

3.7 Vitamin B9

The gut microbiota synthesizes vitamin B9 as tetrahydrofolate (THF) from GTP, erythrose 4-phosphate, and phosphoenolpyruvate. Dihydropteroate synthase catalyzes Folate biosynthesis, which reduces dihydrofolate to tetrahydrofolate [5]. THF is then directly absorbed in the large intestine and distributed throughout the body. Microbiota such as B. fragilis, P. copri, C. difficile, L. plantarum, Lactobacillus reuteri, Lactobacillusdelbrueckii ssp. bulgaricus, and Streptococcus thermophilus and several species of Bifidobacterium spp., F. varium, and S. enterica have folate biosynthetic pathways. It shows that almost all species in all phyla of microbiota produce folate. However, the ability of some of these bacteria to produce and utilize folate varies widely due to strain-dependent properties. Some Bifidobacteria do not produce folate when this vitamin is present, whereas others produce it regardless of vitamin concentration. Microorganisms are also able to increase folate content in a wide variety of foods [2, 19].

3.8 Vitamin B12

Vitamin B12 is a vitamin that contains cobalt, with the active forms being methylcobalamin and adenosylcobalamin. Dietary vitamin B12 forms a complex with dietary protein, further decomposed into free vitamin B12 by pepsin in the stomach. Free vitamin B12 is then absorbed by small intestinal epithelial cells via a gastric glycoprotein, intrinsic factor (IF). In epithelial cells, the IF-vitamin B12 complex is decomposed into free vitamin B12 by lysosomes and then released into the blood, which is further converted into the active form and distributed throughout the body [2].

Vitamin B12 is produced by aerobic and anaerobic pathways, with 30 different enzymes required for its biosynthesis. Several genes express enzymes essential for de novo vitamin B12 biosynthesis [18]. Bacterial vitamin B12 is synthesized from precorrin2 to produce adenosylcobalamin, which is then absorbed directly by the large intestine and distributed throughout the body. Microbiota with a biosynthetic pathway for vitamin B12 include B. fragilis, P. copri, C. difficile, Faecalibacterium prausnitzii, R. lactaris, Bifidobacterium animalis, B. infantis, B. longum, and F. varium. In addition to the microbiota, L. plantarum and Lactobacillus coryniformis also produce vitamin B12 from fermented foods, and B. animalis synthesizes vitamin B12 during milk fermentation [2, 21].

4. Sources of B Vitamins from the diet

Sources of B vitamins can be obtained apart from the biosynthetic mechanism, that is, exogenously from food. Vitamin B1 is found in high concentrations as thiamin pyrophosphate (TPP) in meat, especially pork and chicken; egg; cereals and rice; as well as nuts. The World Health Organization (WHO) / Food and Agriculture Organization (FAO) recommends a daily intake of 1.1–1.2 mg of vitamin B1 for adults [2].

Vitamin B2 is found in many animal sources, such as poultry, fish, liver, and eggs. Dairy products and cheese can also be sources of riboflavin, which makes a significant contribution to children and adults. Vegetable sources such as cereals, grain products, and bread can be a source of riboflavin-rich foods in some developing countries. Leafy greens, such as broccoli, mustard, and turnips, are also good sources of riboflavin. Natural grain products tend to be relatively low in riboflavin, but when intake is increased, these foods increase the bioavailability of riboflavin. WHO/FAO recommends a daily intake of 1.0–1.3 mg of vitamin B2 for adults [2, 15].

Animal foods such as fish and meat contain vitamin B3 as nicotinamide, and plant foods such as nuts and mushrooms contain vitamin B3 as nicotinic acid. WHO/FAO recommends a daily intake of 11–12 mg of vitamin B3 for adults. Vitamin B5 is high in concentrations of CoA or phosphopantetheine in the liver, eggs, milk, chicken, beef, salmon, cereals, grains, and fermented soybeans. WHO/FAO recommends a daily intake of 5 mg of vitamin B5 for adults [1, 2, 11].

Vitamin B6 is abundant in salmon, chicken, tofu, sweet potatoes, potatoes, bananas, and avocados. Vitamin B6 found in food is in the form of PLP or PMP. WHO/FAO recommends a daily intake of 1.3–1.7 mg of vitamin B6 for adults. Vitamin B7 is abundant in foods such as egg yolks, heart, cereals such as oats, nuts, vegetables such as spinach and mushrooms, rice, and vegetable oil. Dairy and breast milk products also contain biotin. Raw egg whites contain large amounts of avidin, which binds tightly to vitamin B7 and prevents its absorption in the intestines. WHO/FAO recommends a daily intake of vitamin B7 of 30 g for adults [1, 2, 14].

