![[Feedback]](/icons/banner/feedbackACT.gif){kind=icon}
![[For Subscribers]](/icons/banner/subscriberACT.gif){kind=icon}
![[Archive]](/icons/banner/archiveACT.gif){kind=icon}
![[Search]](/icons/banner/searchACT.gif){kind=icon}
![[Contents]](/icons/banner/tocACT.gif){kind=icon}
|
|
|
|
|
||
Department of Food Science, Swedish University of Agricultural Sciences, SE-750 07 Uppsala, Sweden;
* TNO-WU Center for Micronutrient Research, 3700 AJ Zeist, the Netherlands; and
TNO Nutrition and Food Research, 3700 AJ Zeist, the Netherlands
2To whom correspondence should be addressed. E-mail: Karin.Arkbage{at}lmv.slu.se.
 |
ABSTRACT |
|---|
 TOP ABSTRACT MATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
KEY WORDS: • folate bioaccessibility • yogurt • pasteurized milk • folate-binding protein • in vitro gastrointestinal model
Over the last decade, the literature on the health effects of folates has grown. It is focused mainly on neural tube defects in the developing fetus, spontaneous abortions, mental fitness and certain forms of cancer (1–5). This resulted in increased recommended dietary intakes of folates, which revealed a gap between actual intake and recommendation in Western populations. To bridge this gap, there is a need for more accurate information on bioavailability of dietary folates from both native and fortified foods. Dairy products, especially fermented milk such as yogurt, are moderate folate sources. Yet, there is a lack of knowledge on the bioavailability of folate and eventual matrix effects caused by different pH, starter cultures and the presence of folate-binding proteins (FBP).2 The dominant folate compound in dairy products is 5-methyltetrahydrofolate (5-CH3-H4folate) of which 
60% is in the monoglutamate form (6). In unprocessed bovine milk, most of the folate is bound to FBP (7).
The possible physiologic functions of the milk FBP remain controversial, especially whether it affects the absorption of folates. Some investigators observed an increase (8,9), some observed no difference (10) and some observed less overall absorption (11,12) of folates when bound to FBP.
Evidence for partial stability and effect of added FBP on folate bioaccessibility during transit through stomach and small intestine was recently obtained using TNO’s dynamic in vitro gastroIntestinal Model (TIM) (13). The term bioaccessibility describes the amount of folate released from the food matrix that is able to pass through the cell membranes with a molecular cut-off of 5 kDa, during transit through a simulated stomach and small intestine, thereby reflecting the availability for absorption in vitro. This study compares the folate bioaccessibility and FBP stability in milk fortified with folic acid or biologically active (6S)-5-CH3-H4folate in the absence or presence of additional FBP in equimolar amounts to folate. Without added FBP, the folate bioaccessibility from milk fortified with folic acid and 5-CH3-H4folate was 58 and 71%, respectively. The addition of FBP significantly decreased the bioaccessibility of folic acid but not of 5-CH3-H4folate. Approximately 15% of the FBP from folic acid-fortified products passed through the gastrointestinal tract intact. In the pasteurized milk fortified with 5-CH3-H4folate, however, only 0–1% was recovered after gastrointestinal passage.
The present study aimed to study the stability of FBP and its effect on the bioaccessibility of added folic acid and (6S)-5-CH3-H4folate in yogurt. Furthermore, we compared the folate bioaccessibility between two dairy matrices, yogurt and pasteurized milk. The lower pH of yogurt (pH 4.2) compared with milk (pH 6.8) (14) might affect the FBP binding activity and its stability during gastrointestinal transit. At a pH < 5, FBP loses its folate-binding capacity, allowing dissociation between FBP and folate in yogurt. Moreover, the starter culture might have proteolytic enzymes that hydrolyze FBP during the gastrointestinal transit. This could result in a different folate bioaccessibility from yogurt compared with milk.
