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Vitamin B6 (pyridoxine) metabolism in diabetes has never been investigated except for a few reports on plasma pyridoxal 5′-phosphate (PLP). These studies indicated that this most active (coenzyme) vitamer can be reduced. The present clinical investigation aimed to measure all vitamers in blood and urine by high performance liquid chromatography as well as important related factors, in women during active reproductive years. Thirty-two insulin-treated type 1 diabetic (T1D) patients, without renal complication, and 27 well-matched healthy controls, aged 30 to 40 years old, were recruited using rigorous criteria. Both groups had normal hemoglobin and serum albumin levels. Plasma PLP and pyridoxal (PL) did not differ significantly in the T1D group but alkaline phosphatase (ALP) activity was greater ( p < 0.01). This produced a shift in plasma PLP-PL profile, as evidenced by a lower plasma PLP/PL ratio ( p < 0.05). Enhanced ALP activity meant more PLP being dephosphorylated to PL (the membrane transfer form), with more ending up in erythrocytes to be rephosphorylated in its active form, as suggested by the significant positive correlation ( p < 0.001) between plasma PL and erythrocyte PLP. More PL into blood circulation also means more oxidation of this vitamer to 4′-pyridoxic acid in kidneys, as confirmed by the positive correlation between plasma PL and urinary 4′-pyridoxic acid ( p < 0.001). The positive correlation ( p < 0.001) between ALP activity and glycosylated hemoglobin indicated a direct effect of the disease. The T1D-induced alteration in vitamin B6 metabolism, consecutive to enhanced ALP activity, may put patients at greater risk of vitamin B6 deficiency and diabetic complications. [ABSTRACT FROM AUTHOR]