ヒトにおける葉酸のBioavailability

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The studies on the determination of absorption and bioavailability of synthetic and dietary folate in humans were reviewed. These investigations were carried out using healthy male volunteers in the 1970s at the metabolic unit of the Department of Nutritional Sciences, University of California, Berkeley. A method sensitive enough to detect the folate absorption following an oral dose of a small quantity of folate was developed in humans. The volunteers were kept under the "uniform folate saturated condition." Urinary folate was measured by microbiological assay using Lactobacillus casei. The bioavailability of synthetic folate derivatives (tetrahydrofolic acid (H_4PteGlu), 5-formylH_4PteGlu, 5-methylH_4PteGlu, pteroyltriglutamate and pteroylheptaglutamate (PteGlu_7)) was similar to that of pteroylmonoglutamate (PteGlu). The bioavailability of folate in various food items was also measured. Among the food measured, bananas, lima beans, liver and yeast contained relatively high amounts of available folate, while the folate bioavailability was poor in orange juice, lettuce and egg yolk. The mechanism of the low bioavailability of folate in orange juice and the inhibitory effect of orange juice on the bioavailability of PteGlu_7 were studied using simulated orange juice with various pHs. The bioavailability of PteGlu_7 was significantly decreased by supplementation of orange juice or citric acid solution (pH3.7). Neutralization of the citric acid solution to pH6.4 reversed this inhibitory effect. In contrast, the bioavailability of PteGlu was not influenced by the supplement of orange juice or citric acid. Data suggest that the low availability of PteGlu_7 is due to the inhibition of intestinal folate conjugase caused by the low pH. The effect of experimental zinc depletion on the intestinal absorption of mono- and polyglutamyl folates was assessed by measuring the increase in serum folate after an oral dose of PteGlu and PteGlu_7 before and after the subjects became zinc-depleted. At the beginning of the study, no differences were detected in the absorption of PteGlu and PteGlu_7. However, after zinc depletion, the rise in serum folate after an oral dose of PteGlu_7 was significantly reduced, while pteroylmonoglutamate absorption appeared to be unchanged. These data suggest that zinc depletion results in a decreased intestinal hydrolysis of pteroylpolyglutamate and that intestinal conjugase in man is a zinc-dependent enzyme.
1989-02-25

ヒトにおける葉酸のBioavailability

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