B vitamin treatments modify the risk of myocardial infarction associated with a MTHFD1 polymorphism in patients with stable angina pectoris.

BACKGROUND: Methylenetetrahydrofolate dehydrogenase (MTHFD1) catalyzes three sequential reactions that metabolize derivatives of tetrahydrofolate (THF) in folate-dependent one-carbon metabolism. Impaired MTHFD1 flux has been linked to disturbed lipid metabolism and oxidative stress. However, limited information is available on its relation to the development of atherothrombotic cardiovascular disease. METHODS AND RESULTS: We explored the association between a MTHFD1 polymorphism (rs1076991 C > T) and acute myocardial infarction (AMI), and potential effect modifications by folic acid/B12 and/or vitamin B6 treatment in suspected stable angina pectoris patients (n = 2381) participating in the randomized Western Norway B Vitamin Intervention Trial (WENBIT). During the median follow-up of 4.9 years 204 participants (8.6%) suffered an AMI. After adjusting for established CVD risk factors, the MTHFD1 polymorphism was significantly associated with AMI (HR: 1.49; 95% CI, 1.23-1.81). A similar association was observed among patients allocated to treatment with vitamin B6 alone (HR: 1.53; 95% CI, 1.01-2.31), and an even stronger relationship was seen in patients treated with both vitamin B6 and folic acid/B12 (HR: 2.35; 95% CI, 1.55-3.57). However, no risk association between the MTHFD1 polymorphism and AMI was seen in patients treated with placebo (HR: 1.29; 95% CI, 0.86-1.93) or folic acid/B12 (1.17; 95% CI, 0.83-1.65). CONCLUSION: A common and functional MTHFD1 polymorphism is associated with increased risk of AMI, although the risk seems to be dependent on specific B vitamin treatment. Further studies are warranted to elucidate the possible mechanisms, also in order to explore potential effect modifications by nutritional factors.

Single oral doses of 13C forms of pteroylmonoglutamic acid and 5-formyltetrahydrofolic acid elicit differences in short-term kinetics of labelled and unlabelled folates in plasma: potential problems in interpretation of folate bioavailability studies.

Single (13)C6-labelled doses of pteroylmonoglutamic acid (PteGlu; 634 nmol) or 5-formyltetrahydrofolic acid (431-569 nmol) were given to fasted adult volunteers, and the rise in total and (13)C-labelled plasma 5-methyltetrahydrofolic acid metabolite monitored over 8 h by HPLC and liquid chromatography-MS. The dose-adjusted area under the curve (AUC) for total (labelled plus unlabelled) plasma 5-methyltetrahydrofolic acid following a 5-formyltetrahydrofolic acid test dose was 155 % that obtained following a PteGlu test dose. Surprisingly, an average 60 and 40 % of the total plasma 5-methyltetrahydrofolic acid response to [(13)C6]PteGlu and [(13)C6]5-formyltetrahydrofolic acid, respectively, was unlabelled; an observation never before reported. Short-term kinetics of plasma [(13)C6]5-methyltetrahydrofolic acid showed a slower initial rate of increase in plasma concentration and longer time to peak following an oral dose of [(13)C6]PteGlu compared with that for an oral dose of [(13)C6]5-formyltetrahydrofolic acid, while the [(13)C6]5-methyltetrahydrofolic acid AUC for [(13)C6]5-formyltetrahydrofolic acid was 221 % that for [(13)C6]PteGlu. These data indicate that PteGlu and 5-formyltetrahydrofolic acid, which are thought to be well absorbed (about 90 %) at physiological doses, exhibit dramatically different rates and patterns of plasma response. A limitation in the rate of reduction of PteGlu before methylation could result in slower mucosal transfer of [(13)C6]5-methyltetrahydrofolic acid derived from [(13)C6]PteGlu into the plasma. This, when coupled with an observed similar plasma clearance rate for [(13)C6]5-methyltetrahydrofolic acid metabolite derived from either folate test dose, would yield a comparatively smaller AUC. These findings suggest potential problems in interpretation of absorption studies using unlabelled or labelled folates where the rate of increase, the maximum increase, or the AUC, of plasma folate is employed for test foods (mainly reduced folates) v. a 'reference dose' of PteGlu.