|
MATERIALS AND METHODS |
|---|
|
TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
Four different yogurt products prepared from yogurt plain containing 3% fat (Arla Foods, Sweden), were used to study folate bioaccessibility (Table 1). The yogurt products were fortified to reach a folate and FBP concentration of 900 nmol/L to achieve a 1:1 mol/L ratio. Folic acid was obtained from Schircks Laboratories (Jona, Switzerland) and (6S)-5-CH3-H4folate (sodium salt) from Merck-Eprova (Schauffhausen, Switzerland). The folate compounds were tested for purity according to Van den Berg et al. (15), using molar extinction coefficients as described by Blakley (16). Stock solutions of (6S)-5-CH3-H4folate (0.73 g/L) and folic acid (1.00 g/L) in 0.1 mol/L phosphate buffer, pH 6.1, containing 10 g ascorbic acid/L were stored at -20°C. Folate fortification was performed on the day of the TIM experiment by the addition of 220 µL of the (6S)-5-CH3-H4folate stock solution or 160 µL of the folic acid stock solution to 400 g of yogurt. FBP was added as 16 g concentrated whey powder containing 75% protein and 15% fat (WPC 75; Arla Foods, Stockholm, Sweden) to 400 g of yogurt. The yogurt product was stirred and placed in the dark at 20°C for 1 h before the start of the TIM experiment. A portion of the yogurt (300 g) was applied to the gastric compartment of the TIM system. The remaining 100 g was stored in subsamples at -20°C for folate and FBP analyses.
|
TIM experiment.
The dynamic in vitro gastrointestinal model, TIM, was described in detail by Minekus et al. (17) and applied in folate studies by Verwei et al. (13). The gastric small-intestinal model represents the stomach, duodenum, jejunum and ileum. The pH curves, peristaltic movements, gastric emptying, intestinal transit and gradual additions of digestive juices are computer-controlled events, comparable to human conditions (17).
Each dairy product was tested in duplicate TIM experiments, each experiment lasting 5 h. At the beginning of each experiment, a test portion of 300 g of yogurt was put into the gastric compartment of TIM. During passage of the food through the TIM system, total jejunal and ileal dialysate were collected during 0–1, 1–2, 2–3 and 3–5 h. The dialysate fractions contained the absorbable (bioaccessible) fraction (membrane cut-off of 5 kDa). The ileal delivery was collected during 0–5 h representing the nonabsorbable (nonbioaccessible) fraction. At the end of each experiment, the residues in the compartments were collected. The same TIM protocol (including passage time, peristaltic movements, pH curves) was used for the yogurt products as that used in a previous study with milk products (13) to enable optimal comparison between the two matrices.
FBP analysis.
FBP concentrations in the food and in TIM samples were analyzed, in duplicate, using a two-site ELISA (18) according to the procedure of Wigertz et al. (19). The antibody against FBP from bovine milk (rabbit anti-bovine FBP 24739) was obtained from the State Serum Institute (Copenhagen, Denmark) and the FBP calibrant from the Central Hospital Hillerød (Hillerød, Denmark). Briefly, 0.09 g of Triton X-100 was added to 3 g of sample. The sample was put on a shaking device, incubated for 45 min at room temperature, diluted to 0.4 nmol FBP/L and applied to a microtiter plate. A calibration curve from 0.002 to 1.1 nmol FBP/L was prepared and included in each assay. A whey protein concentrate containing 65% protein (WPC 65; Arla Foods, Götene, Sweden) was used as an in-house reference material and included in every analysis. The CV between assays did not exceed 15%.
Folate analysis.
The samples from the TIM experiments, milk and yogurt products and the solutions of bile and pancreatic juice "secreted" into TIM (endogenous fraction) were extracted, in duplicate, as described by Strålsjö et al. (20). Briefly, 3 g of test food and TIM sample or 0.7 g of WPC 65 were mixed with 5–8 or 30 volumes, respectively, of extraction buffer (0.1 mol/L phosphate buffer pH 6.1, containing freshly added ascorbic acid 10 g/L and 2-mercaptoethanol 1 mL/L). Samples were heat extracted in a boiling water bath for 12 min, cooled down to 37°C and subjected to conjugase treatment for 3 h [1 mL chicken pancreas conjugase suspension, 10 g/L, (lyophilized chicken pancreas; Difco, Detroit, MI)]. A chicken pancreas blank was included in every analysis to correct for folate from the enzyme suspension. After centrifugation (27,000 x g for 15 min), the supernatants were collected and diluted to 25 or 50 mL. Aliquots of extracts were stored at -20°C until RPBA analysis. To prevent folate oxidation, samples were protected by nitrogen, subdued light and cooled on ice throughout sample preparation. WPC 65 was included in every analysis as an in-house material. The CV between assays did not exceed 10%.
A commercial RPBA kit, SimulTrac SNB Radioassay kit, Vitamin B-12 [57Co]/Folate [125I] (ICN Pharmaceuticals, Costa Mesa, CA) was used for folate quantification. The use of external calibrants and dilution of samples and calibrants were according to Strålsjö et al. (20). Folic acid or (6S)-5-CH3-H4folate was used for preparation of calibration curves in the concentration range of 0.5–10 µg/L (five-point curve, duplicates for each concentration).
