Skip to main content
    • Aa
    • Aa

Folic acid metabolism in human subjects revisited: potential implications for proposed mandatory folic acid fortification in the UK

  • Anthony J. A. Wright (a1), Jack R. Dainty (a1) and Paul M. Finglas (a1)

Following an introduction of the importance of folates and the rationale for seeking to estimate fractional folate absorption from foods (especially for countries not having a mandatory folic acid fortification policy), scientific papers covering the mechanisms of folate absorption and initial biotransformation are discussed. There appears (post-1983) to be a consensus that physiological doses of folic acid undergo biotransformation in the absorptive cells of the upper small intestine to 5-methyltetrahydrofolic acid (as happens for all naturally-occurring reduced 1-carbon-substituted folates). This ‘validates’ short-term experimental protocols assessing ‘relative’ folate absorption in human subjects that use folic acid as the ‘reference’ dose. The underlying scientific premise on which this consensus is based is challenged on three grounds: (i) the apparent absence of a 5-methyltetrahydrofolic acid response in the human hepatic portal vein following absorption of folic acid, (ii) the low dihydrofolate reductase activity peculiar to man and (iii) the implications derived from recent stable-isotope studies of folate absorption. It is concluded that the historically accepted case for folic acid being a suitable ‘reference folate’ for studies of the ‘relative absorption’ of reduced folates in human subjects is invalid. It is hypothesised that the liver, and not the absorptive cells of the upper small intestine, is the initial site of folic acid metabolism in man and that this may have important implications for its use as a supplement or fortificant since human liver's low capacity for reduction may eventually give rise to saturation, resulting in significant (and potentially deleterious) unmetabolised folic acid entering the systemic circulation.

  • View HTML
    • Send article to Kindle

      To send this article to your Kindle, first ensure is added to your Approved Personal Document E-mail List under your Personal Document Settings on the Manage Your Content and Devices page of your Amazon account. Then enter the ‘name’ part of your Kindle email address below. Find out more about sending to your Kindle.

      Note you can select to send to either the or variations. ‘’ emails are free but can only be sent to your device when it is connected to wi-fi. ‘’ emails can be delivered even when you are not connected to wi-fi, but note that service fees apply.

      Find out more about the Kindle Personal Document Service.

      Folic acid metabolism in human subjects revisited: potential implications for proposed mandatory folic acid fortification in the UK
      Available formats
      Send article to Dropbox

      To send this article to your Dropbox account, please select one or more formats and confirm that you agree to abide by our usage policies. If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your Dropbox account. Find out more about sending content to Dropbox.

      Folic acid metabolism in human subjects revisited: potential implications for proposed mandatory folic acid fortification in the UK
      Available formats
      Send article to Google Drive

      To send this article to your Google Drive account, please select one or more formats and confirm that you agree to abide by our usage policies. If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your Google Drive account. Find out more about sending content to Google Drive.

