The effect of wheat bran on the bioavailability of vitamin B-6 in young men

The effect of cooked wheat bran on the bioavailability of vitamin B-6 (B-6) was determined in 10 men, aged 20 to 35 years. The subjects consumed a constant diet with and without the addition of 15 g wheat bran during three successive 18-day periods in a switch-back design. Half of the subjects received the additional bran during periods 1 and 3; the other half consumed the bran during period 2. The bran and nonbran diets supplied, respectively, 1.69 and 1.66 mg of B-6 daily. Plasma total B-6 and pyridoxal phosphate (PLP), urinary 4-pyridoxic acid (4-PA), and urinary and fecal B-6 were determined at regular intervals during each period. Bran significantly increased fecal B-6 (P less than 0.05) and decreased urinary 4-PA (P less than 0.01). Bran also significantly depressed plasma B-6 (P less than 0.01) and PLP (P less than 0.05). These results indicate that the addition of 15 g of wheat bran to the diet decreased the bioavailability of B-6. Since this decrease was modest, never exceeding 17% (based on urinary 4PA and B-6), this amount of bran will not adversely affect B-6 status when intake of the vitamin is adequate.     

Bioavailability of vitamin B-6 in pregnant rats.

The cause for reduced plasma pyridoxal phosphate (PLP) concentration during pregnancy is not well understood. In this study, nonpregnant (control) and pregnant rats were gavaged with [3H]pyridoxine for assessment of the intestinal absorption, tissue distribution, metabolic utilization and urinary excretion of the vitamin. In addition, plasma PLP and pyridoxal levels and erythrocyte aspartate aminotransferase (EAST) activity were measured. There was 50% lower plasma PLP concentration and 50% greater EAST activity in the pregnant rats and no difference in the stimulation of EAST with exogenous PLP. There was no difference in the intestinal absorption, hepatic uptake and retention or urinary excretion of the radioisotope. Less than 3% of the oral dose was detected in fetal/uterine tissue of the pregnant rats. Results of this study indicate that reduced plasma PLP concentration during pregnancy is not a result of diminished total vitamin B-6 body pools or fetal sequestration of vitamin B-6. Low plasma PLP during pregnancy may be a result of altered distribution of PLP between the plasma and erythrocytes.     

Bioavailability of iron from wheat bran in pigs

Iron anemia was induced in pigs immediately after birth by feeding an iron depletion diet containing only 17 mg iron/kg feed. (The requirement for iron in this period is 50 mg iron/kg feed). When Hb concentrations were 5 g/100 ml the pigs were given iron repletion diets. One group received 7% bran in the diet, about 60% of the iron derived from the bran and 40% from ferrous sulfate. The other group received no bran and 80% of the iron from ferrous sulfate. There were no differences, either in the increase of Hb or in the increase of serum iron, in the two groups. In a second experiment, one group received all their iron from cereals, and an addition of 20% bran in the diet. The other group received no bran and 80% of the iron from ferrous sulfate. There was no significant difference in the bioavailability of the iron present in the diets. In our experiments bran seemed to have no inhibitory effect on iron absorption, even when 20% bran was included in the diet.     

Protein utilization by young women consuming animal or plant protein diets at various levels of vitamin B-6 intake

Eight young women consumed a vitamin B-6 depletion diet (egg albumen formula, less than 0.05 mg vitamin B-6/d) for 11-28 d. Subjects (n = 4) then consumed either animal protein (AP, mainly dairy and poultry products) or plant protein (PP, mainly various types of beans) diets with increasing vitamin B-6 intake (0.5, 1.0, 1.5, and 2.0 mg/d) for periods of 14-21 d. All diets provided 1.55 g protein/kg body wt. Apparent protein digestibility of AP (94.6%) was significantly higher than that of PP (88.4%) diets (p less than 0.001). Protein digestibility was not significantly affected by vitamin B-6 intake. Apparent nitrogen balance of subjects consuming AP diets was slightly, though not significantly, higher than that of subjects fed PP diets. N balance was not influenced by vitamin B-6 intake. Data suggest that short-term low vitamin B-6 intake does not affect protein utilization in humans as determined by digestibility and N balance.     