Foods such as beef liver, spinach, and asparagus are high in vitamin B9. Legumes, red meat, and liver also contain high levels of folate. However, up to 70% of folate is lost during cooking due to thermal degradation or dissolution in the cooking water. WHO/FAO recommends a daily intake of 400 g of vitamin B9 for adults. Most humans get vitamin B12 from food, especially from animal protein and fermented foods. Vitamin B12 is found in beef liver, bivalves, salmon, chicken, eggs, beans, and spinach. WHO/FAO recommends a daily intake of 2.4 g of vitamin B12 for adults [2, 18, 22].

From the explanation of the acquisition of vitamins from these foods, malnutrition will significantly inhibit normal metabolism in infants and the elderly. In the case of dietary supplements, vitamin-containing probiotic products are generally produced as freeze-dried powders and formulated as capsules, powders, and tablets [23]. Parameters influencing the manufacture of this dietary supplement include viable cell count and water activity. For the application of probiotics as food and drink, the product is prepared in the form of vegetative cells and added to food products [24]. Probiotic products require special care for the profile of a product, mitigation of health risks due to pathogens, and maintenance of aseptic conditions [25].

5. Role of B Vitamins in gut microbiota

B vitamins are biosynthetic precursors of essential cofactors in various metabolic pathways and play an important role in immunity. In addition to being needed by the host, B vitamins are also needed by some intestinal microbiota for metabolism. Thus, in addition to producing B vitamins, the gut microbiota also consumes B vitamins for primary enzymatic reactions. The gut microbiota is one of the densest microbial communities in the human body. This microbial community consists primarily of four main phyla, that is, Bacteroidetes, Firmicutes, Proteobacteria, and Actinobacteria. In the large intestine, the microbiota are grouped into 2, namely, B-vitamin-producing bacteria and auxotrophic bacteria. The survival of bacteria of auxotrophic species is highly dependent on B vitamins. Although most B vitamins are absorbed in the small intestine, B vitamins are produced and absorbed by bacteria mainly through the large intestine. Many B-vitamin transporters are expressed in the large intestine. As a result, there is competition between the host and bacteria. So that the host experiencing dysbiosis—the gut microbiota is unbalanced, and significantly affects the metabolism of B vitamins in the gut [13].

5.1 Vitamin B1

Thiamine produced in the gut microbiota has a specific role in the composition or function of the gut microbiome. Thiamin is required by certain gut bacteria, Bacteroides thetaiotaomicron and Faecalibacterium spp., which have potential consequences on the host thiamine. Thiamine biosynthesis and its transport system are essential for the growth of B. thetaiotaomicron. Although Faecalibacterium spp. has a vitamin B1 synthesis pathway, this species requires more vitamin B1 for growth than production. Therefore, these bacteria must obtain vitamin B1 from other bacteria or host food via transporters, such as B. thetaoimicron [2, 4]. Intestinal dysbiosis may lead to a predominance of thiamin-only-consuming bacteria, which may contribute to decreased thiamin availability to the host. E. coli in the human fecal microbiota was found to be negatively correlated with fecal thiamin [26].

5.2 Vitamin B2

Riboflavin is vital in the growth of bacteria that are very sensitive to oxygen as an electron transfer agent. Examples of bacteria that are sensitive to oxygen are F. prausnitzii and Roseburia. F. prausnitzii is a major butyrate producer in the human microbiota and has anti-inflammatory properties and gut-protective functions. Vitamin B2 is also an essential precursor to flavin mononucleotide and flavin adenine dinucleotide (FAD), coenzymes of glutathione reductase that protect cells from reactive oxygen species (ROS). Thus, vitamin B2 can act as an indirect antioxidant and modifies the gut microbiota condition through ROS reduction. This condition is also proven to reduce the population of E. coli. Previous studies showed that supplementation of dietary riboflavin for 14 days increased F. prausnitzii and Roseburia and concomitantly reduced E. coli. Phylum Actinobacteria and Firmicutes express riboflavin transporters and enzymes required to form FAD and FMN from free riboflavin [2, 4, 27]. Vitamin B2 also directly affects the fecal microbiome, namely, the genera Alistipes and Clostridium. However, this increase occurred only with high doses of riboflavin supplied directly to the large intestine and may not apply to intakes from foods or dietary supplements that are absorbed mostly in the small intestine [27].