To check for losses of folate during sample preparation and RPBA quantification, known amounts of both folate standards were added to samples before extraction in concentrations of 100% of the native folate content of the samples (n = 2). Recovery was calculated using the equation:
 |
where R (%) is recovery, cs is the folate content in spiked sample, cu is the folate content in the unspiked sample and ct is the theoretical content of the added spike (21). The recovery of folic acid and 5-CH3-H4folate ranged between 90 and 102% in jejunal dialysate, ileal dialysate and ileal delivery samples (n = 2).
Statistics and calculations.
FBP stability after gastrointestinal passage of yogurt is presented as the percentage of FBP found in the ileal delivery sample compared with the FBP content in the test food portion (intake). The bioaccessible and nonbioaccessible folate fractions are given as folate content in dialysate and ileal delivery, respectively, and expressed as the percentage of folate content in the intake.
The data were analyzed by ANOVA, using Minitab Statistical Software release 13.3 for Windows (Minitab, State College, PA), to evaluate the effect of FBP on folate bioaccessibility in yogurt, to compare folate bioaccessibility from yogurt with that from pasteurized milk and to compare folate bioaccessibility data from milk obtained by the two methods of analysis. Tukey’s test was performed for pairwise comparison of data. A P-value <0.05 was considered significant.
|
RESULTS |
|---|
|
TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
The amount of FBP was reduced during passage through the gastrointestinal tract. No FBP was found in the dialysate because the molecular size of intact FBP is too large to pass through the dialysate membrane. From folic acid-fortified yogurt, 34% of the initially added FBP was recovered in the ileal delivery fraction, whereas 17% FBP was recovered from the yogurt fortified with 5-CH3-H4folate (Table 2).
|
90% of the folic acid or 5-CH3-H4folate originally present in the test food (= intake) was recovered in the jejunal dialysate, the ileal dialysate and the ileal delivery together.
|
The samples from previous TIM experiments, in which pasteurized milk with folic acid or 5-CH3-H4folate was tested on folate by HPLC (13) were also analyzed by RPBA (Table 1, 2). Folate bioaccessibility from folate-fortified yogurt and folate-fortified pasteurized milk did not differ (P = 0.10). However, the addition of FBP to yogurt lowered folate bioaccessibility more compared with the addition of FBP to pasteurized milk (P < 0.05).
|
DISCUSSION |
|---|
|
TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
Our study design allows parallel quantification of folate and FBP during passage of fortified food through a simulated gastrointestinal tract. Therefore, we could demonstrate that in a folate-fortified dairy matrix FBP was only partly resistant to the digestive enzymes in stomach and small intestine. We found that this resistance of FBP during TIM passage depended on the folate form present in yogurt. FBP stability in yogurt fortified with folic acid (34%) was twice as high as the stability in yogurt fortified with 5-CH3-H4folate (17%). A similar pattern was observed in pasteurized milk fortified with FBP and folate (13). A relationship between the inhibitory effect of FBP on the bioaccessibility of folic acid and 5-CH3-H4folate, and FBP stability in folic acid and 5-CH3-H4folate fortified milk and yogurt appears to exist. Previous studies found that the ligand binding of FBP increases the folding stability of the protein (22,23). Moreover, because FBP has a lower affinity for 5-CH3-H4folate than for folic acid at pH 5 and 7.4 (23), this could explain the differences found in FBP effect and FBP stability between dairy products fortified with folic acid and 5-CH3-H4folate. Based on these differences, more stable folic acid-FBP complexes seem to exist compared with 5-CH3-H4folate-FBP complexes in both yogurt and milk products. In addition, more FBP-folate complexes appear to exist in yogurt compared with pasteurized milk. In addition to the occurrence of more stable FBP-folate complexes compared with free FBP molecules, another possible explanation could be the presence of the carbohydrate part in bovine FBP, which also is suggested to influence the stability of the molecules.