Stimulation of intestinal cytokinetics and mucin turnover in rats fed wheat bran or cellulose

Rats were fed defined diets containing no fiber, 10% wheat bran or 10% cellulose, and intestinal morphology and cytokinetics were assessed by light microscopy and autoradiography, respectively. In bran-fed animals, there were no differences in morphological appearance of the jejunum, in the number of cells/villus column or in numbers of goblet cells compared to controls. Autoradiographic analysis, at one and 24 h after [3H]thymidine, however, suggested an increased turnover and villus transit of intestinal cells. There was also a 2.5 fold increase in incorporation of labeled sulfate, and a 2-fold increase in [3H]glucose incorporation into total intestinal glycoproteins and mucins. Similar, albeit less dramatic results were obtained in rats fed diets containing cellulose. These studies provide evidence that diets containing certain fiber derivatives can alter aspects of intestinal cell turnover, and support the earlier morphological observations suggesting increased goblet cell secretory activity in response to feeding these fiber derivatives.     

Effects of different levels of vitamin B-6 supplementation on vitamin B-6 concentrations in human milk and vitamin B-6 intakes of breastfed infants

Vitamin B-6 has been reported to vary in concentration in human milk in response to changes in maternal intake of the vitamin. This study examined the effects of such changes on the breastfed infants' intake of vitamin B-6. Lactating mothers received 0, 2.5, 10.0 or 20.0 mg pyridoxine X HCl (PN X HCl) for 3 consecutive days in addition to dietary sources. Dietary intakes of vitamin B-6 (mean = 1.8 +/- 0.2 mg/day) were similar among the four groups. Vitamin B-6 was determined in milk samples from each feeding and milk intakes of infants were estimated by test-weighing. Nonsupplemented mothers had lower vitamin B-6 in their milk (93 +/- 8 micrograms/L) and vitamin B-6 intake of their infants was lower (0.06 +/- 0.01 mg/day) compared to other groups. Maternal supplementation with 2.5, 10.0 or 20.0 mg PN X HCl was paralleled by vitamin B-6 levels in milk of 192 +/- 16, 247 +/- 25 and 413 +/- 45 micrograms/L, respectively, and by vitamin B-6 intakes of breastfed infants of 0.12 +/- 0.02, 0.22 +/- 0.02 and 0.28 +/- 0.03 mg/day, respectively. When maternal intakes of vitamin B-6 approximated 20.0 mg/day, breastfed infants were unlikely to receive the current RDA of 0.3 mg vitamin B-6/day.     

Incomplete utilization of pyridoxine-beta-glucoside as vitamin B-6 in the rat

This research was conducted to determine the bioavailability of 5'-O-(beta-D-glucopyranosyl) pyridoxine (PN-glucoside) during chronic administration in a depletion-repletion bioassay. PN-glucoside was found previously to constitute a major portion of the total vitamin B-6 in many foods of plant origin. Following a 14-d depletion period, rats were fed diets containing graded levels of either free pyridoxine (PN) or PN-glucoside for 17 d. Slope ratio analysis of dose-response curves, on the basis of growth and plasma pyridoxal 5-phosphate (PLP) concentration, indicated 10-34% utilization of PN-glucoside relative to the molar response to PN. Erythrocyte aspartate amino-transferase (AspAT) activity and urinary 4-pyridoxic acid concentration were lower and the stimulation of AspAT activity by exogenous PLP was greater for rats fed PN-glucoside than for those fed PN, which indicated reduced vitamin B-6 nutriture in response to PN-glucoside. A constant 7-9% of the ingested PN-glucoside was detected in urine in intact form at all dosage levels. These results provide further evidence of incomplete bioavailability of PN-glucoside and indicate that its extent of utilization is not influenced by its level of dietary intake.     