Deficiencies of folate and vitamin B(6) exert distinct effects on homocysteine, serine, and methionine kinetics.

Folate and vitamin B(6) act in generating methyl groups for homocysteine remethylation, but the kinetic effects of folate or vitamin B(6) deficiency are not known. We used an intravenous primed, constant infusion of stable isotope-labeled serine, methionine, and leucine to investigate one-carbon metabolism in healthy control (n = 5), folate-deficient (n = 4), and vitamin B(6)-deficient (n = 5) human subjects. The plasma homocysteine concentration in folate-deficient subjects [15.9 +/- 2.1 (SD) micromol/l] was approximately two times that of control (7.4 +/- 1.7 micromol/l) and vitamin B(6)-deficient (7.7 +/- 2.1 micromol/l) subjects. The rate of methionine synthesis by homocysteine remethylation was depressed (P = 0.027) in folate deficiency but not in vitamin B(6) deficiency. For all subjects, the homocysteine remethylation rate was not significantly associated with plasma homocysteine concentration (r = -0.44, P = 0.12). The fractional synthesis rate of homocysteine from methionine was positively correlated with plasma homocysteine concentration (r = 0.60, P = 0.031), and a model incorporating both homocysteine remethylation and synthesis rates closely predicted plasma homocysteine levels (r = 0.85, P = 0.0015). Rates of homocysteine remethylation and serine synthesis were inversely correlated (r = -0.89, P < 0.001). These studies demonstrate distinctly different metabolic consequences of vitamin B(6) and folate deficiencies.

Kinetics of folate turnover in pregnant women (second trimester) and nonpregnant controls during folic acid supplementation: stable-isotopic labeling of plasma folate, urinary folate and folate catabolites shows subtle effects of pregnancy on turnover of folate pools.

To investigate the effects of pregnancy on folate metabolism, we conducted an 84-d study in second-trimester (gestational wk 14-25) pregnant women (n = 6) and nonpregnant controls (n = 6) with stable-isotopic tracer methods. All subjects were fed a diet containing approximately 272 nmol/d (120 microg/d) folate from food, along with supplemental folic acid that contained 15% [3',5'-(2)H(2)] folic acid ([(2)H(2)]folic acid) during d 1--41 and that was unlabeled during d 42--84 to yield a constant total folate intake of 1.02 or 1.93 micromol/d (450 or 850 microg/d). Isotopic enrichment of plasma folate, urinary folate and the urinary folate catabolites para-aminobenzoylglutamate (pABG) and para-acetamidobenzoylglutamate (ApABG) was determined at intervals throughout the study. The labeling of pABG and ApABG reflected that of tissue folate pools from which the catabolites originate. After the intake of labeled folic acid was terminated on d 41, labeling of urinary folate exhibited a biphasic exponential decline with distinct fast and slow components. In contrast, during d 42--84, the enrichment of urinary pABG and ApABG exhibited primarily monophasic exponential decline, and plasma folate underwent little decline of labeling during this period. Pregnant women and controls did not differ in estimates of body folate pool size and most aspects of the excretion of labeled urinary folate and catabolites, rates of decline of excretion, and areas under the curves for folate and catabolite excretion. Pregnant women, however, tended to have a slower rate of decline of pABG than ApABG and higher enrichment at d 42 of ApABG and pABG. These data support and extend our previous findings indicating that pregnancy (gestational wk 14--26) causes subtle changes in folate metabolism but does not elicit substantial increases in the rate or extent of folate turnover at these moderately high folate intakes.

Case study: folate bioavailability.

