Folate status of elderly women following moderate folate depletion responds only to a higher folate intake

Dietary Reference Intakes (DRI) for folate for elderly women have been based primarily on data extrapolated from studies in younger women. This study was conducted to provide the first age-specific data in elderly women (60-85 y) from a controlled metabolic study on which to base folate intake recommendations. Subjects (n = 33) consumed a moderately folate-deplete (118 microg/d) diet for 7 wk, followed by repletion diets providing either 200 or 415 microg folate/d as diet plus folic acid (FA) or a combination of FA and orange juice (OJ) for 7 wk (n = 30). Comparisons among and within groups were made for serum folate (SF), RBC folate and plasma total homocysteine (tHcy) concentrations. SF concentrations decreased significantly (P < 0.001) during depletion (65 +/- 15%). Postrepletion, the adjusted SF concentration for subjects consuming 415 microg folate/d was significantly greater (P = 0.003) than for subjects consuming 200 microg folate/d. RBC folate concentrations decreased (P < 0.001) during depletion (21 +/- 10%) and further (P < 0.001) during repletion (5 +/- 14%). During depletion, plasma tHcy concentrations increased significantly (P < 0.001) and an inverse relationship between SF and plasma tHcy concentrations was observed in 94% of subjects (P < 0.001). Reversal of this inverse relationship was significant only for subjects consuming 415 microg folate/d (P < 0.001). Postrepletion, subjects consuming 200 microg folate/d had a significantly higher (P = 0.009) adjusted plasma tHcy concentration than subjects consuming 415 microg folate/d. These data in elderly women indicate that 415 microg/d folate, provided as a combination of diet, FA and OJ, or diet and FA, normalizes folate status more effectively than does 200 microg/d, thus providing age-specific data for future folate intake recommendations.     

Folate catabolite excretion is responsive to changes in dietary folate intake in elderly women

BACKGROUND: The major route of folate turnover is by catabolic cleavage of the C9-N10 bond producing p-aminobenzoylglutamate (pABG) and its primary excretory form, p-acetamidobenzoylglutamate (ApABG). We hypothesize that total pABG (ApABG + pABG) excretion parallels both the mass of body folate pools from which these catabolites originate and the folate-status indicators. OBJECTIVE: The objective was to determine whether urinary folate catabolite excretion reflects body pool size and parallels the static and functional measures of folate status. DESIGN: Urinary folate catabolite excretion was measured in women (aged 60-85 y) consuming controlled amounts of folate for 14 wk. A low-folate diet (120 microg/d) was consumed (n = 33) for 7 wk, and then subjects were randomly assigned to consume either 200 (n = 14) or 400 (n = 16) microg folate/d. Urinary pABG and ApABG concentrations were measured by HPLC at 0, 7, and 14 wk. RESULTS: Urinary excretion of total pABG was significantly lower (P = 0.001) after depletion (73.9 +/- 4.7 nmol/d) than at baseline (115 +/- 12.7 nmol/d). This rate of decline (approximately 0.7% per day) is consistent with the kinetically measured rate of turnover of total body folate at moderate folate intakes. The average percentage increase in total pABG in response to folate repletion with 400 microg/d (75%) was significant (P = 0.02). Folate catabolite excretion was significantly (P = 0.0001) associated with serum and red blood cell folate, plasma homocysteine, and DNA hypomethylation after depletion and with serum folate (P = 0.001) and plasma homocysteine (P = 0.0002) after repletion with 400 microg folate/d. CONCLUSIONS: Total urinary pABG excretion reflects total body folate pool size and is a long-term indicator that parallels functional measures of folate status.     

