Vitamin B-6 status of middle aged women consuming soymilk versus cow\u27s milk

Vitamin B-6 (B-6), an essential nutrient whose coenzyme form pyridoxal 5\u27-phosphate (PLP) is required by over 100 enzymes, is involved in many important metabolic processes. Pyridoxine glucoside (PNG), a less bioavailable form, is present in plant foods and absent in animal foods. With recognition of beneficial health effects of soy, there has been an increase in soyfoods consumption. Middle aged women frequently have marginal intakes of B-6, and many drink soymilk for its purported benefit in relieving menopausal symptoms. Although the PNG content of soymilk has not been reported, PNG content of other soyfoods ranges from 57-67% of the total B-6. The effect of substituting soymilk for cow\u27s milk on B-6 status was examined in middle aged women (aged 36-52 y; n = 16). The study employed a crossover design: 14-d adjustment period followed by 28-d experimental period, during which half the subjects consumed 4 cups of cow\u27s milk and half consumed 4 cups of soymilk per day; 14-d washout period and second 28-d experimental period when subjects switched to the other milk. Participants followed a self-selected B-6 restricted diet (~1 mg/d) throughout the study by following a list of prohibited or restricted foods. Three consecutive 24-h urine samples were collected and fasting blood was drawn every two weeks. Methionine loads (0.1 g/kg body weight) were given at the end of each experimental period. Plasma PLP, 4-pyridoxic acid (4-PA), urinary 4-PA, and pre- and postload plasma homocysteine were determined by HPLC analysis. Using crossover ANOVA tests, mean plasma 4-PA (P = 0.001), and urinary 4-PA-to-creatinine ratio (P \u3c0.001) were significantly lower when soymilk was consumed. Mean plasma PLP (P = 0.107) and increase in mean plasma total homocysteine concentrations after a methionine load (P = 0.316) were not significantly different between the cow\u27s milk and soymilk treatments. Significant effects may have been seen if the study period was longer (\u3e28 d), the diets more rigidly controlled, and sample size larger (n\u3e16). These results suggest that substituting soymilk for cow\u27s milk has an adverse effect on B-6 status in middle aged women as reflected by the reduced status indicators