Folate nutritional status depends on intake from food and supplements as well as on the bioavailability of the various ingested forms of this vitamin. Although many advances in the understanding of folate bioavailability have occurred in recent years, many areas of uncertainty remain, especially with respect to naturally occurring dietary folate. This review includes a summary of factors that affect folate absorption and utilization, currently used and promising methods suitable for the assessment of bioavailability, significant findings on which current understanding is based and research needs.

Primed, constant infusion with [2H3]serine allows in vivo kinetic measurement of serine turnover, homocysteine remethylation, and transsulfuration processes in human one-carbon metabolism.

BACKGROUND: One-carbon metabolism involves both mitochondrial and cytosolic forms of folate-dependent enzymes in mammalian cells, but few in vivo data exist to characterize the biochemical processes involved. OBJECTIVE: We conducted a stable-isotopic investigation to determine the fates of exogenous serine and serine-derived one-carbon units in homocysteine remethylation in hepatic and whole-body metabolism. DESIGN: A healthy man aged 23 y was administered [2,3,3-(2)H(3)]serine and [5,5,5-(2)H(3)]leucine by intravenous primed, constant infusion. Serial plasma samples were analyzed to determine the isotopic enrichment of free glycine, serine, leucine, methionine, and cystathionine. VLDL apolipoprotein B-100 served as an index of liver free amino acid labeling. RESULTS: [(2)H(1)]Methionine and [(2)H(2)]methionine were labeled through homocysteine remethylation. We propose that [(2)H(2)]methionine occurs by remethylation with [(2)H(2)]methyl groups (as 5-methyltetrahydrofolate) formed only from cytosolic processing of [(2)H(3)]serine, whereas [(2)H(1)]methionine is formed with labeled one-carbon units from mitochondrial oxidation of C-3 serine to [(2)H(1)]formate to yield cytosolic [(2)H(1)]methyl groups. The labeling pattern of cystathionine formed from homocysteine and labeled serine suggests that cystathionine is derived mainly from a serine pool different from that used in apolipoprotein B-100 synthesis. CONCLUSIONS: The appearance of both [(2)H(1)]- and [(2)H(2)]methionine forms indicates that both cytosolic and mitochondrial metabolism of exogenous serine generates carbon units in vivo for methyl group production and homocysteine remethylation. This study also showed the utility of serine infusion and indicated functional roles of cytosolic and mitochondrial compartments in one-carbon metabolism.

Urinary excretion of folate catabolites responds to changes in folate intake more slowly than plasma folate and homocysteine concentrations and lymphocyte DNA methylation in postmenopausal women.

Folate turnover involves urinary excretion, fecal excretion, and catabolism that involves cleavage of the C9-N10 bond to yield pterins and para-aminobenzoylglutamate (pABG). Little is known about the relationship between the function of folate pools and their rates of catabolism. We report here an investigation of excretion of urinary pABG and its primary excretory form, para-acetamidobenzoylglutamate (ApABG) in samples collected during a previously published study of postmenopausal women. Ten women (49-63 y) were fed a low folate diet (56 microg/d) supplemented with folic acid to yield total folate intakes of 195 microg/d (d 1-5), 56 microg/d (d 6-41), 111 microg/d (d 42-69), 286 microg/d (d 70-80) and 516 microg/d (d 81-91). This caused changes in plasma folate, plasma homocysteine and global methylation of lymphocyte DNA. For each subject, a 7-d pooled urine sample was collected over d 1-7, 36-42, 64-70 and 85-91. ApABG constituted >85% of total catabolite excretion, and folate intake did not significantly influence ApABG or pABG excretion. The molar ratio of total catabolite excretion/folate intake varied significantly, with ratios of 1.0 +/- 0.17 (d 1-7), 3.0 +/- 0.55 (d 36-42), 1.1 +/- 0.18 (d 64-70) and 0. 33 +/- 0.054 (d 85-91). These observations indicate that the rate of folate catabolite excretion is related mainly to masses of slow-turnover folate pools governed by long-term folate intake. Folate pools functioning in some forms of methyl group metabolism respond to dietary changes in folate intake much more rapidly.

