pyridoxine (vitamin B6)

introduction

pyridoxine (vitamin B6) is a vitamin which plays a critical role as a cofactor for the production of GABA from glutamic acid.
other actions include:
- acts as a coenzyme in over 100 enzymatic reactions, gluconeogenesis and glycogenolysis are included in its regulation, hence may lower glucose levels
- rescues DNA damage treating impaired PI3K, so it can enhance insulin sensitivity and attenuating insulin resistance
- may reduce formation of advanced glycation end products (AGEs) so as to prevent diabetic complications
- an important coenzyme in the metabolism of homocysteine (Hcy), which was deemed a toxic factor in damaging the endothelial cells in microvessels and inducing vascular diseases through hyperhomocysteinaemia (HHcy). It can work together with other nutrients to degrade Hcy by methylation
- can induce autophagy inhibiting the apoptosis of pancreatic β-cells via the mTOR-dependent pathway
recommended daily intake is 1.7mg
many foods and supplements have much higher levels than this and all can contribute to neuropathic toxicity if > 50mg/d consumed

pyridoxine toxicity may cause:
- reversible sensory axonal neuropathy and perhaps CNS effects, although this is usually following chronic high doses over 100mg per day for months
- it seems that higher vitamin B6 levels (eg >=100mg/day supplements), may lead to the development of a predominantly, if not exclusively, sensory neuropathy of the axonal type however, data is inclonclusive due to small sample sizes. Some advisory bodies are recommending max daily intake of 50mg/day - average dietary intake is 5mg/day ¹⁾ However TGA Australia anecdotal reports of a number of patients developing issues with under 50mg/d ²⁾ It appears the mechanism of pyridoxine induced peripheral neuropathy is necrosis of the dorsal root ganglion and is duration and dose dependent. ³⁾
- irreversible destruction of sensory ganglion cells (neuronopathy) may also occur after a single extremely large parenteral dose
- may cause hypoglycaemia in diabetics ⁴⁾
it is present in thousands of products in Australia as an additive — including medicines, multivitamin and mineral supplements, energy drinks and weight loss shakes
- eg. Red Bull contains ~5mg/250mL ⁵⁾

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