One molecule of FBP binds one molecule of folate at a pH of 7 (24). In addition, the molar ratio between FBP and folates naturally present in milk appeared to be 1:1. Therefore, we aimed for a 1:1 mol/L ratio when fortifying the yogurt and pasteurized milk with FBP and folate in the present study and in the previous study (13). However, a higher proportion of folate than FBP was found in the ileal delivery fraction. This suggests that other constituents in the yogurt could bind folates and prevent them from being absorbed. Jones et al. (25) reported in a rat model that the effect of bovine FBP on folate bioavailability was influenced by the presence of certain milk components. Soluble casein and whey proteins increase folate bioavailability, whereas acid-precipitated casein and a lipid-enriched whey preparation decrease it. Another explanation could be that in our study, the stability of FBP was analyzed by quantifying FBP by an ELISA method based on antibodies raised against FBP. Whether FBP still possesses complete or reduced folate-binding capacity after passage of TIM remains to be determined, although the enhancement of folate in ileal delivery after FBP fortification indirectly suggests that FBP could bind folate and prevent it from being absorbed (pass through the membranes in jejunum and ileum).
Our results seem to agree in part with an in vivo study performed on 6-d-old goat kids (26). That study showed that gastric acidity and gastrointestinal digestive enzymes only slightly affected the folate-binding capacity of FBP in goat’s milk. In addition, Tani et al. (10) found in rats that the folate binding activity of FBP recovered fully in jejunum after being reversibly inactivated under the gastric acidic conditions. However, contradictory results were obtained in an in vitro study (27) in which half of the folic acid-binding capacity was lost during pepsin treatment and all of the folic acid-binding capacity was lost after further digestion with trypsin.
Dietary folates are difficult to analyze due to low natural concentrations, the presence of nonstable native folate forms and the food matrix (28). By applying both HPLC and RPBA methods for analysis of all samples from the pasteurized milk products subjected to TIM, we could compare the two analytical methods. The methods of analysis did not differ in either the bioaccessible (P = 0.21) or the nonbioaccessible (P = 0.77) folate fraction. The bioaccessibility pattern from the different milk products was similar with both methods of analysis. On average, the RPBA method gave 16% higher values in the jejunal dialysate samples and 3% higher values in the ileal delivery samples, which could be due in part to the difference in quantifying the total folate content (RBPA method) rather than the individual folate forms (HPLC method). The residues denoted strong matrix effects resulting in a false high response with the RPBA method. Except for the residue samples, we found that the values deriving from HPLC were confirmed by RPBA (Fig. 2). This implies that the results from this study can be compared with those of the previous milk study and with data in the literature.
|
The TIM system provides an excellent opportunity for comparison of folate bioaccessibility from different (dairy) matrices. The model is strictly controlled for variables such as pH curves, enzyme activities, peristaltic movements and transit times, compared with the human system. Therefore, biases from pathologic or physiologic conditions, for example, are avoided. The TIM system mimicked the in vivo situation kinetically because the absorption of free folate was higher in jejunum than in ileum (12). Moreover, maximum folate absorption occurred 1–2 h after application of the test food to the TIM system, which is in accordance with data on folate absorption in humans (29). Folate bioaccessibility studies in TIM provide a good complement to more costly and time-consuming folate absorption studies in humans, but require more validation against human experiments. Several in vivo and in vitro studies indicated that bovine FBP contribute to the absorption and/or retention of folates from milk, especially during the neonatal period (8,9,12,27,30–33). It was reported that free folates are absorbed mainly in jejunum (7), whereas FBP-bound folates are absorbed more efficiently in ileum (11,31). However, because the TIM system has no intestinal receptor systems, it cannot answer the question whether FBP-bound folates can be absorbed. For this purpose, in vitro studies with cultured intestinal cells or intestinal segments could be performed to study FBP-bound or free folate transport across the intestinal wall.
In conclusion, both folic acid and (6S)-5-CH3-H4folate in fortified yogurt are highly bioaccessible (82%). The addition of FBP to yogurt (P < 0.05) decreased the folate bioaccessibility with a more pronounced effect in yogurt fortified with folic acid than in yogurt fortified with (6S)-5-CH3-H4folate. In addition, the inhibiting effect of FBP on folate bioaccessibility was higher (P < 0.05) in yogurt compared with milk. The stability of FBP during gastrointestinal transport of yogurt depended on the folate form used for fortification, and ranged between 17 to 34%; it appeared to be higher than the FBP stability in pasteurized milk (0–15%).
Further studies are warranted to elucidate the stability of FBP during gastrointestinal passage and its effect on folate bioavailability. Work is now in progress in our laboratory to validate the folate bioavailability from both fermented milk and pasteurized milk in adults in the presence of equimolar amounts of FBP and folate using a human ileostomy model.
|
ACKNOWLEDGMENTS |
|---|
|
FOOTNOTES |
|---|
3 Abbreviations used: 5-CH3-H4folate, 5-methyltetrahydrofolate; FBP, folate-binding protein; RPBA, radio protein-binding assay; TIM, TNO gastrointestinal model; WPC, whey protein concentrate.