      Folic acid metabolism in human subjects revisited: potential implications for proposed mandatory folic acid fortification in the UK
      Available formats
Corresponding author
*Corresponding author: Mr A. J. A. Wright, fax +44 1603 507723, email
Hide All
Perry J (1971) Folate analogues in normal mixed diets. Br J Nutr 21, 435441.
Scott JM & Weir DG (1976) Folate composition, synthesis and function in natural materials. Clin Haematol 5, 547568.
Suh J-R, Herbig AK & Stover PJ (2001) New perspectives on folate catabolism. Annu Rev Nutr 21, 255282.
Shane B (1995) Folate chemistry and metabolism. In Folate in Health and Disease, pp. 122 [Bailey LB, editor]. New York: Marcel Dekker.
Scott JM (1999) Folate and vitamin B12. Proc Nutr Soc 58, 441448.
Anonymous (1991) Prevention of neural tube defects: results of the Medical Research Council Vitamin Study. Lancet 338, 131137.
Czeizel AE & Dudas I (1992) Prevention of the first occurrence of neural tube defects by periconceptual vitamin supplementation. N Engl J Med 327, 3235.
Wald DS, Law M & Morris JK (2002) Homocysteine and cardiovascular disease: evidence on causality from a meta-analysis. Br Med J 325, 12021208.
Casas JP, Bautista LE, Smeeth L, Sharma P & Hingorani A (2005) Homocysteine and stroke: evidence on a causal link from mendelian randomisation. Lancet 365, 224232.
Seshadri S, Beiser A, Selhub J, Jacques PF, Rosenberg IH, D'Agostino RB, Wilson PW & Wolf PA (2002) Plasma homocysteine as a risk factor for dementia and Alzheimer's disease. N Engl J Med 346, 476483.
Choi S-W & Mason JB (2002) Folate status: effects on pathways of colorectal carcinogenesis. J Nutr 132, 2413S2418S.
Blount BC, Mack MM, Wehr CM, MacGregor JT, Hiatt RA, Wang G, Wickramasinghe SN, Everson RB & Ames BN (1997) Folate deficiency causes uracil misincorporation into human DNA and chromosome breakage: Implications for cancer and neuronal damage. Proc Natl Acad Sci USA 94, 32903295.
Sanderson P, McNulty H, Mastroiacovo P, McDowell IFW, Melse-Boonstra A, Finglas PM & Gregory JF lll (2003) Folate bioavailability: UK Food Standards Agency workshop report. Br J Nutr 90, 473479.
Chandler CJ, Wang TTY & Halsted CH (1986) Pteroylpolyglutamate hydrolase from human jejunal brush borders. J Biol Chem 261, 928933.
Strum WB (1979) Enzymatic reduction and methylation of folate following pH-dependant, carrier-mediated transport in rat jejunum. Biochim Biophys Acta 554, 249257.
Mason JB (1990) Intestinal transport of monoglutamyl folates in mammalian systems. In Contemporary issues in clinical nutrition, 13. Folic acid metabolism in health and disease, pp. 4763 [Picciano MF, Stokstad ELR and Gregory JF, editors]. New York: Wiley-Liss Inc.
Kelly P, McPartlin J, Goggins M, Weir DG & Scott JM (1997) Unmetabolized folic acid in serum: acute studies in subjects consuming fortified food and supplements. Am J Clin Nutr 65, 17901795.
Rogers LM, Pfeiffer CM, Bailey LB & Gregory JF (1997) A dual-label stable-isotopic protocol is suitable for determination of folate bioavailability: Evaluation of urinary excretion and plasma folate kinetics of intravenous and oral doses of [13C5] and [2H2]folic acid. J Nutr 127, 23212327.
Gregory JF (2001) Case Study: Folate Bioavailability. J Nutr 131, 1376S1382S.
Steinberg SE, Campbell CL & Hillman RS (1979) Kinetics of the normal folate enterohepatic cycle. J Clin Invest 64, 8388.
Steinberg SE (1984) Mechanisms of folate homeostasis. Am J Physiol 246, G319G324.
Krumdieck CL, Fukushima K, Fukushima T, Shiota T & Butterworth CE (1978) A long-term study of the excretion of folate and pterins in a human subject after ingestion of 14C folic acid, with observations on the effect of diphenylhydantoin administration. Am J Clin Nutr 31, 8893.
Clifford AJ, Arjomand A, Dueker SR, Schneider PD, Buchholz BA & Vogel JS (1998) The dynamics of folic acid metabolism in an adult given a small tracer dose of 14C-folic acid. Adv Exp Med Biol 445, 239251.
Bhandari SD & Gregory JF (1992) Folic acid, 5-methyl-tetrahydrofolate and 5-formyl-tetrahydrofolate exhibit equivalent intestinal absorption, metabolism and in vivo kinetics in rats. J Nutr 122, 18471854.