Folate absorption in women with a history of neural tube defect-affected pregnancy.

BACKGROUND: The risk of neural tube defects (NTDs) is significantly reduced by supplemental folic acid. NTD risk may be associated with impaired absorption of polyglutamyl folate, the primary form of naturally occurring food folate, and of folic acid in supplements or fortified food. Stable-isotope methods provide the specificity needed to test this hypothesis. OBJECTIVE: We determined whether women who had an NTD-affected pregnancy had a reduced ability compared with control women to absorb polyglutamyl folate relative to folic acid. DESIGN: Healthy, nonpregnant women with a history of an NTD-affected pregnancy (cases; n = 11) and control women (n = 11) were administered an oral dose containing a mixture of [(2)H]pteroylpentaglutamate ([(2)H(2)]PteGlu(5); 233 nmol) and [(13)C]pteroylmonoglutamate ([(13)C(5)]PteGlu(1); 567 nmol) after a 30-d saturation protocol (2 mg unlabeled folic acid/d). Relative extents of absorption were evaluated by urinary excretion of (2)H(2)- and (13)C(5)-labeled folates 48 h postdose. RESULTS: During the first 24 h postdose, cases excreted less (f1.gif" BORDER="0"> +/- SD) [(2)H(2)]PteGlu(5) (21 +/- 12% compared with 37 +/- 19%; P = 0.01) and [(13)C(5)]PteGlu(1) (17 +/- 8% compared with 31 +/- 14%; P = 0.007) than did controls. No significant differences between cases and controls were detected in the percentage of [(2)H(2)]PteGlu(5) or [(13)C(5)]PteGlu(1) excreted during the second 24 h postdose or when the data were averaged over 48 h. However, excretion of the [(2)H(2)]folates tended to be lower in cases than in controls over the 48-h period (33 +/- 13% compared with 45 +/- 26%; P = 0.21). A similar trend (P = 0.29) for lower excretion of [(13)C(5)]folates in cases was also observed (31 +/- 16% compared with 39 +/- 17%). The ratio of urinary [(2)H(2)]folates to [(13)C(5)]folates did not differ significantly between cases and controls. CONCLUSION: These data suggest the need for a larger-scale study using stable-isotope methods to further investigate this hypothesis.

Vitamin B-6 deficiency in rats reduces hepatic serine hydroxymethyltransferase and cystathionine beta-synthase activities and rates of in vivo protein turnover, homocysteine remethylation and transsulfuration.

Vitamin B-6 deficiency causes mild elevation in plasma homocysteine, but the mechanism has not been clearly established. Serine is a substrate in one-carbon metabolism and in the transsulfuration pathway of homocysteine catabolism, and pyridoxal phosphate (PLP) plays a key role as coenzyme for serine hydroxymethyltransferase (SHMT) and enzymes of transsulfuration. In this study we used [(2)H(3)]serine as a primary tracer to examine the remethylation pathway in adequately nourished and vitamin B-6-deficient rats [7 and 0.1 mg pyridoxine (PN)/kg diet]. [(2)H(3)]Leucine and [1-(13)C]methionine were also used to examine turnover of protein and methionine pools, respectively. All tracers were injected intraperitoneally as a bolus dose, and then rats were killed (n = 4/time point) after 30, 60 and 120 min. Rats fed the low-PN diet had significantly lower growth and plasma and liver PLP concentrations, reduced liver SHMT activity, greater plasma and liver total homocysteine concentration, and reduced liver S-adenosylmethionine concentration. Hepatic and whole body protein turnover were reduced in vitamin B-6-deficient rats as evidenced by greater isotopic enrichment of [(2)H(3)]leucine. Hepatic [(2)H(2)]methionine production from [(2)H(3)]serine via cytosolic SHMT and the remethylation pathway was reduced by 80.6% in vitamin B-6 deficiency. The deficiency did not significantly reduce hepatic cystathionine-beta-synthase activity, and in vivo hepatic transsulfuration flux shown by production of [(2)H(3)]cysteine from the [(2)H(3)]serine increased over twofold. In contrast, plasma appearance of [(2)H(3)]cysteine was decreased by 89% in vitamin B-6 deficiency. The rate of hepatic homocysteine production shown by the ratio of [1-(13)C]homocysteine/[1-(13)C]methionine areas under enrichment vs. time curves was not affected by vitamin B-6 deficiency. Overall, these results indicate that vitamin B-6 deficiency substantially affects one-carbon metabolism by impairing both methyl group production for homocysteine remethylation and flux through whole-body transsulfuration.

