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      Folic acid handling by the human gut: implications for food fortification and supplementation 1 2 3

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          Abstract

          Background: Current thinking, which is based mainly on rodent studies, is that physiologic doses of folic acid (pterylmonoglutamic acid), such as dietary vitamin folates, are biotransformed in the intestinal mucosa and transferred to the portal vein as the natural circulating plasma folate, 5-methyltetrahydrofolic acid (5-MTHF) before entering the liver and the wider systemic blood supply.

          Objective: We tested the assumption that, in humans, folic acid is biotransformed (reduced and methylated) to 5-MTHF in the intestinal mucosa.

          Design: We conducted a crossover study in which we sampled portal and peripheral veins for labeled folate concentrations after oral ingestion with physiologic doses of stable-isotope–labeled folic acid or the reduced folate 5-formyltetrahydrofolic acid (5-FormylTHF) in 6 subjects with a transjugular intrahepatic porto systemic shunt (TIPSS) in situ. The TIPSS allowed blood samples to be taken from the portal vein.

          Results: Fifteen minutes after a dose of folic acid, 80 ± 12% of labeled folate in the hepatic portal vein was unmodified folic acid. In contrast, after a dose of labeled 5-FormylTHF, only 4 ± 18% of labeled folate in the portal vein was unmodified 5-FormylTHF, and the rest had been converted to 5-MTHF after 15 min (postdose).

          Conclusions: The human gut appears to have a very efficient capacity to convert reduced dietary folates to 5-MTHF but limited ability to reduce folic acid. Therefore, large amounts of unmodified folic acid in the portal vein are probably attributable to an extremely limited mucosal cell dihydrofolate reductase (DHFR) capacity that is necessary to produce tetrahydrofolic acid before sequential methylation to 5-MTHF. This process would suggest that humans are reliant on the liver for folic acid reduction even though it has a low and highly variable DHFR activity. Therefore, chronic liver exposure to folic acid in humans may induce saturation, which would possibly explain reports of systemic circulation of unmetabolized folic acid. This trial was registered at clinicaltrials.gov as NCT02135393.

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          Most cited references 22

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          Folate and vitamin B-12 status in relation to anemia, macrocytosis, and cognitive impairment in older Americans in the age of folic acid fortification.

          Historic reports on the treatment of pernicious anemia with folic acid suggest that high-level folic acid fortification delays the diagnosis of or exacerbates the effects of vitamin B-12 deficiency, which affects many seniors. This idea is controversial, however, because observational data are few and inconclusive. Furthermore, experimental investigation is unethical. We examined the relations between serum folate and vitamin B-12 status relative to anemia, macrocytosis, and cognitive impairment (ie, Digit Symbol-Coding score 210 nmol/L-the maximum of the reference range for serum vitamin B-12-replete participants with normal creatinine. After control for demographic characteristics, cancer, smoking, alcohol intake, serum ferritin, and serum creatinine, low versus normal vitamin B-12 status was associated with anemia [odds ratio (OR): 2.7; 95% CI: 1.7, 4.2], macrocytosis (OR: 1.8; 95% CI: 1.01, 3.3), and cognitive impairment (OR: 2.5; 95% CI: 1.6, 3.8). In the group with a low vitamin B-12 status, serum folate > 59 nmol/L (80th percentile), as opposed to < or = 59 nmol/L, was associated with anemia (OR: 3.1; 95% CI: 1.5, 6.6) and cognitive impairment (OR: 2.6; 95% CI: 1.1, 6.1). In the normal vitamin B-12 group, ORs relating high versus normal serum folate to these outcomes were < 1.0 (P(interaction) < 0.05), but significantly < 1.0 only for cognitive impairment (0.4; 95% CI: 0.2, 0.9). In seniors with low vitamin B-12 status, high serum folate was associated with anemia and cognitive impairment. When vitamin B-12 status was normal, however, high serum folate was associated with protection against cognitive impairment.
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            Homocysteine and stroke: evidence on a causal link from mendelian randomisation.

