Toxicity
Adverse effects have only been documented from vitamin B6 supplements and never from food sources. This article only discusses the safety of the common supplemental form of vitamin B6, pyridoxine (for a full discussion please see pyridoxine). Toxicologic animal studies identify specific destruction of the dorsal root ganglia which is documented in human cases of overdosage of pyridoxine. Although it is a water-soluble vitamin and is excreted in the urine, doses of pyridoxine in excess of the RDI over long periods of time result in painful and ultimately irreversible neurological problems.
The primary symptoms are pain and numbness of the extremities, and in severe cases, difficulty walking. Sensory neuropathy typically develops at doses of pyridoxine in excess of 1,000 mg per day. However, a few individuals who developed sensory neuropathies at doses of less than 500 mg daily over a period of months have been reported. None of the studies, in which an objective neurological examination was performed, found evidence of sensory nerve damage at intakes below 200 mg/day. This condition is usually reversible when supplementation is stopped.
Existing authorisations and valuations vary considerably worldwide. In 1993, the European Community Scientific Committee on Food defined intakes of 50 mg of vitamin B6 per day as harmful and established a tolerable upper intake level of 25 mg/day for adults in 2000.
The Expert Group on Vitamins and Minerals of the Food Standard Agency UK (UK EVM) derived a safe upper level (SUL) of 10 mg/day for a 60-kg adult in 2003.
The tolerable upper limit has been set by the US FDA at 100 mg/day in 2000. The nutrient reference values in Australia and New Zealand recommend an upper limit of 50 mg a day in adults. "The same figure was set for pregnancy and lactation as there is no evidence of teratogenicity at this level. The UL was set based on metabolic body size and growth considerations for all other ages and life stages except infancy. It was not possible to set a UL for infants, so intake is recommended in the form of food, milk or formula." "The ULs were set using results of studies involving long-term oral administration of pyridoxine at doses of less than 1g/day (Berger & Schaumburg 1984, Bernstein & Lobitz 1988, Dalton 1985, Dalton & Dalton 1987, Del Tredici et al 1985, FNB:IOM 1998, Parry & Bredesen 1985). A NOAEL of 200 mg/day was identified from the studies of Bernstein & Lobitz (1988) and Del Tredici et al (1985). These studies involved subjects who had generally been on the supplements for five to six months or less. The study of Dalton and Dalton (1987), however, suggested the symptoms might take substantially longer than this to appear. In this latter retrospective survey, subjects who reported symptoms had been on supplements for 2.9 years, on average. Those reporting no symptoms had taken supplements for 1.9 years."
Because no placebo-controlled studies show therapeutic benefits of high doses of pyridoxine, and the well-documented occurrence of significant toxic effects, there is little reason to exceed the RDI using supplements unless under medical supervision e.g. in treatment of primary hyperoxaluria.
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