Vitamin B6 - Benefits And Functions
Vitamin B6 is the term used for a group of water-soluble vitamins. Water-soluble vitamins are poorly stored in the body and are quickly excreted through urine. Therefore, they need to be regularly obtained through diet.
The group of vitamin B6 compounds includes naturally occurring compounds 1:
- as well as the phosphorylated forms of pyridoxine (pyridoxine phosphate), pyridoxal (pyridoxal phosphate), and pyridoxamine (pyridoxamine phosphate)
Pyridoxine, pyridoxal, and pyridoxamine are mainly converted to pyridoxal phosphate (also known as pyridoxal-5-phosphate) in the body. Pyridoxal phosphate is the most common and biologically active form of vitamin B6 in the body 2. The highest concentrations of it are found in muscle tissue.
In small amounts, vitamin B6 is found in the muscles (75-80%), liver (5-10%), brain, as well as in plasma, erythrocytes, and other organs. 3 4. The body reserves of vitamin B6 are estimated to be 167 mg (3.7 μg per gram of tissue). 5 6.
Vitamin B2 Required For the Formation of the Bioactive Form of Vitamin B6
There is evidence that the daily intake of riboflavin (vitamin B2) has an impact on the plasma levels of vitamin B6 9. This is because vitamin B2 is involved in the conversion of vitamin B6 into its bioactive form 10 11. For foods containing vitamin B2, click here (vitamin B2 foods table).
Vitamin B6 is primarily needed by the body for the metabolism of amino acids (obtained from proteins). In its bioactive form (pyridoxal phosphate), pyridoxal serves as a coenzyme and regulator for over 140 enzymes 12. Coenzymes are required for the activity of enzymes, which drive chemical reactions in the body. Processes in which pyridoxal phosphate is essential include transamination, decarboxylation, racemization, and elimination 13. Approximately 4% of cellular enzymes and their reactions are influenced by pyridoxal phosphate 14.
Vitamin B6 plays an important role in C1 metabolism. In this context, vitamin B6 serves as a coenzyme for the enzymes serine hydroxymethyltransferase, cystathionine-beta-synthase, and cystathionine-gamma-lyase 15. C1 metabolism is necessary for DNA synthesis, DNA repair, DNA methylation, protection against oxidative stress, and detoxification.
Vitamin B6 is involved in the simultaneous conversion of serine to glycine and tetrahydrofolate to 5,10-methylenetetrahydrofolate through its function as a coenzyme for the enzyme serine hydroxymethyltransferase 16. Vitamin B6 intervenes in the metabolism of folate (vitamin B9), thereby indirectly contributing to the formation of nucleic acids. Nucleic acids are macromolecules that are components of DNA and RNA.
Through transsulfuration, the amino acid homocysteine is converted into the amino acid cysteine. Two vitamin B6-dependent enzymes (cystathionine β-synthase and cystathionine γ-lyase) are required for this process. Homocysteine is formed as an intermediate product in protein metabolism during the breakdown of the sulfur-containing amino acid methionine. Elevated homocysteine levels in the blood have been associated with an increased risk of systemic inflammation, damage to blood vessels, and the development of cardiovascular diseases 17 18. Furthermore, elevated homocysteine levels can also be a risk factor for cerebrovascular diseases and have direct toxic effects on neurons in the central nervous system. However, not all studies associate higher homocysteine levels with a modest increase in the risk of heart disease and stroke 19. Therefore, a conclusive evaluation of this matter is still pending.
In pyridoxal phosphate-dependent decarboxylation reactions, CO2 is removed from an amino acid, resulting in the formation of amines. These amines function as neurotransmitters or hormones such as dopamine, taurine, histamine, serotonin, norepinephrine, adrenaline (epinephrine), and γ-aminobutyric acid (GABA) 20 21. Neurotransmitters and hormones are chemical messengers. Neurotransmitters transmit and, if necessary, amplify or modify information in the nervous system, while hormones travel through the bloodstream. Other amines are involved in DNA metabolism regulation 22. Serotonin, dopamine, and norepinephrine are considered "happiness hormones" as they positively influence mood and concentration. Vitamin B6 could potentially be helpful in alleviating symptoms of depression 23. However, studies on this topic are still pending.
Pyridoxamine phosphate serves as a coenzyme for aminotransferase. Aminotransferases, also known as transaminases, accelerate the transfer of an amino group (−NH2) from an amino acid to an α-keto acid. These reactions occur in the formation of nonessential amino acids and in the metabolism of all amino acids, except for lysine, as stated by the 24.
Racemizations lead to the formation of racemic mixtures of D- and L-amino acids, which play a role in signaling during brain development as stated by the 25.
Furthermore, vitamin B6 is also required for the formation of hemoglobin and other oxygen-transporting proteins 26. Vitamin B6 assists in the incorporation of iron into hemoglobin 27. Hemoglobin is the iron-containing red pigment in red blood cells (erythrocytes).
An adequate intake of vitamin B6 is important for the maintenance of immune function 28 29. Vitamin B6 is involved in the production of antibodies as it promotes the formation of lymphocytes and interleukin-2 30 31.