Whitehead VM, Pratt R, Viallet A & Cooper BA (1972) Intestinal conversion of folinic acid to 5-methyltetrahydrofolic acid in man. Br J Haematol 22, 6372.
Smith ME, Matty AJ & Blair JA (1970) The transport of pteroylglutamic acid across the small intestine of the rat. Biochim Biophys Acta 219, 3746.
Selhub J, Brin H & Grossowicz N (1973) Uptake and reduction of radioactive folate by everted sacs of rat small intestine. Eur J Biochem 33, 433438.
Olinger EJ, Bertino HJ & Binder HJ (1973) Intestinal folate conversion. II. Conversion and retention of pteroylmonoglutamate by jejunum. J Clin Invest 52, 21382145.
Tani M & Iwai Z (1983) High-performance liquid chromatographic separation of physiological folate monoglutamate compounds. Investigation of absorption and conversion of pteroylglutamic acid in the small intestine of the rat in situ. J Chromatogr 267, 175181.
Rosenberg IH (1976) Absorption and malabsorption of folates. Clin Haematol 5, 589618.
Whitehead VM & Cooper BA (1967) Absorption of unaltered folic acid from the gastro-intestinal tract in man. Br J Haematol 13, 679686.
Melikian V, Paton A, Leeming RJ & Portman-Graham H (1971) Site of reduction and methylation of folic acid in man. Lancet 2, 955957.
Gregory JF & Quinlivan EP (2002) In vivo kinetics of folate metabolism. Annu Rev Nutr 22, 199200.
Selhub J, Dhar GJ & Rosenburg IH (1983) Gastrointestinal absorption of folates and antifolates. Pharmacol Ther 20, 397418.
Gregory JF (1997) Bioavailability of folate. Eur J Clin Nutr 51, S54S59.
Perry J & Chanarin I (1970) Intestinal absorption of reduced folate compounds in man. Br J Haematol 18, 329339.
Perry J & Chanarin I (1972) Observations on folate absorption with particular reference to folate polyglutamate and possible inhibitors to its absorption. Gut 13, 544550.
Brown JP, Scott JM, Foster FG & Weir DG (1973) Ingestion and absorption of naturally occurring pteroylmonoglutamates (folates) in man. Gastroenterology 64, 223232.
Lucock MD, Wild J, Smithells RW & Hartley R (1989) In vivo characterization of the absorption and biotransformation of pteroylmonoglutamic acid in man: a model for future studies. Biochem Med Metab Biol 42, 3042.
Bower C, Stanley FJ, Croft M, De Clerk NH, Davis RE & Nicol DJ (1993) Absorption of pteroylpolyglutamates in mothers of infants with neural tube defects. Br J Nutr 69, 827834.
Markkanen T (1968) Absorption tests with natural folate material in controls and in gastrectomized patients. Am J Clin Nutr 21, 473481.
Bailey LB, Barton LE, Hillier SE & Cerda JJ (1988) Bioavailability of mono and polyglutamyl folate in human subjects. Nutr Rep Int 38, 509518.
Fenech M, Noakes N, Clifton P & Topping D (1999) Aleurone flour is a rich source of bioavailable folate in humans. J Nutr 129, 11141119.
Prinz-Langenohl R, Bronstrup A, Thorand B, Hages M & Pietrzik K (1999) Availability of food folate in humans. J Nutr 129, 913916.
Konings EJM, Troost FJ, Castenmiller JJM, Roomans HHS, van den Brandt PA & Saris WHM (2002) Intestinal absorption of different types of folate in healthy subjects with an ileostomy. Br J Nutr 88, 235242.
Rychlik M, Netzel M, Pfannebecker I, Frank T & Bitsch I (2003) Application of stable isotope dilution assays based on liquid chromatography-tandem mass spectrometry for the assessment of folate bioavailability. J Chromatogr B Analyt Technol Biomed Life Sci 792, 167176.
Pentieva K, McNulty H, Reichert R, et al. (2004) The short-term bioavailabilities of (6S)-5-methyltetrahydrofolate and folic acid are equivalent in men. J Nutr 134, 580585.
Willems FF, Boers GHJ, Blom HJ, Aengevaeren WRM & Verheugt FWA (2004) Pharmacokinetic study on the utilisation of 5-methyltetrahydrofolate and folic acid in patients with coronary artery disease. Br J Pharmacol 141, 825830.
Pietrzik K, Hages M & Remer T (1990) Methodological aspects in vitamin bioavailability testing. J Micronutr Anal 7, 207222.
Wright AJA, Finglas PM, Dainty JR, Hart DJ, Wolfe CA, Southon S & Gregory JF (2003) 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. Br J Nutr 90, 363371.