Polymorphisms of methylenetetrahydrofolate reductase and other enzymes: metabolic significance, risks and impact on folate requirement.

A common genetic polymorphism results from a C-->T substitution in the gene encoding methylenetetrahydrofolate reductase (MTHFR), the enzyme that produces 5-methyltetrahydrofolate (5-methyl-THF) required for the conversion of homocysteine to methionine. In individuals with the T/T genotype (T/T), functional metabolic effects include changes in one-carbon folate derivatives, elevations in plasma homocysteine and differences in response to folic acid supplementation compared with normal (C/C) or heterozygous (C/T) genotypes. The metabolic changes associated with the T/T genotype are postulated to modify risk for chronic disease (e.g., vascular disease and cancer) and neural tube defects (NTD) when accompanied by folate deficiency. The modulation of these metabolic abnormalities by increasing folate intake suggests that folate requirements may be different in affected individuals (T/T) relative to normal (C/C) or heterozygous (C/T) individuals. The complex interaction between this common genetic polymorphism of MTHFR and folate intake is the focus of intense investigation.

Folate metabolism and requirements.

Folate functions in multiple coenzyme forms in acceptance, redox processing and transfer of one-carbon units, including nucleotides and certain amino acids. Folate-requiring metabolic processes are influenced by folate intake, intake of other essential nutrients, including vitamins B-12 and B-6, and at least one common genetic polymorphism. Estimates of folate requirements have been based on intakes associated with maintenance of normal plasma and erythrocyte folate concentrations and functional tests that reflect abnormalities in folate-dependent reactions. Dietary Reference Intakes for folate that have been developed recently are based primarily on metabolic studies in which erythrocyte folate concentration was considered the major indicator of adequacy. For adults >/=19 y, the Recommended Dietary Allowance (RDA) is 400 microg/d of dietary folate equivalents (DFE); for lactating and pregnant women, the RDAs include an additional 100 and 200 microg of DFE/d, respectively.

Urinary excretion of [2H4]folate by nonpregnant women following a single oral dose of [2H4]folic acid is a functional index of folate nutritional status.

In a 10-wk study with nonpregnant women (21-27 y, n = 5-6 per group), subjects were fed a diet containing approximately 68 nmol/d (30 microg/d) folate from food that was supplemented with folic acid in apple juice to yield a constant intake of 454, 680 or 907 nmol/d (200, 300 or 400 microg/d) to evaluate folate status and long-term in vivo kinetics. Reported here is an additional phase of this protocol conducted to determine the relationship between short-term urinary excretion after a single isotopically labeled dose and various measures of folate nutritional status. It was hypothesized that urinary excretion from a single [glutamate-2H4]folic acid ([2H4]folic acid) dose would increase in proportion to folate nutritional status due to saturable cellular uptake and retention processes along with saturation of renal reabsorption. Each subject was given 1.13 micromol (500 microg) of [2H4]folic acid orally on the morning of d 70 of the study, followed by a complete 24-h urine collection. Urine was analyzed to determine the isotopic enrichment of urinary folate by gas chromatography-mass spectrometry and the concentration of urinary folate by HPLC. Urinary excretion of [2H4]folate was greatest at the 907 nmol/d intake and was positively correlated with serum folate concentration but was not correlated with erythrocyte folate. Excretion of [2H4]folate tended to be greatest when plasma homocysteine concentrations were low (<8 micromol/L), although this relation was not significant. These results suggest that 24-h urinary excretion after a single oral dose of isotopically labeled folate is a functional indicator of folate nutritional status that complements other measures of folate nutriture.

