Non-nutritional uses of vitamin B6
Several studies have suggested that oral contraceptives cause vitamin B6 deficiency. As a result of this, supple-ments of vitamin B6 of 50–100 mg/day, and some-times higher, have been used to overcome the side-effects of oral contraceptives. Similar supplements have also been recommended for the treatment of the premenstrual syndrome, although there is little evi-dence of efficacy from placebo-controlled trials.
All of the studies that suggested that oral contra-ceptives cause vitamin B6 deficiency used the metabo-lism of tryptophan as a means of assessing vitamin B6 nutritional status. When other biochemical markers of status were also assessed, they were not affected by oral contraceptive use. Furthermore, most of these studies were performed using the now obsolete high-dose contraceptive pills.
Oral contraceptives do not cause vitamin B6 defi-ciency. The problem is that estrogen metabolites inhibit kynureninase and reduce the activity of kyn-urenine hydroxylase. This results in the excretion of abnormal amounts of tryptophan metabolites, similar to what is seen in vitamin B6 deficiency, but for a dif-ferent reason.
Doses of 50–200 mg of vitamin B6/day have an antiemetic effect, and the vitamin is widely used, alone or in conjunction with other antiemetics, to minimize the nausea associated with radiotherapy and to treat pregnancy sickness. There is no evidence that vitamin B6 has any beneficial effect in pregnancy sickness, or that women who suffer from morning sickness have lower vitamin B6 nutritional status than other pregnant women.
Doses of vitamin B6 of 100 mg/day have been reported to be beneficial in the treatment of the carpal tunnel syndrome or tenosynovitis. However, most of the reports originate from one centre and there appears to be little independent confirmation of the usefulness of the vitamin in this condition.
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