Vitamin B-6: food or medicine?

The recent spat between the House of Commons Agriculture Select Committee and the government’s Food Advisory Committee about vitamin B-6 was inevitable.¹ The problem started in June 1997 when the Food Advisory Committee, on the advice of the Department of Health’s Committee on Toxicity of Chemicals in Food, Consumer Products, and the Environment, recommended that vitamin B-6 should be seen either as a food supplement, in which case the daily dose would be limited to 10 mg, or as a medicine, when it could be available in higher doses. The draft regulations, issued in April 1998 with a 26 June deadline for comments, stipulated that over the counter sales of vitamin B-6 from pharmacies should be limited to daily doses of 11-49 mg; for doses of 50 mg and over the vitamin would have to be prescribed. This position was fiercely contested by the Agriculture Select Committee in its report, conveniently published on 23 June. This argued that limiting the daily “food” dose to 10 mg/day was scientifically unsound and infringed individuals’ rights to decide what they ate.²

Whether or not the Food Advisory Committee has got it right—and it probably has—some guidelines were certainly needed. The daily dietary requirement of pyridoxine, which plays an essential part in amino acid metabolism, is around 1.4 mg for men and 1.2 mg for women, and in those who are B-6 deficient the British National Formulary recommends up to 150 mg daily. Good evidence also exists that vitamin B-6 can prevent (daily dose 10 mg) or reverse (150 mg/day) isoniazid induced neuropathy and treat idiopathic sideroblastic anaemia (up to 400 mg/day). Here then are the bona fide food and medicinal requirements for vitamin B-6.

A survey for the Food Advisory Committee in 1997 showed that of the 400 vitamin B-6 products available as dietary supplements in the United Kingdom, around 50 contained daily doses of over 50 mg and four contained doses of 250 mg.³ What these higher doses were being used for is not clear, but vitamin B-6 is often taken to reverse symptoms of the premenstrual syndrome, the menopause, and depression: for none of these is the evidence of benefit persuasive. Here then is the nub: at the doses at which vitamin B-6 is being used as a medicine—for conditions such as premenstrual tension—there is, according to the British National Formulary, “little sound evidence to support the claims.”

How then can a substance be used in such high doses as a foodstuff? Legally it is somewhat arbitrary whether a substance is a medicine or a foodstuff, and the decision often depends on how the manufacturer presents the product. Guidelines have been published,⁴ but even so a judgment by the licensing authority may be challenged in the courts. Briefly, a product should be classified as a medicine if it is manufactured or supplied wholly or mainly for treating or preventing disease. Support for its being a medicine would come if its sale was accompanied by curative or remedial claims. A manufacturer who uses such claims would know that the product should be classed as a medicine. A second manufacturer, who argues that the product is purely a food supplement, might be permitted to market accordingly. The new recommendations make it clear that for vitamin B-6 the government has decided that at doses over 10 mg/day vitamin B-6 is a medicine.

Once a product moves from being a food to a medicine the benefit:risk analysis to which it is subject alters. While there is little in British food regulation relating to formal benefit:risk assessment, for medicines the science of assessment is mature. The licensing authority aims to balance the potential benefit from the product with both its unwanted effects and the inherent dangers of the underlying condition itself. Accordingly, the risk of serious unwanted effects from a drug used to treat an otherwise fatal cancer might be more acceptable than a trivial side effect from a drug used for a minor symptom. Into the equation must also go the numbers of patients likely to be damaged. The potent anti-inflammatory drug phenylbutazone was withdrawn from general use in arthritis because it caused deaths from bone marrow suppression at a rate of around 5.8 times per million scripts; no such limitation was applied to indomethacin, for which the equivalent figure was 1.4 per million scripts.⁵ These are fine judgments, which can result in denial of a medicine to many thousands to avoid a serious unwanted effect in one.

But is there a risk in taking vitamin B-6? This is probably the most contentious part of the current debate. The Committee on Toxicity argued that there was.² It is recognised that vitamin B-6 can cause peripheral neuropathy in high daily doses. It seemed most persuaded by a study from 1987 showing that over half of a group of 172 women taking vitamin B-6 for around three years in doses averaging 117 mg/day developed symptoms such as parasthesia, hyperaesthesia, weakness, or numbness which reversed when the vitamin was stopped. The results were not consistent with other studies and the trial design was weak, but it seems inescapable that with such large numbers some women will indeed have developed neuropathy at these low doses. Once this step was taken the next follows easily: for conditions where there is no clear benefit from the vitamin any exposure of a woman to risk, even a small one should only be permitted after discussion with a prescriber.

The debate between the rights and wrongs of central (government) and peripheral (individual) decision making is important and is bound to surface again when the new National Institute for Clinical Excellence starts advising prescribers on best treatment. Clearly decisions should be made as closely as possible to individual consumers, but when major technical assessments are involved and when the results apply to communities rather than individuals the place for decisions lies more centrally. The Commons Agriculture Committee argues that if the product is adequately labelled, individuals should be left to decide for themselves. However, I find the Committee on Toxicity’s centralist and restrictive position on vitamin B-6 more appealing.