Vitamin B6 (Pyridoxine) - Nutrition

What is Vitamin B₆ (Pyridoxine)?

Vitamin B₆, also commonly called pyridoxine, is an essential water-soluble vitamin used as a cofactor in a number of enzymatic reactions. Vitamin B₆ in the diet is found as pyridoxine, pyridoxal, and pyridoxamine all of which are hydrolysed by a phosphatase before absorption across the intestinal mucosa by passive diffusion. Once within the enterocyte the hydrolysed vitamin B₆ derivative is phosphorylated by pyridoxine kinase and transported to the liver for continued metabolism to its active form, pyridoxal-5-phosphate (PLP). In the liver PLP is bound to albumin and then transported to target tissues. Large concentrations of pyridoxine are found in muscle, but this depot is not available during periods of Vitamin B₆ deficiency and is only released during muscle catabolism. Vitamin B₆ derivatives are freely filtered by the renal tubules.

Why is it Important?

The active form of Vitamin B₆, PLP, participates in a number of reactions involving amino acid metabolism (e.g. aminotransferases, decarboxylases, racemizations) as well as various synthetic pathways and gluconeogenesis.

Roles in the Body

Vitamin B₆ as PLP is an essential cofactor for glycogen phosphorylase during gluconeogenesis. It is also a cofactor in serine palmitoyl transferase used sphingolipid synthesis, a cofactor in γ-aminolevulinic acid synthentase, the first and rate-limiting step in heme synthesis. PLP is also required for synthesis of taurine, dopamine, histamine, and norepinephrine, and is involved in multiple steps in the conversion of tryptophan to niacin.

Consequences of Vitamin B₆ Deficiency

Dogs:

Induced vitamin B₆ deficiency has been reported to cause anorexia, weight loss, and microcytic hypo chromic anaemia in puppies, and if left untreated could result in death. Ataxia, hematochezia, seizures, muscle fasciculations, cardiac dilation and hypertrophy of both the left and right auricles and ventricles (with more pronounced changes to the right), and demyelination of peripheral nerves have also been described in adult dogs fed Vitamin B₆ deficient diets.

Cats:

Induced vitamin B₆ deficiency in kittens was reported to result in poor growth rate, microcytic hypochromic anaemia, and calcium oxalate crystalluria. Increased dietary intake of protein has also been show to increase Vitamin B₆ requirement in cats. Vitamin B₆ is light sensitive and prone to degradation with heat processing. Conditions associated with diuresis (e.g., chronic disease, such as renal disease or diabetes, or therapeutic intervention, such as intravenous fluids or increased water intake with management of lower urinary diseases) can result in increased loss of Vitamin B₆ and may increase daily requirements. Patients on chronic haemodialysis are at an increased risk for development of deficiency.

Toxicity

There are no published toxicity reports in cats, but chronic high intake (i.e., >100x the requirement) of Vitamin B₆ can result in ataxia and tonic convulsions in dogs within 1 week of daily administration.

Dietary Sources

Vitamin B₆ is naturally occurring in muscle and organ meats, eggs, dairy, and vegetables. Vitamin B₆ is also supplemented into all commercially-prepared complete and balanced dog and cat foods.

Diagnosing Pyridoxine Deficiency

Diagnosis of pyridoxine deficiency is based on presence of microcytic hypochromic anaemia, potential presence of calcium oxalate crystals in urine sediment evaluation, and measurement of increased plasma tyrosine concentration. Diagnosis is also made on clinical signs consistent with deficiency and direct measurement pyridoxine levels of diet.