BRIEF CRITICAL REVIEWS
ROLE OF GLYCOSYLATED VITAMIN B, IN HUMAN NUTRITION A study of lactating women consuming mixed diets showed that the quantity of dietary glycosylated vitamin Be had little influence on the vitamin Be nutritional status of the mothers or their infants.
The existence of naturally occurring “bound” forms of vitamin B, in foods of plant origin has long been recognized. Although considerable progress has been made recently, the chemical and nutritional properties of these forms of the vitamin are still not completely understood. The first “bound” form of vitamin B, to be identified was 5’-0-(6-D-g Iucopyranosyl) pyridoxine (PNG), which was isolated from rice bran by Yasumoto et a1.l The existence of several minor glycosylated forms of vitamin B, also has been r e p ~ r t e d , ~but - ~ the significance of these is uncertain. The results of both microbiologic6 and high-pressure Iiqu id chromatography (HPLC)7 assay methods indicated that glycosylated vitamin B, comprises 5 7 5 % (mean, 3 0 4 0 % ) of the total vitamin B, in plant-derived foods. The primary glycosylated form of vitamin B, in many foods is PNG.7 Because a substantial proportion of dietary vitamin B, may be glycosylated, the bioavailability of glycosylated vitamin B, is an important issue. However, information concerning the nutritional properties of glycosylated vitamin B, is incomplete. Studies done with rats indicated that purified PNG is comparatively well absorbed, undergoes little utilization in metabolism of vitamin B, and is rapidly excreted in the ~ r i n e . ~Most ,~ studies have indicated 20-30% net bioavailability of PNG relative to pyridoxine in rats,6-10 whereas one report indicated nearly equivalent bioavaiIability of PNG and pyridoxine.ll Kabir et a1.12 employed a bioassay with human subjects and observed a negative correlation between the apparent bioavailability of vitamin B, and the proportion of
glycosylated vitamin B, in several foods. Further studies with additional foods indicated an inconsistent r e l a t i ~ n s h i p ,which ’~ suggested that the composition of food also influences the utilization of PNG. Preliminary reports of stable-isotope experiments in humans indicate that the mean bioavailability of purified PNG is -55100%,14s15which is substantially greater than that observed in the rat. Information is lacking concerning the overall utilization of glycosylated vitamin B, by free-living humans consuming mixed diets. Andon et a1.16 recently reported a study of the relationship between diet and vitamin B, nutriture in 30 lactating omnivorous women and their infants. The women were approximately two months postpartum and did not consume vitamin supplements. An important aspect of the study was the collection of three-day diet composites for determination of total and glycosylated vitamin B., The vitamin B, status of the women was assessed by measurement of the concentration of pyridoxal phosphate in plasma as well as total urinary vitamin B, and 4-pyridoxic acid. The concentration of pyridoxal phosphate in plasma of the infants was also determined. Samples of milk were collected and analyzed for total and glycosylated vitamin B., The total dietary vitamin B6 intake of the women was 8.33 ? 4.04 pmol/day (mean ? SD), and the intake of glycosylated vitamin B, was 1.33 & 0.85 pmol/day. Thus, a mean of 16% of the vitamin B, in these typical mixed diets was glycosylated. All of the women had adequate vitamin B, status with respect to the indexes measured, and all of the infants exhibited plasma concenNUTRITION REVIEWSIVOL 48,NO GIJUNE 1990 251