Kinetic model of folate metabolism in nonpregnant women consuming [2H2]folic acid: isotopic labeling of urinary folate and the catabolite para-acetamidobenzoylglutamate indicates slow, intake-dependent, turnover of folate pools.

In a 10-wk study of folate metabolism in nonpregnant women (21-27 y, n -6 per group), subjects were fed a diet containing approximately 68 nmol/d (30 microg/d) folate from food. The remainder of the ingested folate was provided as folic acid in apple juice (as nonlabeled during wk 1-2, as [2H2]folic acid during wk 3-10) to yield a constant intake of 454, 680 or 907 nmol/d (200, 300 or 400 microg/d). Isotopic enrichment of total urinary folate and the primary catabolite para-acetamidobenzoylglutamate (ApABG) was determined. Isotopic enrichment of ApABG served as an indicator of labeling of tissue folates. A kinetic model consisting of fast- and slow-turnover nonsaturable pools and a saturable slow-turnover pool, with provisions for urinary and fecal excretion, catabolism and enterohepatic circulation, yielded a close fit to the data. Mean residence times for total body folate were 212, 169 and 124 d for folate intakes of 454, 680, and 907 nmol/d, respectively. The model predicted that variation in folate intake over this range had little effect on the mass of the large saturable folate pool; however, the fast-turnover nonsaturable pools increased in proportion to folate intake, whereas the slow nonsaturable pool also tended to increase. This model will aid in evaluation of folate turnover and in predicting kinetic consequences of physiologic conditions associated with altered folate requirements.

Nutritional Properties and significance of vitamin glycosides.

Glycosylated forms of pyridoxine, vitamin D, niacin, pantothenate, and riboflavin exist in nature, whereas glycosides of retinol and ascorbic acid are products of in vitro transglycosidation. Beta-Glucosides of pyridoxine (a) are prevalent in plant-derived foods, (b) contribute to human nutrition as partially available sources of vitamin B6, (c) undergo partial hydrolysis by a novel mammalian cytosolic beta-glucosidase, and (d) exert a weak antagonistic effect on the utilization of free pyridoxine. Niacin exists in grains as complexed forms with low bioavailability, whereas vitamin D glycosides are toxic components of certain calcinogenic plants of importance in animal health. Glycosides of pantothenate and riboflavin appear to be minor products of mammalian metabolism. Glycosylation of retinol or other hydrophobic alcohols may facilitate glycolipid turnover, whereas a stable ascorbyl glucoside may have nutritional applications. Glycosylation of vitamins exerts widely ranging chemical and biological effects, with great nutritional and metabolic significance.

Folate catabolism in pregnant and nonpregnant women with controlled folate intakes.

Measurement of the urinary folate catabolites, para-aminobenzoylglutamate (pABG) and the more predominant acetylated form, acetamidobenzoylglutamate (apABG), has been used to assess folate requirements in both pregnant and nonpregnant women. Folate catabolite excretion has been reported to be significantly higher in pregnant women (second trimester) compared with nonpregnant controls. The primary goals of this study were to determine if pregnant women in a controlled metabolic study excreted higher quantities of urinary folate catabolites than nonpregnant controls and if catabolite excretion was influenced by folate intake. We evaluated the effect of gestation and folate intake on the urinary excretion of apABG and pABG in pregnant women (n = 12; wk 14-26 gestation) and nonpregnant controls (n = 12) assigned to consume folate levels approximating the current (400 microg/d) and previous (800 microg/d) RDA. Subjects were fed a controlled diet containing 120 microg folate/d and either 330 or 730 microg synthetic folic acid/d. In contrast to previously reported data, no differences in mean folate catabolite excretion were detected between pregnant and nonpregnant subjects. Catabolite excretion (pABG + apABG) decreased significantly relative to initial values in pregnant women consuming 450 microg folate/d (-40 +/- 20%; mean +/- SD) and final mean excretion was significantly lower in the pregnant women consuming 450 microg folate/d (86 +/- 32 nmol/d) compared with 850 microg folate/d (148 +/- 20 nmol/d). Data from this study indicate that second trimester pregnant women do not excrete more folate catabolites than nonpregnant controls and that consumption of 450 vs. 850 microg folate/d results in a significant reduction in the quantity of folate catabolites excreted.

