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Licensed Unlicensed Requires Authentication Published by De Gruyter June 1, 2013

Folic acid supplementation does not reduce intracellular homocysteine, and may disturb intracellular one-carbon metabolism

  • Desirée E.C. Smith , Jacqueline M. Hornstra , Robert M. Kok , Henk J. Blom and Yvo M. Smulders EMAIL logo


Background: In randomized trails, folic acid (FA) lowered plasma homocysteine, but failed to reduce cardiovascular risk. We hypothesize this is due to a discrepancy between plasma and intracellular effects of FA.

Methods: In a double-blind trial, 50 volunteers were randomized to received 500 µg FA daily for 8 weeks, or placebo. Plasma and peripheral blood mononuclear cell (PBMC) concentrations of homocysteine, S-adenosylmethionine (SAM), S-adenosylhomocysteine, methionine, cystathionine and 5-methyltetrahydrofolate (bioactive folate) were measured by liquid chromatography-tandem mass spectrometry (LC-MS/MS). PBMCs were used as a cellular model since they display the full spectrum of one-carbon (1C) enzymes and reactions.

Results: At baseline, plasma concentrations were a poor reflection of intracellular concentrations for most 1C metabolites, except 5-methyltetrahydrofolate (R=0.33, p=0.02), homocysteine (Hcy) (R=0.35, p=0.01), and cystathionine (R=0.45, p=0.001). FA significantly lowered plasma homocysteine (p=0.00), but failed to lower intracellular homocysteine or change the concentrations of any of the other PBMC 1C metabolites. At baseline, PBMC homocysteine concentrations correlated to PBMC SAM. After FA supplementation, PBMC homocysteine no longer correlated with PBMC SAM, suggesting a loss of SAM’s regulatory function. In vitro experiments in lymphoblasts confirmed that at higher folate substrate concentrations, physiological concentrations of SAM no longer effectively inhibit the key regulatory enzyme methylenetetrahydrofolate reductase (MTHFR).

Conclusions: FA supplementation does not reduce intracellular concentrations of Hcy or any of its closely related substances. Rather, FA may disturb physiological regulation of intracellular 1C metabolism by interfering with SAM’s inhibitory effect on MTHFR activity.

Corresponding author: Prof. Dr. Yvo M. Smulders, Department of Internal Medicine, VU University Medical Center, De Boelelaan 1117, 1007 MB Amsterdam, The Netherlands, Phone: +31 20 4444307, Fax: +31 20 4444313

The authors wish to acknowledge the partial funding by Unilever, Rotterdam, The Netherlands.

Conflict of interest statement

Authors’ conflict of interest disclosure: The authors stated that there are no conflicts of interest regarding the publication of this article.

Research funding: None declared.

Employment or leadership: None declared.

Honorarium: None declared.


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Received: 2012-10-12
Accepted: 2012-12-25
Published Online: 2013-06-01
Published in Print: 2013-08-01

©2013 by Walter de Gruyter Berlin Boston

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