trations of pyridoxal phosphate within typical ranges. (Definitive guidelines for the assessment of plasma PLP in infants have not been developed, however.) Regression analysis showed statistically significant correlations between total dietary vitamin B, and maternal plaSma pyridoxal phosphate, urinary 4-pyridoxic acid, and total urinary vitamin Be. When the glycosylated component was excluded from the dietary intake of vitamin Be in regression analysis, similar relationships were observed with these indicators of vitamin B, status. This finding suggests that the glycosylated component of these diets contributed little to the vitamin B, nutriture of the women. Whether this experimental design provides the statistical power and sensitivity to detect partial utilization of glycosylated vitamin B, is unclear. The high degree of variability among the subjects may have precluded detection of the possible (partial) utilization of glycosylated vitamin B6 in these diets. Analysis of breast-milk samples revealed that glycosylated vitamin B, represented a mean of only 2.5% of the total vitamin. Significant correlations were found between total vitamin B6 in breast milk and the plasma pyridoxal phosphate level of the infants. The lactation process in healthy, well-nourished women appears to discriminate between free and glycosylated species with respect to the secretion of various forms of vitamin B, into milk. Similar findings with respect to secretion of glycosylated vitamin B, in milk have been observed in studies of PNG metabolism in lactating rats.17 Previous HPLC analysis of samples of human milk from well-nourished women also indicated the presence of little or no glycosylated vitamin B,.’ In contrast, Reynolds et a1.18 reported that milk samples from Nepalese vegetarian women contained 15.3% of the total vitamin B, in glycosylated form, which corresponded to the percentage of glycosylated vitamin B6 in the diet. The reasons for these differences in the vitamin B, composition of milk from American and Nepalese women are unclear. Overall, these studies indicate that 252 NUTRITION REVIEWSIVOL 48,NO GIJUNE 1990
the relatively low proportion of glycosylated vitamin B, in milk presents little or no problem with respect to the bioavailability of vitamin B6 for the nursing infant. Of additional interest was the observation that all of the infants in the study of Andon et a1.16 had apparently normal nutritional and developmental status despite the fact that breast milk provided only 29% of the recommended dietary allowance (RDA)lg of vitamin B, when calculated on the basis of the measured concentration of vitamin B, and an assumed secretion of 715 mUday at two months postpartum. These findings suggest that the RDA for vitamin B6 in infants may be high. The question of the nutritional significance of glycosylated vitamin B, and its potentially Iimited bioavailabi Iity remains unclear, particularly with respect to vegetarians. Schultz and Leklem20 evaluated free-living vegetarians and nonvegetarians who consumed quantities of total vitamin B, calculated to be equivalent.20 The proportion of dietary glycosylated vitamin B, in each group was not determined, although it would presumably be higher in the vegetarian diet. No significant differences between groups were found with respect to plasma pyridoxal phosphate, urinary 4-pyridoxic acid, and total urinary vitamin B., These results suggest that the bioavailability of vitamin B, in vegetarian diets was similar to that in mixed diets. This similarity is consistent with the reported effective utilization of purified PNG in controlled studies with human subject^.^^^^^ Further studies should determine the factors responsible for the contradictory findings concerning the bioavailability of glycosylated vitamin B., In addition, further information is needed concerning the concentration of glycosylated forms of vitamin B, in common foods. 1. Yasumoto K, Tsuji H, lwarni K, Mitsuda H. Isolation from rice bran of a bound form of vitamin B, and its identification as 5’-O-(p-~-glucopyranosyl) pyridoxine. Agric Biol Chem 1977;41:106167 2. Tadera K, Nagano K, Yagi F, Kobayashi A, lrnada K, Tanaka S. Isolation and structure of a new rne-