Bates CJ, Kerry SJ & Bluck LJC (2004) Stable isotope-labelled vitamin C as a probe for vitamin C absorption by human subjects. Br J Nutr 91, 699705.
Bluck LJC, Jones KS, Coward WA & Bates CJ (2005) The ‘anomolous’ absorption of labelled and unlabelled vitamin C in man. Br J Nutr 93, 627632.
Mathews CK & Huennekens FM (1963) Further studies on dihydrofolate reductase. J Biol Chem 238, 34363442.
Kok RM, Smith DEC, Dainty JR, van den Akker JT, Finglas PM, Smulders YM, Jacobs C & de Meer K (2004) 5-Methyltetrahydrofolic acid and folic acid measured in plasma with liquid chromatography tandem mass spectrometry: applications to folate absorption and metabolism. Anal Biochem 326, 129138.
Wright AJA, Finglas PM, Dainty JR, Wolfe CA, Hart DJ, Wright DM & Gregory JF (2005) Differential kinetic behaviour and distribution of pteroylglutamic acid and reduced folates: a revised hypothesis of the primary site of PteGlu metabolism in humans. J Nutr 135, 619623.
Zakrzewski SF & Nichol CA (1960) Evidence for a single enzyme reducing folate and dihydrofolate. J Biol Chem 235, 29842988.
Jarabak J & Bachur NR (1971) A soluble dihydrofolate reductase from human placenta: purification and properties. Arch Biochem Biophys 142, 417425.
Nylen PA, Abelson HA, Whitehead VM, Dolnick B, Petersen DW & Kamen BA (1984) Quantitation and lack of dihydrofolate reductase (DHFR) in human tissue in comparison to cultured human and animal cell lines in vitro and in vivo. Proc Amer Assoc Cancer Res 25, 309.
Kamen BA, Nylen PA, Whitehead VM, Abelson HT, Dolnick BJ & Pterson DW (1985) Lack of dihydrofolate reductase in human tumor and leukaemia cells in vivo. Cancer Drug Deliv 2, 133–138.
Whitehead VM, Kamen BA & Beaulieu D (1987) Levels of dihydrofolate reductase in livers of birds, animals, primates and man. Cancer Drug Deliv 4, 185189.
Ganeshaguru K & Hoffbrand AV (1978) The effect of deoxyuridine, vitamin B12, folate and alcohol on the uptake of thymidine and on the deoxynucleoside triphosphate concentrations in normal and megaloblastic cells. Br J Haematol 40, 29–41.
Bailey SW, Syslo MC & Ayling JE (2002) An assay for dihydrofolate reductase in human tissues by HPLC with fluorometric detection. FASEB J 16, A267 (abstr. # 217.4).
Bailey SW, Manilow MR, Hess DL, Duell PB, Upson BM, Graf EG, Ivin-Jones A, Syslo MC & Ayling JE (2003) Unreduced folic acid in plasma of subjects consuming either folic acid or 5-methyltetrahydrofolate. FASEB J 17, A311 (abstr. # 191.16).
Troen AM, Mitchell B, Sorensen B, et al. (2006) Unmetabolised folic acid in plasma is associated with reduced natural killer cell cytotoxicity among postmenopausal women. J Nutr 136, 189194.
Lavoie A & Cooper BA (1974) Rapid transfer of folic acid from blood to bile in man, and its conversion into folate coenzymes and into a pteroylglutamate with little biological activity. Clin Sci Mol Med 46, 729741.
Lin Y, Dueker SR, Follett JR, et al. (2004) Quantitation of in vivo human folate metabolism. Am J Clin Nutr 80, 680691.
Garratt LC, Ortori CA, Tucker GA, Sablitzky F, Bennett MJ & Barrett DA (2005) Comprehensive metabolic profiling of mono- and polyglutamated folates and their precursors in plant and animal tissue using liquid chromatography/negative ion electrospray ionisation tandem mass spectrometry. Rapid Commun Mass Spectrom 19, 23902398.
Smith DEC, Kok RM, Teerlink T, Jacobs C & Smulders YM (2006) Quantitative determination of erythrocyte folate vitamer distribution by liquid chromatography-tandem mass spectrometry. Clin Chem Lab Med 44, 450–459.
Lamers Y, Prinz-Langenohl R, Bramswig S & Pietrzik K (2006) Red blood cell folate concentrations increase more after supplementation with (6S)-5-methyltetrahydrofolic acid than with folic acid in women of childbearing age. Am J Clin Nutr 84, 156–161.
Pfeiffer CM, Fazili Z, McCoy L, Zhang M & Gunter EW (2004) Determination of folate vitamers in human serum by stable-isotope-dilution tandem mass spectrometry and comparison with radioassay and microbiologic assay. Clin. Chem. 50, 423–432.