Vitamin B-6 metabolism as affected by exercise in trained and untrained women fed diets differing in carbohydrate and vitamin B-6 content

To assess the effect of exercise on vitamin B-6 metabolism in women, five young-trained, five young-untrained, and five postmenopausal-untrained women were alternately fed four diets and then exercised (80% VO2 max; 20 min) after each diet. The following diets were fed: 2 wk moderate carbohydrate (CHO) (49%) (2.3 mg B-6); 1 wk high CHO (64%) (2.4 mg B-6); 2 wk moderate CHO + B-6 (10.3 mg B-6); 1 wk high CHO + B-6); 1 wk high CHO + B-6 (10.4 mg B-6). Blood was drawn pre- and postexercise and post-30 min and post-60 min of exercise and analyzed for plasma pyridoxal 5'-phosphate (PLP) and total B-6. Twenty-four-hour urines were analyzed for 4-pyridoxic acid and urinary B-6. PLP and total B-6 increased significantly with exercise and decreased significantly from post- to post-60 min for all diets. Four-pyridoxic acid increased significantly from the preexercise day to the day of exercise for all diets. Although there was a significant change in B-6 metabolism with exercise, neither training, increased CHO, nor age had an effect on this change.     

Effects of vitamin B-6 deficiency on selenium metabolism in the rat

Rats were fed for 23 d diets adequate or deficient in vitamin B-6 and containing selenium as either sodium selenite, selenocysteine (SeCys) or selenomethionine (SeMet). They were then injected with 75Se of the same chemical form and killed 2 d later. Tissue deposition of stable and radiotracer selenium and the activity of glutathione peroxidase (GSHPx) were used to assess selenium utilization. Erythrocyte levels of selenium and GSHPx were lower in vitamin B-6--deficient animals for all forms of selenium; however, 75Se deposition in erythrocytes was not affected by vitamin B-6 status. The activities of cystathionine lyase, aspartate aminotransferase and selenocysteine lyase were lower in livers of vitamin B-6--deficient rats than in vitamin B-6--supplemented rats. The proportion of liver and kidney 75Se soluble in 5% trichloroacetic acid and 0.1 M 2-mercaptoethanol was consistently lower in vitamin B-6--deficient animals, but cation-exchange chromatography of tissue extracts did not identify a specific low-molecular-weight species. Tissue retention of 75Se provided as SeMet was increased in vitamin B-6--deficient animals, but the proportion of 75Se retained in muscle and liver as SeCys was significantly reduced. These findings suggest that the conversion of SeMet to a form available for GSHPx synthesis is reduced by vitamin B-6 deficiency.     

Vitamin B-6-supplemented diets compared with a low vitamin B-6 diet suppress azoxymethane-induced colon tumorigenesis in mice by reducing cell proliferation

Male ICR mice were examined for the effect of vitamin B-6 [pyridoxine (PN) HCl] on azoxymethane-induced colon tumorigenesis. Mice were fed the diets containing 1, 7, 14 or 35 mg PN HCl/kg for 22 wk, and given a weekly injection of azoxymethane (5 mg/kg body) for the initial 10 wk. Compared with the 1 mg PN HCl/kg diet, 7, 14 and 35 mg PN HCl/kg diets significantly suppressed the incidence and number of colon tumors, colon cell proliferation and expressions of c-myc and c-fos proteins. For some variables, 14 and 35 mg PN HCl/kg diets were more effective than the 7 mg/kg diet. Supplemental vitamin B-6 had no influence on the number of colon apoptotic cells. The results suggest that elevating dietary vitamin B-6 suppresses colon tumorigenesis by reducing cell proliferation.     

Response of vitamin B-6 content of muscle to changes in vitamin B-6 intake in men

Previous reports indicated that in growing rats the vitamin B-6 pool in muscle was relatively stable during deficiency but increased in response to increased vitamin B-6 intake. To determine whether human muscle would show a similar response 10 college-aged males received a low vitamin B-6 diet (1.76 mumol/d) for 6 wk followed by 6 wk on a self-selected diet supplemented with 0.98 mmol pyridoxine HCl/d. During depletion, excretion of pyridoxic acid rapidly adjusted to approximate the intake. Plasma pyridoxal phosphate concentrations at the end of the baseline, depletion, and supplementation periods were 81 +/- 51, 9 +/- 3, and 455 +/- 129 nmol/L, respectively, whereas muscle concentrations were 21 +/- 9, 20 +/- 4, and 25 +/- 7 nmol/g, respectively and total vitamin B-6 in muscle was 28 +/- 10, 27 +/- 4, and 35 +/- 10 nmol/g, respectively. These data provide further confirmation that the vitamin B-6 pools in skeletal muscle are resistant to depletion. They also demonstrate that in humans with constant body weight, vitamin B-6 supplementation is not associated with marked increases in vitamin B-6 in muscle.     