5.3 Vitamin B3

Vitamin B3 is the only B vitamin that humans can synthesize. It is a precursor to nicotinamide adenine dinucleotide (NAD), a coenzyme in cellular oxidation–reduction reactions with a central role in aerobic respiration. Niacin acts as an agonist for the cell surface receptor niacin receptor 1, which pairs with G-proteins. Niacin also has strong antioxidant and anti-inflammatory properties and can function as a modulator of gut protection and prevent bacterial endotoxin production. Thus, niacin deficiency causes intestinal inflammation and diarrhea, which has a direct impact on the gut microbiota population. Vitamin B3 deficiency impacts the diversity and low number of Bacteroidetes, especially in obese individuals. Supplementation with tryptophan and niacin has been shown to restore the composition of the gut microbiota through the angiotensin I (peptidyl-dipeptidase A)-converting enzyme. Furthermore, intake of niacin microcapsules (900 to 3000 mg) significantly increased the Bacteroidetes population. These results suggest that niacin has a beneficial effect on gut microbial composition in humans [2, 3, 4].

5.4 Vitamin B5

Lactobacillus spp., Streptococcus spp., and Enterococcus spp. are members of the phylum Firmicutes that do not produce pantothenate but require pantothenic acid for their growth. This indicates that there is a symbiosis in the distal intestine between pantothenic acid-eating bacteria and pantothenic acid-producing bacteria. A study has demonstrated the uptake of pantothenic acid and biotin by the sodium-dependent multivitamin transporter (SMVT, SLC5A6) across the intestinal loop. Mostly Fusobacterium and Bifidobacterium spp., some strains of C. difficile, and Faecalibacterium spp., lack the vitamin B5 synthesis pathway but express the pantothenic acid transporter to utilize vitamin B5 as an energy producer [2, 4].

5.5 Vitamin B6

Vitamin B6 can play an essential role in shaping microbiota composition and metabolic capacity. In bacteria such as E. coli, vitamin B6 is synthesized in the PLP form from various precursors, including, deoxyxylulose 5-phosphate, 4-phosphohydroxy-L-threonine, glyceraldehyde-3-phosphate, and D-ribulose 5-phosphate. PLP produced by commensal bacteria works with ribonucleotide metabolism to facilitate the effects of 5-fluorouracil, a drug used to treat colorectal cancer. Vitamin B6 deficiency results in a marked decrease in intestinal arginine biosynthesis, and disruption of this metabolic pathway leads to the selective growth of certain gut bacteria, namely, the Bacteroidaceae family, and an increase in Lachnospiraceae. An increase in vitamin B6-producing bacteria such as Bacteroides acidifaciens has been shown to weaken the colonization of S. typhimurium and promote recovery from intestinal inflammation [4, 28].

5.6 Vitamin B7

Free biotin can affect the composition of the gut microbiota because it is required for the growth and survival of some microbiota. Prevotella spp., Bifidobacterium spp., Clostridium, Ruminococcus, Faecalibacterium, and Lactobacillus sp. do not have the vitamin B7 synthesis pathway because it lacks the essential biotin biosynthetic gene. However, they express a free biotin transporter, indicating a need for biotin. These results indicate that these bacteria also utilize biotin from food and bacteria to compete with the host. Therefore, it is necessary to control diseases related to some of these microbes. Another study showed that enzymes in the biotin biosynthetic pathway were overexpressed in the Bacteroides enterotype. Biotin uptake in the small and large intestines occurs via a carrier-mediated process involving the SMVT system encoded by the SLC5A6 gene [2, 4]. SMVT dysfunction reduces biotin in the intestine, causing dysbiosis, and induces Nox and ROS, which cause damage to enterocyte apoptosis. This mechanism causes the intestinal villi to shrink and increases intestinal permeability, inflammation, and dysplasia, all of which induce dysbiosis [29].

5.7 Vitamin B9

Folate is essential for several metabolic processes, including carbon transfer, thymidylate synthesis, purine synthesis, and the synthesis of several amino acids. Once absorbed, folate also participates in nucleotide synthesis, DNA repair, and methylation [30]. This function applies to both the host and the microbiota that require folate. The biosynthesis and expression of folate transporters in the gut microbiota are influenced by gut microbes, such as Bifidobacteria. In commensal bacteria, a vitamin B9 metabolite, 6-formylpterin (6-FP), is produced by the photodegradation of folic acid. This metabolite cannot activate MAIT cells (mucosal-associated invariant T), suppressing excessive MAIT cell responses and preventing excessive allergic and inflammatory responses [4].