Manuscript received 17 May 2003. Initial review completed 1 July 2003. Revision accepted 13 August 2003.
|
LITERATURE CITED |
|---|
|
TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
1. Berry, R. J., Li, Z., Erickson, D., Li, S., Moore, C. A., Wang, H., Mulinare, J., Zhao, P., Wong, L. C., Gindler, J., Hong, S. & Correa, A. (1999) Prevention of neural-tube defects with folic acid in China. N. Engl. J. Med. 341:1485-1490.
2. Honein, M. A., Paulozzi, L. J., Mathews, T. J., Erickson, D. J. & Wong, L.-Y.C. (2001) Impact of folic acid fortification of the US food supply on the occurrence of neural tube defects. J. Am. Med. Assoc. 285:2981-2986.
3. Richter, B., Stegmann, K., Röper, B., Böddeker, I., Ngo, E.T.K.M. & Koch, M. C. (2001) Interaction of folate and homocysteine pathway genotypes evaluated in susceptibility to neural tube defects (NTD) in a German population. J. Hum. Genet. 46:105-109.[Medline]
4. Giovannucci, E., Rimm, E. B., Ascherio, A., Stampfer, M. J., Colditz, G. A. & Willett, W. C. (1995) Alcohol, low-methionine-low-folate diets, and risk of colon cancer in men. J. Natl. Cancer Inst. 87:265-273.
5. Seshadri, S., Beiser, A., Selhub, J., Jacques, P. F., Rosenberg, I. H., D’Agostino, R. B., Wilson, P.W.F. & Wolf, P. A. (2002) Plasma homocysteine as a risk factor for dementia and Alzheimer’s disease. N. Engl. J. Med. 346:476-483.
6. Selhub, J. (1989) Determination of tissue folate composition by affinity chromatography followed by high pressure ion pair liquid chromatography. Anal. Biochem. 182:84-93.[Medline]
7. Gregory, J. F. (1995) The bioavailability of folate. Bailey, L. B. eds. Folate in Health and Disease 1995 Marcel Dekker New York, NY. .
8. Salter, D. N. & Blakeborough, P. (1988) Influence of goat’s-milk folate-binding protein on transport of 5-methyltetrahydrofolate in neonatal-goat small intestinal brush-border-membrane vesicles. Br. J. Nutr. 59:497-507.[Medline]
9. Colman, N., Hettiarachchy, N. & Herbert, V. (1981) Detection of a milk factor that facilitates folate uptake by intestinal cells. Science (Washington DC) 211:1427-1429.
10. Tani, M., Fushiki, T. & Iwai, K. (1983) Influence of folate-binding protein from bovine milk on the absorption of folate in gastrointestinal tract of rat. Biochim. Biophys. Acta 757:274-281.[Medline]
11. Izak, G., Galewski, K., Rachmilewitz, M. & Grossowicz, N. (1972) The absorption of milk-bound pteroylglutamic acid from small intestine segments. Proc. Soc. Exp. Biol. Med. 140:248-250.
12. Said, H. M., Horne, D. W. & Wagner, C. (1986) Effect of human-milk folate- binding protein on folate intestinal transport. Arch. Biochem. Biophys. 251:114-120.[Medline]
13. Verwei, M., Arkbåge, K., Havenaar, R., Van den Berg, H., Witthöft, C. & Schaafsma, G. (2003) Folic acid and 5-methyltetrahydrofolate in fortified milk are bioaccessible as determined in a dynamic in vitro gastrointestinal model. J. Nutr. 133:2377-2383.
14. Renner, E. (1989) Micronutrients in Milk and Milk-Based Food Products 1989 Elsevier Applied Science London, UK.
15. Van den Berg, H., Finglas, P. M. & Bates, C. (1994) FLAIR intercomparisons on serum and red cell folate. Int. J. Vitam. Nutr. Res. 64:288-293.[Medline]
16. Blakley, R. L. (1969) The biochemistry of folic acid and related pteridines. Frontiers of Biology 1969 North Holland Publishing Amsterdam, The Netherlands.
17. Minekus, M., Marteau, P., Havenaar, R. & Huisintveld, J.H.J. (1995) A multicompartmental dynamic computer-controlled model simulating the stomach and small-intestine. Altern. Lab. Anim. 23:197-209.
