Less severe cases present with metabolic disease associated with insufficient activity of the coenzyme pyridoxal 5’ phosphate (PLP). [1] The most prominent of the lesions is due to impaired tryptophan– niacin conversion. This can be detected based on urinary excretion of xanthurenic acid after an oral tryptophan load. Vitamin B 6 deficiency can also result in impaired transsulfuration of methionine to cysteine. The PLP-dependent transaminases and glycogen phosphorylase provide the vitamin with its role in gluconeogenesis, so deprivation of vitamin B 6 results in impaired glucose tolerance. [1] [15] Diagnosis [ edit ] Vitamin B 6 is one of the B vitamins, and thus an essential nutrient. [1] [2] [3] [4] The term refers to a group of six chemically similar compounds, i.e., " vitamers", which can be interconverted in biological systems. Its active form, pyridoxal 5′-phosphate, serves as a coenzyme in more than 140 enzyme reactions in amino acid, glucose, and lipid metabolism. [1] [2] [3] a b c d e f "USDA Food Data Central. Standard Reference, Legacy Foods". USDA Food Data Central. April 2018 . Retrieved 18 August 2021.

One study in 156 adults with high homocysteine levels and mild cognitive impairment found that taking high doses of B6, B12 and folate (B9) decreased homocysteine and reduced wasting in some regions of the brain that are vulnerable to Alzheimer’s ( 16). Ristilä M, Strid H, Eriksson LA, Strid A, Sävenstrand H (March 2011). "The role of the pyridoxine (vitamin B6) biosynthesis enzyme PDX1 in ultraviolet-B radiation responses in plants". Plant Physiol Biochem. 49 (3): 284–92. doi: 10.1016/j.plaphy.2011.01.003. PMID 21288732.More research that includes randomized trials and not merely observational studies is needed to assess the exact role of vitamin B6 in cancer prevention. Lheureux P, Penaloza A, Gris M (April 2005). "Pyridoxine in clinical toxicology: a review". Eur J Emerg Med. 12 (2): 78–85. doi: 10.1097/00063110-200504000-00007. PMID 15756083. S2CID 39197646.

Ulvik A, Midttun Ø, Pedersen ER, Eussen SJ, Nygård O, Ueland PM (July 2014). "Evidence for increased catabolism of vitamin B-6 during systemic inflammation". The American Journal of Clinical Nutrition. 100 (1): 250–5. doi: 10.3945/ajcn.114.083196. PMID 24808485. Kobayashi D (2019). "[Food Poisoning by Ginkgo Seeds through Vitamin B6 Depletion]". Yakugaku Zasshi (in Japanese). 139 (1): 1–6. doi: 10.1248/yakushi.18-00136. PMID 30606915. However, vitamin B6 deficiency is thought to be rare in most healthy adults, so there is limited research on using B6 to treat anemia.

The group that took B6 and folic acid had lower homocysteine levels and less abnormal heart tests during exercise than the placebo group, putting them at an overall lower risk of heart disease ( 31). a b c Bachmann T, Rychlik M (August 2018). "Synthesis of [13C₃]-B6 Vitamers Labelled at Three Consecutive Positions Starting from [13C₃]-Propionic Acid". Molecules. 23 (9). doi: 10.3390/molecules23092117. PMC 6225105. PMID 30142892.