Konjac mannan improves the vitamin B-6 status of rats fed a vitamin B-6-deficient diet

To investigate how dietary fiber in the diet affects vitamin B-6 nutriture of rats which have been deprived of vitamin B-6, rats were made vitamin B-6-deficient by feeding a vitamin B-6-deficient 70% casein diet. They were fed 2% cellulose powder-based vitamin B-6-deficient diets supplemented with 3% of additional dietary fiber sources (agar, konjac mannan, pectin and cellulose powder) for subsequent 18 days. Vitamin B-6 status was evaluated according to several biological criteria (weight gain, urinary excretion of xanthurenic acid after tryptophan loading, plasma pyridoxal 5'-phosphate, apparent pyridoxal 5'-phosphate-saturation of liver kynureninase, urinary excretion of 4-pyridoxic acid and fecal output of vitamin B-6). Vitamin B-6 status evaluated by these criteria was considerably improved in the konjac mannan-fed group, when compared with the respective data of the vitamin B-6 supplemented group. The relative mean effect of the konjac mannan diet was about 40% of the vitamin B-6 supplemented diet. In conclusion, konjac mannan was effective for improving the vitamin B-6 nutritional state in vitamin B-6-deprived rats.     

Response of B-6 vitamers in plasma, erythrocytes and tissues to vitamin B-6 depletion and repletion in the rat

We determined the response patterns of B-6 vitamers in blood and tissues to vitamin B-6 depletion and repletion. B-6 vitamers were measured in plasma, erythrocytes, liver, muscle, kidney, heart, brain, spleen and lung by reverse-phase high performance liquid chromatography in male rats pair-fed control or vitamin B-6-deficient diets for 2 or 4 wk, or for 4 wk followed by 1 wk of repletion with the control diet (n = 4/group). Food intake (15.6 +/- 0.3 g/d, mean +/- SEM; n = 28) and body weight (190 +/- 2 and 290 +/- 5 g at wk 0 and 5, respectively; n = 28) of control groups were not different from those of deficient groups throughout the study. After 2 wk of vitamin B-6 depletion, tissue concentrations of pyridoxal phosphate (PLP) and pyridoxamine phosphate (PMP) were about 50% and 10-40% lower, respectively, in the deficient than in the control group (except for spleen PMP); in plasma and erythrocytes, PLP and pyridoxal concentrations were about 90% lower in the deficient group. Differences in vitamer concentrations between control and deficient groups were not larger after 4 wk of depletion than after 2 wk. Vitamer concentrations in plasma, erythrocytes and all tissues returned to control levels after 1 wk of repletion with the control diet. These results demonstrate that B-6 vitamers in blood and tissues of the rat respond quickly and reversibly to changes in dietary vitamin B-6, with larger percentage changes occurring in plasma and erythrocytes than in tissues.     

Assessment of vitamin B-6 status in young women consuming a controlled diet containing four levels of vitamin B-6 provides an estimated average requirement and recommended dietary allowance

The Recommended Dietary Allowance (RDA) of vitamin B-6 for young women was recently reduced from 1.6 to 1.3 mg/d based on an adequate plasma pyridoxal phosphate (PLP) concentration of 20 nmol/L. To assess vitamin B-6 requirements and suggest recommendations for intake, seven healthy young women consumed a controlled diet providing 1.2 g protein/kg body weight for a 7-d adjustment period (1.0 mg vitamin B-6/d) and three successive 14-d experimental periods (1.5, 2.1 and 2.7 mg/d, respectively). Direct and indirect vitamin B-6 status indicators were measured in plasma, erythrocytes and urine. Indicators most strongly correlated with vitamin B-6 intake [i.e., plasma and erythrocyte PLP, urinary 4-pyridoxic acid (4-PA) and total vitamin B-6] were regressed on vitamin B-6 intake and the dietary vitamin B-6 to protein ratio. Inverse prediction using adequate and baseline values estimated vitamin B-6 requirement. Adequate values were determined for plasma PLP and urinary 4-PA from baseline values of 60 previous subjects, using the statistical method suggested by Sauberlich. The current study suggests a vitamin B-6 Estimated Average Requirement (EAR) for young women of 1.1 mg/d or 0.016 mg/g protein, and a RDA of 1.5 mg/d or 0.020 mg/g protein. When results from this study are combined with data from four other recent studies, the combined data predict an EAR of 1.2 mg/d or 0.015 mg/g protein, and a RDA of 1.7 mg/d or 0.018 mg/g protein. This study suggests that the current vitamin B-6 RDA may not be adequate.     