5.8 Vitamin B12

Microorganisms use various forms of cyanocobalamin in many reactions, including methionine synthesis, carbon skeleton mutation, elimination reactions, amino mutations, and acetate and methane synthesis [31]. As many as 83% of the microbiota (260/313 species) encode cobalamin-dependent enzymes. Most of these species also lack the genes needed to synthesize cobalamin. In another report, 75.9% of bacteria utilized cobalamin, and only half possessed the cobalamin biosynthetic pathway. An example is B. thetaiotaomicron, which does not encode the cobalamin biosynthetic pathway gene but has three homologous cobalamin transporters. This statement indicates that this microbiota depend on the cobalamin absorption mechanism to maintain survival. Supplementation of 3.94 g/ml cyanocobalamin was shown to increase fecal cobalamin, with a lower Bacteroides population condition. Cobalamin and its derivatives also determine pathogenicity in the host gut. The bacterial transcription factor EutR requires ethanol amine, cobalamin precursors, and cobalamin-derived adenosylcobalamin for transcription of virulence factors required for host infection and spread of enterohemorrhagic E. coli (EHEC) serotypes O157:H7 and Salmonella [4].

6. The role of B Vitamins on human digestive health

The B vitamins in our body have several crucial physiological functions. The functional roles of these micronutrients are diverse, ranging from cellular energy-producing reactions such as the mitochondrial citric acid cycle to respiratory oxidation, immunity, neurotransmitter synthesis, cell signaling, and nucleic acid biosynthesis. Thus, a deficiency of B vitamins will impair normal cell metabolism and trigger the development of several chronic diseases in humans [4, 32]. Apart from being essential for the human body, B vitamins are also crucial in shaping the diversity and richness of the gut microbiota. A wealth of evidence has shown that a healthy gut lies in a healthy microbial ecology [3, 4].

6.1 Vitamin B1

Thiamin is a precursor of thiamin pyrophosphate, which is essential for carbohydrate metabolism and nerve function. Energy metabolism, particularly the balance between glycolysis and the citric acid cycle, is related to the functional control of immune cells, which is referred to as immunometabolism. Immunometabolism by vitamin B1 is vital for glycolysis-dependent digestive cells, especially Peyer’s patch. In the gut, nave immunoglobulin (Ig)M+ B cells differentiate into IgA+ B cells in the Peyer patch, and then, IgA+ B cells differentiate into IgA-producing plasma cells in the lamina propria. The naive B cells in Peyer’s patch prefer to use the vitamin B1-dependent citric acid cycle to generate ATP. However, once B cells differentiate into IgA-producing plasma cells, they switch to using glycolysis to generate ATP [2, 3].

Consistent with the importance of vitamin B1 in generating energy in the gut, mice fed a vitamin B1-deficient diet showed impaired maintenance of naive B cells in Peyer’s patches. In addition to reducing the number of naive B cells in Peyer’s patch, thiamin deficiency also reduces the size of B cell follicles, evidenced by the reduction of naive B cells in female Balb/c experimental animals. The researchers also showed that feeding the mice a vitamin B1-deficient diet caused the vitamin B1 deficiency to last for only a week. Vitamin B1 deficiency that affects the host immune response through the regulation of differentiation and proliferation of these immune cells ultimately affects the gut microbiota [2, 3].

6.2 Vitamin B2

Riboflavin is required for the development of the gastrointestinal tract after birth and is linked to crypt hypertrophy, crypt bifurcation dysfunction, and a loss of proliferative potential in intestinal cells. These changes are visible during the postnatal and post-weaning stages. These changes were irreversible, even after the experimental administration of riboflavin in vivo and in vitro. Riboflavin deficiency has been shown to reduce the number of villi but, on the other hand, can increase the length of the villi. Reduction of riboflavin in humans has also been associated with shortened duodenal crypts and reduced cell division. In vitro studies using Caco-2, HCT116, and HT29 cells demonstrated a potential mechanism for the riboflavin deficiency phenotype, which led to the result that riboflavin inhibited cell growth by reducing cellular ATP generation and increasing oxidative stress. This impairs mitosis and accumulates aneuploidy cells. These changes in gut morphology may also be associated with an adaptive response to stress-induced deficiency [4].