Absorption of folate from fortified cereal-grain products and of supplemental folate consumed with or without food determined by using a dual-label stable-isotope protocol.

The absorption of folic acid in fortified white and whole-wheat bread, rice, or pasta or in solution was evaluated in human subjects with use of a single-dose, dual-label, stable-isotope protocol that did not involve prior loading of subjects with nonlabeled folate. In each of five sequential trials, 14 adults received a single oral dose of [13C5]folic acid in one of the four fortified cereal-grain products or in water concurrently with an intravenous injection of [2H2]folic acid. In two additional trials, subjects received oral [13C5]folic acid with or without a light breakfast meal. In all trials, urine was collected 24-36 h postdosing and the isotopic labeling of urinary folates determined. Isotope excretion ratios of urinary folates (% [13C5]folate dose/% [2H2]folate dose), which were used as criteria of absorption, indicated no significant differences among the various fortified foods and the control (P = 0.607). Because statistical power was sufficient to have detected a 50% difference from the control, these results suggest that [13C5]folic acid in these fortified cereal-grain foods was highly available. This study also suggests that fortification will contribute effectively to the folate status of the population. Consuming [13C5]folic acid after a light breakfast meal led to a small reduction in absorption relative to the control without food (P < 0.085). Between-subject variation in this protocol exceeded that observed in previous studies conducted using prior saturation of subjects with nonlabeled folic acid. We recommend that either prior saturation or multiple doses be used in future applications of this technique to improve precision.

Pyridoxine-5'-beta--glucoside exhibits incomplete bioavailability as a source of vitamin B-6 and partially inhibits the utilization of co-ingested pyridoxine in humans.

This research was conducted to investigate 1) the bioavailability of pyridoxine-5'-beta-D-glucoside (PN-glucoside) relative to that of pyridoxine (PN) in human subjects, and 2) the competitive effect of PN-glucoside on the metabolism of co-ingested PN. To evaluate PN-glucoside bioavailability, the subjects were administered a single oral dose of either deuterium-labeled ([2H2]) PN (Trial 1) or [2H2] PN-glucoside (Trial 2), and the urinary excretion rates of labeled 4-pyridoxic acid (4PA) were measured. The [2H2]4PA derived from [2H2] PN or [2H2]PN-glucoside was excreted mainly in the first 8 h after the dose. Excretion of [2H2]4PA during the 48-h postdose period indicated that the bioavailability of PN-glucoside was approximately 50% relative to PN, which is consistent with our previous report of 58% bioavailability determined using a different protocol and fewer subjects. To assess the effects of PN-glucoside on PN utilization, the subjects were administered different ratios of nonlabeled PN-glucoside with [2H2]PN in Trials 3 and 4. Comparing Trial 1 with Trials 3 and 4, the quantity of nonlabeled PN-glucoside, as a fraction of total vitamin B-6 administered, ranged from 0 to 40% (on the basis of pyridoxine equivalents), with a constant dose of [2H2]PN in each. In these trials, the rate but not the total extent of the excretion of [2H2]4PA derived from [2H2]PN was inversely related to the proportion of co-ingested nonlabeled PN-glucoside. Thus, antagonistic effects of PN-glucoside on PN metabolism do occur in humans, although the effect is less pronounced than that seen previously in rats. Such interactive effects must be considered in evaluating the net bioavailability of dietary forms of vitamin B-6.