Short-term folate,vitamin B-12 or vitamin B-6 supplementation slightly affects memory performance but not mood in women of various ages

Based on research demonstrating associations between folate, B-12 and B-6 vitamins and cognition and mood, we investigated the effects of short-term supplementation in 211 healthy younger, middle-aged and older women who took either 750 microg of folate, 15 microg of vitamin B-12, 75 mg of vitamin B-6 or a placebo daily for 35 d. In addition, we examined associations between dietary intake of these vitamins and cognition and mood. Usual dietary intake status was estimated using a retrospective, self-report, quantified food frequency questionnaire. Participants completed alternate forms of standardized tests of cognitive processing resources, memory, executive function, verbal ability and self-report mood measures before and after supplementation. Supplementation had a significant positive effect on some measures of memory performance only, and no effect on mood. Dietary intake status was associated with speed of processing, recall and recognition and verbal ability.     

Efficacy of the rat bioassay for the determination of biologically available vitamin B-6

Research was conducted to evaluate the merits of rat bioassays in studies concerning the bioavailability of vitamin B-6. A protocol was devised, which included pair-feeding and prevention of coprophagy. Plasma pyridoxal 5'-phosphate (PLP) was measured by a rapid chromatographic method as an indicator of vitamin B-6 status. Pyridoxamine and pyridoxal exhibited 70% molar activity relative to pyridoxine at suboptimal dietary levels. Selected foods (spinach, cornmeal and potato) were evaluated at three dietary levels with this protocol. Analysis of dose-response curves (dietary vitamin B-6 vs. plasma PLP) by slope-ratio methods yielded imprecise estimates of the relative vitamin B-6 bioavailability. This low precision, in addition to the potential for interference by direct absorption of B-6 vitamers synthesized by intestinal microflora, indicates the need for improved methods for measurement of biologically available vitamin B-6. It was concluded that rat bioassay methods, even as modified here, may be frequently unsuitable for use in studies of vitamin B-6 bioavailability. Results presented provide further support for the use of plasma PLP as an indicator of vitamin B-6 nutriture.     

Metabolism of vitamin B-6 by human liver

The enzymes that metabolize vitamin B-6 were analyzed in liver biopsy samples from five patients without hepatic disease by using methods optimized for small samples. Pyridoxal kinase (EC 2.7.1.35) activities were 11.2 +/- 3.6 nmol/minute per gram of tissue and 0.16 +/- 0.05 nmol/minute per milligram of soluble protein (mean +/- SD); a clear dependence of the activity on zinc as the divalent cation was observed. Pyridoxine (pyridoxamine) 5'-phosphate oxidase (EC 1.4.3.5) activities, when using N-(5'-phosphopyridoxyl)-[3H]tryptamine as the substrate, were 0.64 +/- 0.22 pmol/minute per milligram of protein and 47 +/- 19 pmol/minute per gram of tissue. The activities were 63 +/- 18% lower when riboflavin 5'-phosphate was omitted from the assay; hence, it appears the oxidase is only partially saturated with its cofactor. The pyridoxal 5'-phosphate hydrolase(s) activities at alkaline pH were 282 +/- 183 nmol/minute per gram of tissue and 4.0 +/- 3.2 nmol/minute per milligram of particulate protein. Pyridoxal was rapidly oxidized to pyridoxic acid (28.1 +/- 19.8 nmol/minute per gram of tissue and 0.37 +/- 0.24 nmol/minute per milligram of soluble protein) by soluble enzyme(s), and the rate was unaffected by pyridine nucleotides. These experiments constitute the first quantitative analyses of the enzymes responsible for metabolizing vitamin B-6 in human liver, and provide data for interpreting the pharmacokinetics of B-6 utilization by humans, as well as methods for investigating diseases with aberrant metabolism of this nutrient.