Riboflavin is also essential for methylation reactions, nucleotide synthesis, and DNA stability and repair. A cohort study in the Netherlands on a diet in cancer suggested that riboflavin was likely to be associated with a reduced risk of proximal colon cancer among women (RR = 0.61; P-trend = 0.07). This finding is reinforced by results from the Women’s Health Initiative cohort observational study, which showed that higher total riboflavin intake was associated with a reduced risk of colorectal cancer (HR = 0.81; 95% CI: 0.66–0.99) [1].

6.3 Vitamin B3

Human and mouse colonic epithelial cells possess efficient and specific mechanisms for vitamin B3 absorption. This vitamin plays an essential role in reducing inflammation, so a deficiency will lead to inflammatory bowel diseases such as ulcerative colitis. Vitamin B3 controls inflammation by inhibiting vascular permeability in intestinal tissue by activating PGD2/DP1 signaling in endothelial cells. This vitamin also modulates the inflammatory response by increasing the rate of ATP generation in Caco-2 cells. In addition, vitamin B3 is involved in various cellular oxidation–reduction metabolic reactions and rapamycin signaling pathways, thereby suppressing colon inflammation [3, 32].

Vitamin B3 synthesized by the gut microbiota contributes to local colonocyte nutrition and maintains intestinal stem cell morphology. Vitamin B3 is also known to protect colonic epithelial cells against dextran-sulfate-sodium (DSS)-induced apoptosis and promote cell proliferation in experimental animals. The mechanism of protection of the intestinal epithelium by vitamin B3 is by activating the prostanoid D 1 (DP1) receptor on macrophages and endothelial and colonic epithelial cells. One study found that retention of vitamin B3-containing enemas effectively promoted mucosal healing in patients with ulcerative colitis, with possible mechanisms of downregulation of colonic inflammatory cytokines and suppression of pro-inflammatory gene expression [3, 4].

6.4 Vitamin B5/pantothenic acid

Vitamin B5, or pantothenic acid, is an essential coenzyme A (CoA) precursor and acts as a carrier protein. This vitamin is involved in various metabolic pathways, such as the citric acid cycle, cell growth, neurotransmitter synthesis, and fatty acid oxidation [4]. Dietary pantothenic acid supplementation also affects the gut microbial profile. Increased intake of pantothenic acid increased the relative numbers of Prevotella and Actinobacteria [3].

6.5 Vitamin B6/pyridoxine

Vitamin B6 functions primarily as a cofactor for the biosynthesis and catabolism of amino acids. In addition, this vitamin is also involved in fatty acid biosynthesis and neurotransmitter biosynthesis and as an antioxidant [3]. Relative pyridoxine deficiency is found in 10–25% of Inflammatory Bowel Disease (IBD) cases. Plasma levels of B6 are considered a risk factor for thrombosis in patients with IBD because they have an inverse relationship with homocysteine [1].

A cross-sectional study has shown an association between the severity of intestinal irritation and low dietary vitamin B6 intake. The mechanism of this phenomenon is that the lack of vitamin B6 affects the balance of anti-inflammatory and pro-inflammatory cytokines. Vitamin B6 deficiency also reduces microbial diversity and significantly alters gut metabolites such as short-chain fatty acids, which also play an essential role in triggering this irritation. In addition, studies of vitamin B6 deficiency in animals have shown a significant reduction in the number of mucus-secreting cells, an important factor in maintaining gut health. Vitamin B6 also decreases cell calcium transport but does not affect the basic morphology of enterocytes, such as cell viability, cell volume, membrane permeability, and protein content [4].

Vitamin B6 can influence colorectal carcinogenesis through its role in DNA synthesis and methylation. Animal studies have shown that this vitamin can inhibit angiogenesis, suppress nitric oxide, and reduce oxidative stress [1].

6.6 Vitamin B7/Biotin

Vitamin B7 acts as a coenzyme for several biochemical reactions, such as glycolysis, cell signaling, and epigenetic regulation. This vitamin also controls the expression of genes, including nuclear factor kappa B (NF-kB), through a histone-binding mechanism known as biotinylation. Therefore, this vitamin may also have an anti-inflammatory effect on the gastrointestinal mucosa [32].

6.7 Vitamin B9/Folate

Vitamin B9 is essential for the replication and regeneration of nucleic acids, affecting cells’ survival rate. Folate is involved in synthesizing S-adenosylmethionine (SAM), which is required for cellular biosynthesis and DNA methylation. This vitamin is essential for replicating and recovering nucleic acids, influencing survival rates through cell proliferation and regeneration. In addition, folate regulates gene activity, regenerates the intestinal lining, reduces lymphocyte growth, and reduces NK cell cytotoxicity [3]. Thus, every living cell, including gastrointestinal cells, requires folate to carry out these various biochemical and biosynthetic processes [4].