Mills JL, Von Kohorn I, Conley MR, Zeller JA, Cox C, Williamson RE & Dufour DR (2003) Low vitamin B-12 concentrations in patients without anaemia: the effect of folic acid fortification of grain. Am J Clin Nutr 77, 14741477.
Schneider JA, Tangney CC & Morris MC (2006) Folic acid and cognition in older persons. Expert Opin Drug Saf 5, 511522.
Morris MC, Evans DA, Bienias JL, Tangney CC, Hebert LE, Scherr PA & Schneider JA (2005) Dietary folate and vitamin B12 intake and cognitive decline among community-dwelling older persons. Arch Neurol 62, 641–645.
Durga J, van Boxtel MPJ, Schouten EG, Kok FJ, Jolles J, Katan MB & Verhoef P (2007) Effect of 3-year folic acid supplementation on cognitive function in older adults in the FACIT trial: a randomised, double-blind, controlled trial. Lancet 369, 208–216.
Morris MS, Jacques PF, Rosenberg IH & Selhub J (2007) Folate and vitamin B-12 status in relation to anemia, macrocytosis, and cognitive impairment in older Americans in the age of folic acid fortification. Am J Clin Nutr 85, 193–200.
Smith AD (2007) Folic acid fortification: the good, the bad, and the puzzle of vitamin B-12. Am J Clin Nutr 85, 3–5.
Slansky JE, Li Y, Kaelin WG & Farnham PJ (1993) A protein synthesis-dependent increase in E2F1 mRNA correlates with growth regulation of the dihydrofolate reductase promoter. Molec Cell Biol 13, 16101618.
Obama K, Kanai M, Kawai Y, Fukushima M & Takabayashi A (2002) Role of retinoblastoma protein and E2F-1 transcription factor in the acquisition of 5-fluorouracil reistance by colon cancer cells. Int J Oncol 21, 309314.
Nijhout HF, Reed MC, Budu P & Ulrich CM (2004) A mathematical model of the folate cycle. J Biol Chem 279, 5500855016.
Cole BF, Baron JA, Sandler RS, et al. (2007) A randomised trial of folic acid for the prevention of colorectal adenomas. JAMA (In the Press).
Van Guelpen B, Hultdin J, Johansson I, Hallmans G, Stenling R, Riboli E, Winkvist A & Palmqvist R (2005) Low folate levels may protect against colorectal cancer. Gut 55, 14611466.
Stolzenberg-Solomon RZ, Chang SC, Leitzmann MF, Johnson KA, Johnson C, Buys SS, Hoover RN & Zeigler RG (2006) Folate intake, alcohol use, and postmenopausal breast cancer risk in the Prostate, Lung, Colorectal, and Ovarian Cancer Screening Trial. Am J Clin Nutr 83, 895–904.
Lange H, Suryapranata H, de Luca G, Borner C, Dille J, Kallmeyer K, Pasalary MN, Scherer E & Dambrink J-HE (2004) Folate theraphy and in-stent restenosis after coronary stenting. N Engl J Med 350, 26732681.
Khanna D, Park GS, Paulus HE, Simpson KM, Elashoff D, Cohen SB, Emery P, Dorrier C & Furst DE (2005) Reduction of the efficacy of methotrexate by the use of folic acid: post hoc analysis from two randomised controlled studies. Arthritis Rheum 52, 30303038.
Takacs P & Rodriguez L (2005) High folic acid levels and failure of single-dose methotrexate treatment in ectopic pregnancy. Int J Gynecol Obstet 89, 301–302.
Li Z, Gindler J, Wang H, Berry RJ, Li S, Correa A, Zheng J-C, Erickson JD & Wang Y (2003) Folic acid supplements during early pregnancy and likelihood of multiple births: a population-based cohort study. Lancet 361, 380–384.
Haggarty P, McCallum H, McBain H, Andrews K, Duthie S, McNeill G, Templton A, Haites N, Campbell D & Bhattacharya S (2006) Effect of B vitamins and genetics on success of in-vitro fertilisation: prospective cohort study. Lancet 367, 15131519.
Janeway CA, Travers P, Walport MJ & Capra JD (1999) Immunobiology: the immune system in health and disease, 4th ed. New York: Elsevier Science Ltd/Garland Publishing.
Recommend this journal

Email your librarian or administrator to recommend adding this journal to your organisation's collection.

British Journal of Nutrition
  • ISSN: 0007-1145
  • EISSN: 1475-2662
  • URL: /core/journals/british-journal-of-nutrition
Please enter your name
Please enter a valid email address
Who would you like to send this to? *



Altmetric attention score

Full text views

Total number of HTML views: 54
Total number of PDF views: 353 *
Loading metrics...

Abstract views

Total abstract views: 541 *
Loading metrics...

* Views captured on Cambridge Core between September 2016 - 23rd October 2017. This data will be updated every 24 hours.