18. Høier-Madsen, M., Hansen, S. I. & Holm, J. (1986) Rabbit antibodies against the folate-binding protein from cows milk - production, characterization and use for development of an enzyme-linked-immunosorbent-assay (ELISA). Biosci. Rep. 6:895-900.[Medline]
19. Wigertz, K., Svensson, U. K. & Jägerstad, M. (1997) Folate and folate-binding protein content in dairy products. J. Dairy. Res. 64:239-252.[Medline]
20. Strålsjö, L., Arkbåge, K., Witthöft, C. M. & Jägerstad, M. (2002) Evaluation of a radioprotein-binding assay (RPBA) for folate analysis in berries and milk. Food Chem 79:525-534.
21. Rodriguez, L. C., Campana, A.M.G., Barrero, F. A., Linares, C. J. & Ceba, M. R. (1995) Validation of an analytical instrumental method by standard addition methodology. J. Am. Off. Anal. Chem. Int. 78:471-476.
22. Kaarsholm, N. C., Kolatrup, A., Danielsen, S. E., Holm, J. & Hansen, S. I. (1993) Ligand-induced conformation change in folate-binding protein. Biochem. J. 292:921-925.
23. Holm, J. & Hansen, S. I. (2001) Binding of radiolabeled folate and 5-methyltetrahydrofolate to cow’s milk folate binding protein at pH 7.4 and 5.0. Relationship to concentration and polymerization equilibrium of the purified protein. Biosci. Rep. 21:733-743.[Medline]
24. Salter, D. N., Scott, K. J., Slade, H. & Andrews, P. (1981) The preparation and properties of folate-binding protein from cow’s milk. Biochem. J. 193:469-476.[Medline]
25. Jones, M. L., Treloar, T. & Nixon, P. F. (2003) Dietary interactions influence the effects of bovine folate-binding protein on the bioavailability of tetrahydrofolates in rats. J. Nutr. 133:489-495.
26. Salter, D. N. & Mowlem, A. (1983) Neonatal role of milk folate-binding protein: studies on the course of digestion of goat’s milk folate binder in the 6-d-old kid. Br. J. Nutr. 50:589-596.[Medline]
27. Ford, J. E. (1974) Some observations on the possible nutritional significance of vitamin B12- and folate-binding proteins in milk. Br. J. Nutr. 31:243-257.[Medline]
28. Witthöft, C. M., Forssén, K., Johannesson, L. & Jägerstad, M. (1999) Folates—food sources, analysis, retention and bioavailability. Scand. J. Nutr. 43:138-146.
29. Prinz-Langenohl, R., Brönstrup, A., Thorand, B., Hages, M. & Pietrzik, K. (1999) Availability of food folate in humans. J. Nutr. 129:913-916.
30. Ford, J. E., Knaggs, G. S., Salter, D. N. & Scott, K. J. (1972) Folate nutrition in the kid. Br. J. Nutr. 27:571-583.[Medline]
31. Tani, M. & Iwai, K. (1984) Some nutritional effects of folate-binding protein in bovine milk on the bioavailability of folates to rats. J. Nutr. 114:778-785.
32. Mason, J. B. & Selhub, J. (1988) Folate-binding protein and the absorption of folic acid in the small intestine of the suckling rat. Am. J. Clin. Nutr. 48:620-625.
33. Swiatlo, N., Oconnor, D. L., Andrews, J. & Picciano, M. F. (1990) Relative folate bioavailability from diets containing human, bovine and goat milk. J. Nutr. 120:172-177.
This article has been cited by other articles:
|
|
 |
|
  G. Papastoyiannidis, A. Polychroniadou, A.-M. Michaelidou, and E. Alichanidis Fermented Milks Fortified with B-group Vitamins: Vitamin Stability and Effect on Resulting Products Food Science and Technology International, December 1, 2006; 12(6): 521 - 529. [Abstract] [PDF]
|
|
|
|
 |
|
 M. Verwei, A. P. Freidig, R. Havenaar, and J. P. Groten Predicted Serum Folate Concentrations Based on In Vitro Studies and Kinetic Modeling are Consistent with Measured Folate Concentrations in Humans J. Nutr., December 1, 2006; 136(12): 3074 - 3078. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. Verwei, K. Arkbage, H. Mocking, R. Havenaar, and J. Groten The Binding of Folic Acid and 5-Methyltetrahydrofolate to Folate-Binding Proteins during Gastric Passage Differs in a Dynamic In Vitro Gastrointestinal Model J. Nutr., January 1, 2004; 134(1): 31 - 37. [Abstract] [Full Text] [PDF]
|
|
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