Because of its essential role in producing methyl donors, folate deficiency significantly impairs DNA replication. Folate deficiency causes an increase in the crypt depth of the intestinal mucosa in the duodenum and jejunum, resulting in a decreased villi-to-crypt ratio. In experimental animals, induced methyl donor deficiency by feeding a folate-deficient diet accompanied by the antibiotic succinylsulfathiazole 1% has also increased crypt depth and altered gut cell differentiation. In this study, folate deficiency also caused megaloblastic changes in the epithelial cell nuclei and reduced crypt mitosis. These changes are seen more prominently in the ileum with elongation of the crypts, an increase in goblet cells, and a decrease in Paneth cells. Deficiencies of folate, riboflavin, vitamin B6, and vitamin B12 concomitantly alter Wnt signaling in the experimental colon and decrease apoptosis in epithelial cells. Unexpectedly, these changes are irreversible, even with an excess of folate [4].

Folate deficiency significantly alters intestinal cell morphology and is associated with increased intestinal carcinogenesis [3]. Two case–control studies demonstrated that folate supplementation and high red blood cell folate levels significantly reduced the risk of dysplasia and neoplasia in patients with ulcerative colitis. Folate supplementation, in combination with sulfasalazine administration, has a protective effect on the development of colorectal cancer in patients with chronic ulcerative colitis. The mechanism of folate protection against colorectal cancer is to prevent aberrations in DNA synthesis and aberrations in DNA methylation. However, it should be noted that folate deficiency can also occur due to IBD therapy, such as methotrexate and sulfasalazine. A recent meta-analysis of folate supplementation has found that folate plays a role in preventing pancreatic cancer. Individuals with a high dietary folate intake were 34% less likely to develop pancreatic cancer than individuals with a low folate intake [1].

6.8 Vitamin B12

Like folate, cobalamin is involved in the synthesis of methyl donors. These donors are essential for the nucleic acid synthesis and protein and lipid metabolism. Vitamin B12 also acts as a cofactor for methionine synthase in sulfur amino acid metabolism to recycle homocysteine into methionine. The effect of vitamin B12 deficiency on colonic morphology is similar to that of folate deficiency because of its association with several cellular metabolic reactions. Vitamin B12 deficiency protects against DSS-induced inflammation in C57BL/6 mice. Other studies have shown reduced cell differentiation and gut protective factors in vitamin B12-deficient mice. In addition, in patients with vitamin B12 deficiency, the villi become shorter with a reduced villi/crypt ratio. Deficiency or excess of vitamin B12 affects the growth of gut microbiota. However, vitamin B12 deficiency did not relatively change the gut microbiota composition in healthy mice but did change it in DSS-induced colitis mice [4, 6, 32].

Vitamin B12, together with vitamins B9 and B6, influence the occurrence of colorectal carcinoma through its role in DNA synthesis and methylation. In addition, these three vitamins have been shown to inhibit angiogenesis, suppress nitric oxide, and reduce oxidative stress in animal models. However, an experimental study that administered a combination of folic acid (2.5 mg), vitamin B6 (50 mg), and vitamin B12 (1 mg) in 1470 subjects did not reduce the risk of colorectal carcinoma after a follow-up period of 9.3 years. Patients with celiac disease also have higher total plasma homocysteine levels than the general population, indicating lower serum levels of vitamins B6, B9, and B12 [1].

The liver is the physiological reservoir of cyanocobalamin in humans. Vitamin B12 deficiency is observed in several liver diseases such as hepatitis, cirrhosis, and hepatocellular carcinoma. Vitamin B12 inhibits HCV through the inhibition of ribosome entry sites. Vitamin B12 is also associated with aphthous stomatitis (Figure 1) [1].

Figure 1.

Summary of the main gut bacteria synthesizing B vitamins and the effects of their deficiency on gut health [3].

7. Conclusion

B vitamins act as cofactors for several cellular metabolic reactions. In addition to vitamin B3, other B vitamins must be obtained through dietary intake, supplementation, and synthesis of the gut microbiota. The biosynthesis of B vitamins in the gut is influenced by several factors, including exposure to antibiotics and free radicals, genetic makeup, dietary habits, and lifestyle. Competition between gut microbiota, pathogenic microbes, and the host leads to deficiency conditions, especially if exogenous intake is not optimal. A deficiency of B vitamins ultimately affects the gut microbial composition, gut health, and overall host metabolism.