tabolite of pyridoxine in seedlings of Pisum safivum L. Agric Biol Chem 1983;47:1357-9 Tadera K, Mori E, Yagi F, Kobayashi A, lmada K, lmabeppu M. Isolation and structure of a minor metabolite of pyridoxine in seedlings of Pisum safivum L. J Nutr Sci Vitaminol 1985;31:403-8 Tadera K, Kaneko T, Yagi F. Evidence for the occurrence and distribution of a new type of vitamin B, conjugate in plant foods. Agric Biol Chem 1986;50:2933-4 Tadera K, Kaneko T, Yagi F. Isolation and structural elucidation of three new pyridoxine-glycosides in rice bran. J Nutr Sci Vitaminol 1988; 341167-77 Kabir H, Leklem J, Miller L. Measurement of glycosylated vitamin B, in foods. J Food Sci 1983;43:1422-5 Gregory JF, Ink SL. Identification and quantification of pyridoxine-p-glucoside as a major form of vitamin B, in plant-derived foods. J Agric Food Chem 1987;35:76-82 Ink SL, Gregory JF 111, Sartain DB. Determination of pyridoxine P-glucoside bioavailability using intrinsic and extrinsic labeling in the rat. J Agric Food Chem 1986;34:857-62 Trumbo PR, Gregory JF 111. Metabolic utilization of pyridoxine-p-glucoside in rats: influence of vitamin B, status and route of administration. J Nutr 1988;118:1336-42 Trumbo PR, Gregory JF 111, Sartain DB. Incomplete utilization of pyridoxine-P-glucoside as vitamin B, in the rat. J Nutr 1988;118:170-5 Tsuji H,Okada J, lwami K, Yasumoto K, Mitsuda H. Availability of vitamin B, and small intestinal absorption of pyridoxine-P-o-glucoside in rats. Vitamins 1977;51:153-9 Kabir H, Leklem JE, Miller LT. Relationship of the glycosylated vitamin B, content of foods to vita-
min 8, bioavailability in humans. Nutr Rept Int 1983;28:709- 15 Bills ND, Leklem JE, Miller LT. Vitamin B, bioavailability in plant foods is inversely correlated with percentage glycosylated vitamin B., Fed Proc 1987;46:1487 Trumbo PR, Gregory JF 111, Sartain DB, Toth JP, Bailey LB, Cerda JJ. The bioavailability of pyridoxine-beta-glucoside in the rat and human [abstract]. FASEB J 1988;2:A1086. Gregory JF 111, Trumbo PR, Bailey LB, Toth JP, Cerda JJ, Baumgartner TG. Bioavailability of oral and intravenous deuterium-labeled pyridoxinebeta-glucoside in human subjects. FASEB J 1989;3:A454 Andon MB, Reynolds RD, Moser-Veillon PB, Howard MP. Dietary intake of total and glycosylated vitamin B, and the vitamin B, nutritional status of unsupplemented lactating women and their infants. Am J Clin Nutr 1989;50:1050-8
17. Trumbo PR, Gregory JF 111. The fate of dietary pyridoxine-p-glucoside in the lactating rat. J Nutr 1989;119:36-9 18. Reynolds RD, Acharya S,Leklem JE, Moser PB. Effects of low maternal dietary intake of calcium, selenium and vitamin B, upon breast milk composition in Nepal. In: Hamosh M, Goldman AS, eds. Human lactation. Vol. 2. Maternal and environmental factors. New York: Plenum Press, 19861205-13 19. Food and Nutrition Board, National Academy of Sciences-National Research Council. Recommended dietary allowances. 10th ed. Washington, DC: National Academy Press, 1989 20.
Schultz TD, Leklem JE. Vitamin B, status and bioavailability in vegetarian women. Am J Clin Nutr 1987;46:647-51
PLASMA DOPA IS UNAFFECTED BY DIET Significant amounts of bound 3,4-dihydroxyphenylalanine (DOPA), when present in dog food, did not affect plasma levels of DOPA or catecholamines in dogs. Thus, plasma DOPA is a useful index of activity of tyrosine hydroxylase, the rate-limiting enzyme in catecholamine synthesis in these animals.
It has recently been reported’ that the concentration in plasma of 3,4-dihydroxyphenylalanine (DOPA), the precursor of catecholamines, varies in response to the
activity of the sympathetic nervous system. Thus, plasma DOPA concentrations dropped after chemical sympathectomy, as well as after anesthesia. Plasma DOPA NUTRITION REVIEWSIVOL 48,NO GIJUNE 1990 253