In addition, vitamin B6 is involved in the formation of niacin (vitamin B3) from the essential amino acid tryptophan 32.
Furthermore, the vitamin B6-dependent enzyme δ-6-desaturase is also involved in lipid metabolism 33 34. This enzyme is, in turn, involved in the formation of polyunsaturated fatty acids such as eicosapentaenoic acid and docosahexaenoic acid from alpha-linolenic acid, and arachidonic acid from linoleic acid.
Pyridoxal phosphate-dependent enzymes are involved in amino acid metabolism, glycolysis, gluconeogenesis, glycogenolysis, transsulfuration, as well as the biosynthesis of polyamines and the formation of sphingoid bases (required for myelin formation) and a precursor of delta-aminolevulinic acid 35. More information about this is available in the next section.
Energy Supply of the Body
During physical activity, amino acids can be released from the muscles and converted into energy. The conversion of released amino acids into energy (glucose) involves gluconeogenesis, which, in turn, depends on the presence of pyridoxal phosphate. Additionally, lactate is metabolized into glucose in the liver 36. Glucose is a simple carbohydrate and the primary source of energy in the body.
In energy metabolism, pyridoxal phosphate-dependent aminotransferase enables the activation or inactivation (interconversion) of amino acids and intermediates in energy-generating processes 37.
Furthermore, vitamin B6 is necessary as a coenzyme for the enzyme glycogen phosphorylase, which allows for the breakdown or conversion of muscle glycogen into glucose-1-phosphate 38. This process is known as glycogenolysis and occurs when there is a short-term absence of dietary glucose. Glycogen serves as the short- to medium-term storage form of glucose, i.e., stored energy. The muscles utilize the released glucose (blood sugar) for energy supply. Therefore, vitamin B6 is directly involved in muscle energy metabolism.
Overall, vitamin B6 is involved in energy production and supply. Due to this and higher intake of amino acids, the vitamin B6 requirements for athletes are slightly higher.
When is Vitamin B6 used?
Pyridoxine is used in the treatment of vitamin B6 deficiency, vitamin B6-related cramps, anemia, and inherited metabolic disorders 39. Vitamin B6 also provides additional benefits in relieving nausea and vomiting in pregnant women.
Vitamin B6 helps with the symptoms of premenstrual syndrome and premenstrual depression. This is supported by studies involving 940 patients. 40. Dosages of up to 100 mg of Vitamin B6 per day have shown benefits in treatment.
Furthermore, there is evidence that higher plasma levels of Vitamin B6 and vitamin B2 in the blood reduce the risk of breast cancer, especially in premenopausal women. 41
Vitamin B6 is absorbed in the small intestine, specifically the jejunum, unless there is malabsorption. 42.
Plant-based foods mainly contain pyridoxine and its phosphorylated form – pyridoxine phosphate. 43. Phosphorylated forms of vitamin B6 are freed from phosphorus and then absorbed in pure form through passive diffusion. 44.
In rare cases, pyridoxine is also present in the form of a glucoside 45. In this bound form, vitamin B6 is less readily absorbed by the body 46 47. Cruciferous vegetables such as cauliflower, broccoli, kohlrabi, and red cabbage are particularly rich in pyridoxine glucosides 48.
The presence of pyridoxine glucoside reduces bioavailability by 50-80% 49 50 51. The advantage here is that the bound vitamin B6 is more stable and less susceptible to heat 52. In addition, dietary fiber also reduces bioavailability by 5-10% 53 54. Foods high in dietary fiber can be found in this table.
Do Vegans Need To Consider Anything?
With a varied vegan diet, it is very easy to meet the need for vitamin B6. Numerous studies with vegans have also confirmed this.
According to a large study, vegans have nearly the same intake levels as omnivores (vegan women: 2.08 mg, omnivorous women: 2.17 mg; vegan men: 2.23 mg, omnivorous men: 2.26 mg) 55. So the differences are small.
In another study, the values only differed slightly from the intake levels of omnivores (vegan women: 2.63 mg, omnivorous women: 2.31 mg; vegan men: 2.81 mg, omnivorous men: 2.59 mg) 56.
In a Danish study, vegans showed higher vitamin B6 intake levels (women: 1.3 to 2.1; men: 1.8 to 2.8 mg per day) compared to the general population (women: 1.1 to 1.6; men: 1.5 to 2.0 mg per day) 57.
According to a small German study from 2006, only 4% of 93 vegans had vitamin B6 intake levels below the recommended values for Germany 58. However, a majority of the participants did not reach satisfactory vitamin B6 levels in their blood. This is mainly attributed to pyridoxine glucosides, which can be poorly absorbed. Scientists point out foods with high vitamin B6 bioavailability, such as those found in beans, lentils, and bananas.
If the need for vitamin B6 should be met quickly and easily, cruciferous vegetables should be avoided, and starchy foods like potatoes should be consumed instead. For a comprehensive article on foods containing vitamin B6, click here. You can view the table of vitamin B6 foods here.