1 Twenty‐four Asian vegetarians had significantly lower 25‐ hydroxyvitamin D (25‐OHD) levels and longer antipyrine half‐lives than twenty white non‐vegetarians (P less than 0.001). 2 Treatment with oral antipyrine over 4 or 5 weeks in seven vegetarian Asians and five racially different non‐vegetarians increased drug oxidation significantly in both groups as measured by a fall in antipyrine half‐ lives and a rise in serum gamma‐glutamyltranspeptidase levels and urinary 6 beta‐hydroxycortisol/17‐hydroxycorticosteroid ratios. 3 Antipyrine treatment produced a fall in circulating 25‐hydroxyvitamin D of around 60% in all subjects in whom pretreatment levels could be measured, independent of race and diet. 4. In the Caucasian non‐ vegetarian group 1,25 dihidroxyvitamin D levels, the most active metabolite of vitamin D, were also measured and remained unaltered despite a substantial fall in 25‐hydroxy substrate. 5 The acute fall in 25‐hydroxyvitamin D concentration with a maintained level of 1,25 dihidroxyvitamin D may represent the early changes of drug‐induced osteomalacia.
Vitamin D2 is absorbed by the gut and vitamin D3 is synthesized in the skin. Both are hydroxylated in the liver to 25-hydroxyvitamin D (25-OHD), the major circulating metabolite. This is further converted by renal 1 a hydroxylase to the active hormone, 1,25-dihydroxyvitamin D.In a preliminary study Asian vegetarians, who showed significant impairment of antipyrine hydroxylation compared with a group of Caucasian nonvegetarians (Asians -n = 24; antipyrine Ti 16.3 ± 0.8 h; Caucasians -n=20; antipyrine Ti 9.1+0.5 h; P<0.001), also had reduced circulating 25-OHD levels (Asians 4.8±0.7 .tg/l, Caucasians 13.7 +1.7 ±g/l; P< 0.001).As antipyrine and vitamin D are both hydroxylated in the hepatic endoplasmic reticulum it is possible that the increase in antipyrine half-lives and reduction in 25-OHD levels noted in the Asian vegetarians are due to impairment of the same enzyme system. To test this hypothesis a further 7 vegetarian Asians and 5 racially different non-vegetarians were treated chronically with antipyrine to induce hepatic monooxygenase activity (antipyrine 600 mg p.o. daily for 5 and 4 weeks respectively). Placebo controls were included in each group. A significant fall in antipyrine half-lives and rise in serum gamma glutamyl transpeptides (yGT) levels during the study periods indicated hepatic enzyme induction (vegetarians -
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