PLP is a coenzyme needed for the proper function of the enzymes cystathionine synthase and cystathionase. These enzymes catalyze reactions in the catabolism of methionine. Part of this pathway (the reaction catalyzed by cystathionase) also produces cysteine. These amounts of B6 are significantly higher than the RDA and sometimes combined with other B vitamins. It’s difficult to assess if increasing intake of B6 from dietary sources has the same benefits for certain conditions that supplements may provide. For US food and dietary supplement labeling purposes the amount in a serving is expressed as a percent of Daily Value. For vitamin B 6 labeling purposes 100% of the Daily Value was 2.0mg, but as of May 27, 2016 it was revised to 1.7mg to bring it into agreement with the adult RDA. [26] [27] A table of the old and new adult daily values is provided at Reference Daily Intake. György P (1934). "Vitamin B2 and the Pellagra-like Dermatitis in Rats". Nature. 133 (3361): 498–9. Bibcode: 1934Natur.133..498G. doi: 10.1038/133498a0. S2CID 4118476. Dietary deficiency is rare. Classic clinical symptoms include rash and inflammation around the mouth and eyes, plus neurological effects that include drowsiness and peripheral neuropathy affecting sensory and motor nerves in the hands and feet. In addition to dietary shortfall, deficiency can be the result of anti-vitamin drugs. There are also rare genetic defects that can trigger vitamin B 6 deficiency-dependent epileptic seizures in infants. These are responsive to pyridoxal 5'-phosphate therapy. [8] Definition [ edit ] Pyridoxine (PN) Pyridoxamine (PM) Pyridoxal (PL)

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A review of 12 studies found that both adequate dietary intake and blood levels of B6 were associated with lower risks of colorectal cancer. Individuals with the highest blood levels of B6 had an almost 50% lower risk of developing this type of cancer ( 36). In addition, a review of 19 studies concluded that supplementing with B6, B12 and folate alone or in combination did not improve brain function or reduce the risk of Alzheimer’s ( 18). Ford AH, Almeida OP (May 2019). "Effect of Vitamin B Supplementation on Cognitive Function in the Elderly: A Systematic Review and Meta-Analysis". Drugs Aging. 36 (5): 419–34. doi: 10.1007/s40266-019-00649-w. PMID 30949983. S2CID 96435344. Transaminases break down amino acids with PLP as a cofactor. The proper activity of these enzymes is crucial for the process of moving amine groups from one amino acid to another. To function as a transaminase coenzyme, PLP bound to a lysine of the enzyme then binds to a free amino acid via formation of a Schiff's base. The process then dissociates the amine group from the amino acid, releasing a keto acid, then transfers the amine group to a different keto acid to create a new amino acid. [3]

Harada K (2016). "食品中の機能性成分解析"[Analysis of Functional Ingredients in Foods]. Bunseki Kagaku (in Japanese). The Japan Society for Analytical Chemistry. 65 (6): 309–319. doi: 10.2116/bunsekikagaku.65.309. ISSN 0525-1931. Mei N, Guo X, Ren Z, Kobayashi D, Wada K, Guo L (January 2017). "Review of Ginkgo biloba-induced toxicity, from experimental studies to human case reports". J Environ Sci Health C Environ Carcinog Ecotoxicol Rev. 35 (1): 1–28. doi: 10.1080/10590501.2016.1278298. PMC 6373469. PMID 28055331. Studies that highlight the use of vitamin B6 to prevent and treat health issues focus on supplements rather than food sources. Vitamin B 6 is absorbed in the jejunum of the small intestine by passive diffusion. [1] [4] Even extremely large amounts are well absorbed. Absorption of the phosphate forms involves their dephosphorylation catalyzed by the enzyme alkaline phosphatase. [15] Most of the vitamin is taken up by the liver. There, the dephosphorylated vitamins are converted to the phosphorylated PLP, PNP and PMP, with the two latter converted to PLP. In the liver, PLP is bound to proteins, primarily albumin. The PLP-albumin complex is what is released by the liver to circulate in plasma. [4] Protein-binding capacity is the limiting factor for vitamin storage. Total body stores, the majority in muscle, with a lesser amount in liver, have been estimated to be in the range of 61 to 167mg. [4] A case study in a 72-year-old woman with anemia due to low B6 found that treatment with the most active form of vitamin B6 improved symptoms ( 22).Brasky TM, White E, Chen CL (October 2017). "Long-Term, Supplemental, One-Carbon Metabolism-Related Vitamin B Use in Relation to Lung Cancer Risk in the Vitamins and Lifestyle (VITAL) Cohort". Journal of Clinical Oncology. 35 (30): 3440–3448. doi: 10.1200/JCO.2017.72.7735. ISSN 1527-7755. PMC 5648175. PMID 28829668.