VITAMIN B6 TOXICITY
Vitamin B6 (also known as pyridoxine) became “front page news” when it was discovered that, in megadoses, it causes such serious nerve damage and difficulty in walking that people have had to give up their jobs.
It has been thought that pyridoxine nerve damage would occur only with daily doses in the range of 2000-6000 mg (two to six grams), or 1000 times the recommended daily allowance (RDA) of two to four mg. Now, however, according to correspondence in the New England Journal of Medicine (311:986), it has even taken place in an otherwise healthy young woman who took only 500 mg every day. After 12 months on this dosage, she experienced shooting pains and increasing numbness in her limbs, and such difficulty in balancing on her feet that it became impossible to walk unaided in the dark or with her eyes closed. These are typical of the symptoms experienced by people who have been taking megadoses of vitamin B6. Brief exposure to very high dosage therefore appears to have the same toxic effect as longer exposure to doses of a more moderate size. Either way, the dosage involved have always been a few hundred times larger than the RDA. Consumers need to be alert to this danger since 500 mg tablets of vitamin B6 can still be purchased without a doctor’s prescription.
Like all other “water-soluble” vitamins, vitamin B6 has traditionally been considered harmless, regardless of the dosage. This experience, however, shows that in megadoses it is definitely unsafe.
After normal doses of pyridoxine (two to four milligrams daily), the body converts it into pyridoxal phosphate, the “active” form of vitamin B6. Pyridoxal phosphate is then taken up by the tissues and becomes a part of many enzymes (the chemical “machinery” of cells).
The Journal speculates that the body is chemically so overwhelmed by megadoses of pyridoxine that it cannot convert more than a small fraction of it into the active phosphate form. Unaltered pyridoxine, it goes on to suggest, then floods the tissues in such abundance that the cells take it up instead of the phosphate. In this way, the excess of plain pyridoxine could block the cells’ enzymes and prevent them from taking up the small normal amounts of pyridoxal phosphate still available.
Alternatively, the Journal suggests, manufactured pyridoxine may contain small amounts of a poisonous impurity. Normal doses of pyridoxine would not provide sufficient impurity to do harm, but in megadoses, the amount of impurity could be dangerous. Either way, pyridoxine in daily doses 100-1000 greater than the RDA of two to four mg has proved to be very toxic. Taking 100-1000 times the usual dose of almost anything might be expected to cause trouble, and megadosing, in general, seems to be a rather risky fad.
*214\143\2*