            Individuals homozygous for the T allele of the MTHFR C677T polymorphism have higher plasma homocysteine concentrations (the phenotype) than those with the CC genotype, which, if pathogenetic, should put them at increased risk of stroke. Since this polymorphism is distributed randomly during gamete formation, its association with stroke should not be biased or confounded. We investigated consistency between the expected odds ratio for stroke among TT homozygotes, extrapolated from genotype-phenotype and phenotype-disease studies, and the observed odds ratio from a meta-analysis of genotype-disease association studies. We searched MEDLINE and EMBASE up to June, 2003, for all relevant studies on the association between homocysteine concentration and the MTHFR polymorphism, and until December, 2003, for those on the association between the polymorphism and the risk of stroke. Pooled odds ratios and 95% CI were calculated by random-effects and fixed-effects models. Consistency between expected and observed odds ratios was assessed by interaction test. 111 studies met the selection criteria. Among 15635 people without cardiovascular disease, the weighted mean difference in homocysteine concentration between TT and CC homozygotes was 1.93 micromol/L (95% CI 1.38 to 2.47). The expected odds ratio for stroke corresponding to this difference based on previous observational studies was 1.20 (1.10 to 1.31). In our genetic meta-analysis (n=13928) the odds ratio for stroke was 1.26 (1.14 to 1.40) for TT versus CC homozygotes, similar to the expected odds ratio (p=0.29). Consistency between the odds ratios was preserved in analyses by age-group, ethnic background, and geographical location. The observed increase in risk of stroke among individuals homozygous for the MTHFR T allele is close to that predicted from the differences in homocysteine concentration conferred by this variant. This concordance is consistent with a causal relation between homocysteine concentration and stroke.
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              Folate-vitamin B-12 interaction in relation to cognitive impairment, anemia, and biochemical indicators of vitamin B-12 deficiency.

              Previous reports on pernicious anemia treatment suggested that high folic acid intake adversely influences the natural history of vitamin B-12 deficiency, which affects many elderly individuals. However, experimental investigation of this hypothesis is unethical, and the few existing observational data are inconclusive. With the use of data from the 1999-2002 National Health and Nutrition Examination Survey (NHANES), we evaluated the interaction between high serum folate and low vitamin B-12 status [ie, plasma vitamin B-12 210 nmol/L] with respect to anemia and cognitive impairment. With subjects having both plasma folate 59 nmol/L. Among subjects with low vitamin B-12 status, mean circulating vitamin B-12 was 228 pmol/L for the normal-folate subgroup and 354 pmol/L for the high-folate subgroup. We subsequently showed increases in circulating homocysteine and MMA concentrations with increasing serum folate among NHANES participants with serum vitamin B-12 or = 148 pmol/L. These interactions, which were not seen in NHANES III before fortification, imply that, in vitamin B-12 deficiency, high folate status is associated with impaired activity of the 2 vitamin B-12-dependent enzymes, methionine synthase and MMA-coenzyme A mutase.
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                Author and article information

                Journal
                Am J Clin Nutr
                Am. J. Clin. Nutr
                ajcn
                The American Journal of Clinical Nutrition
                American Society for Nutrition
                0002-9165
                1938-3207
                August 2014
                18 June 2014
                18 June 2014
                : 100
                : 2
                : 593-599
                Affiliations
                [1 ]From the Institute of Cellular Medicine, Newcastle University, Newcastle upon Tyne, United Kingdom (IP, MH, and DEJ); the Institute of Food Research, Norwich Research Park, Norwich, United Kingdom (MJK, MP, JRD, AJAW, and PMF); the Centre for Analytical Bioscience, School of Pharmacy, University of Nottingham, Nottingham, United Kingdom (DAB); and the Department of Radiology, Freeman Hospital, Newcastle upon Tyne, United Kingdom (JR and RJ).
                Author notes
                [2]

                Supported by the UK Biotechnology and Biological Sciences Research Council (grants BB/F014104/1 and BB/FO14457/1). This is an open access article distributed under the CC-BY license ( http://creativecommons.org/licenses/by/3.0/).

                [3 ]Address correspondence to DE Jones, Institute of Cellular Medicine, Fourth Floor, William Leech Building, Medical School, Framlington Place, Newcastle upon Tyne NE20 0SU, United Kingdom. E-mail: david.jones@ 123456ncl.ac.uk .
                Article
                080507
                10.3945/ajcn.113.080507
                4095662
                24944062

                This is an open access article distributed under the CC-BY license ( http://creativecommons.org/licenses/by/3.0/).

                Page count
                Pages: 7
                Categories
                Vitamins, Minerals, and Phytochemicals

                Nutrition & Dietetics

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