The prevalence of low and very high vitamin B6 (Pyridoxal 5’-phosphate) concentrations in hospital patients

Abstract

ABSTRACT Pyridoxal 5’-phosphate (PLP) is the active, coenzyme form of vitamin B₆, and is involved in >100 enzymatic reactions, including metabolism of amino acids, carbohydrates, neurotransmitters and lipids. Low vitamin B₆ status has been associated with severe malnutrition and venous thromboembolism whilst very high doses of pyridoxine (B₆) supplementation lead to toxicity presenting as sensory neuropathy. Pyridoxine is used in the treatment of many conditions e.g. cystinuria, homocystinuria, seizures or peripheral neuropathy associated with isoniazid and hydralazine therapy. Pyridoxine is thought to cause toxicity when intake exceeds 2g per day (~1000x RDA). The European Commission has recommended the upper limit for pyridoxine intake from supplements as 25 mg/day. The corresponding PLP concentrations in blood are unknown.

We retrospectively assessed the prevalence of low and very high PLP concentrations in specimens received for analysis between March 2010 and July 2014 from Guy’s and St. Thomas’ Hospital NHS Foundation Trust patients. Whole blood PLP was measured by HPLC with fluorescence detection. A reference range of 35.2-110.1 nmol/L was applied (Chromsystems).

We processed 269 samples; 146 (54%) were from females. 47 (17%) samples were from <18 year olds and 42 (16%) from >65. The main clinical indications for assessing PLP were cystinuria 68 (25%), peripheral neuropathy 26 (10%), and malnutrition/malabsorption 9 (3%). Four (2%) patients had PLP below and 103 (38%) above the lower and upper limits of the reference range respectively, including 19 (7%) patients with concentrations >540 nmol/L (5x upper limit). Of these 19 patients, sensory neuropathy was present 3 cases.

Low vitamin B₆ status was present in a small number of patients. High/very high PLP concentrations were more common suggesting supplementation and monitoring compliance/response to treatment. Correlating PLP concentrations with patients’ symptoms and dose/duration of pyridoxine may help to establish ‘safe’ limits for PLP levels to prevent sensory neuropathy.

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This article is available to download as a PDF file by clicking below, having previously been published in "Biomedical Scientist" - Sep 2015 edition.