Folate nutriture alters choline status of women and men fed low choline diets

Choline and folate share methylation pathways and, in studies of rats, were shown to be metabolically inter-related. To determine whether choline status is related to folate intake in humans, we measured the effect of controlled folate depletion and repletion on the plasma choline and phosphatidylcholine concentrations of 11 healthy men (33-46 y) and 10 healthy women (49-63 y) fed low-choline diets in two separate metabolic unit studies. Total folate intake was varied by supplementing low folate (25 and 56 microg/d for men and women, respectively) and low choline (238 and 147 mg/d for men and women, respectively) diets with pteroylglutamic acid for 2-6 wk following folate-depletion periods of 4-5 wk. The low folate/choline intakes resulted in subclinical folate deficiencies; mean plasma choline decreases of 28 and 25% in the men and women, respectively; and a plasma phosphatidylcholine decrease of 26% in the men (P < 0. 05). No functional choline deficiency occurred, as measured by serum transaminase and lipid concentrations. The decreases in choline status measures returned to baseline or higher upon moderate folate repletion and were more responsive to folate repletion than plasma folate and homocysteine. Feeding methionine supplements to the men did not prevent plasma choline depletion, indicating that folate is a more limiting nutrient for these methylation pathways. The results indicate that 1) choline is utilized as a methyl donor when folate intake is low, 2) the de novo synthesis of phosphatidylcholine is insufficient to maintain choline status when intakes of folate and choline are low, and 3) dietary choline is required by adults in an amount > 250 mg/d to maintain plasma choline and phosphatidylcholine when folate intake is low.     

Folate status response to controlled folate intake is affected by the methylenetetrahydrofolate reductase 677C-->T polymorphism in young women

This study was designed to evaluate the effect of the methylenetetrahydrofolate reductase (MTHFR) 677C-->T polymorphism on folate and homocysteine response in non-Hispanic women consuming a low folate diet followed by a diet providing the Recommended Dietary Allowance (RDA) for folate. Women (aged 20-30 y old) with either the TT (n = 19) or CC (n = 22) MTHFR 677C-->T genotype participated in a folate depletion-repletion study (7 wk, 115 microg dietary folate equivalents (DFE)/d; 7 wk, 400 microg DFE/d). Overall serum folate decreased (P < 0.0001) during depletion and increased (P < 0.0001) during repletion with lower (P = 0.03) postdepletion serum folate in women with the TT versus CC genotype. Folate status was low (serum folate < 13.6 nmol/L) in more women with the TT (59%) compared with the CC genotype (15%) postdepletion. Red blood cell folate for all subjects decreased during depletion (P < 0.0001) and repletion (P = 0.02) with lower (P = 0.04) red blood cell folate in women with the TT compared with the CC genotype postrepletion. Homocysteine increased (P < 0.0001) for both genotype groups postdepletion and decreased (P = 0.02) postrepletion for the CC genotype group only. Homocysteine concentrations tended to be higher (P = 0.09) in the TT versus CC genotype group postdepletion and postrepletion. These data suggest that the MTHFR 677C-->T polymorphism negatively affects the folate and homocysteine response in women consuming low folate diets followed by repletion with the RDA. These results may be important when evaluating the impact of the MTHFR 677C-->T polymorphism in countries in which low folate diets are chronically consumed.     

Ethnicity and race influence the folate status response to controlled folate intakes in young women

Population-based studies report differences in folate status indicators among Mexican American (MA), African American (AA) and Caucasian (CA) women. It is unclear, however, whether these differences are due to variations in dietary folate intake. The present study was designed to investigate the influence of ethnicity/race on folate status parameters in MA, AA, and CA women (18-45 y; n = 14 in each group) under conditions of strictly controlled folate intake. In addition, the adequacy of the 1998 folate U.S. recommended dietary allowance (RDA), 400 micro g/d as dietary folate equivalents (DFE), for non-Caucasian women was assessed. Subjects (n = 42) with the methylenetetrahydrofolate reductase 677 CC genotype consumed a low-folate diet (135 micro g DFE/d) for 7 wk followed by repletion with 400 (7 MA, 7 AA, 7 CA) or 800 micro g DFE/d (7 MA, 7 AA, 7 CA) for 7 wk. AA women had lower (P 相似文献   

Genomic methylation of peripheral blood leukocyte DNA: influences of arsenic and folate in Bangladeshi adults

BACKGROUND: Studies in cell culture and animal models indicate that arsenic exposure induces modifications in DNA methylation, including genome-wide DNA hypomethylation. It is not known whether arsenic exposure influences genomic DNA methylation in human populations chronically exposed to arsenic-contaminated drinking water. OBJECTIVE: The objective of this study was to determine whether arsenic is associated with genomic hypomethylation of peripheral blood leukocyte (PBL) DNA in Bangladeshi adults who are chronically exposed to arsenic. We also investigated whether arsenic-induced alterations in DNA methylation may be influenced by folate nutritional status. DESIGN: PBL DNA methylation and concentrations of plasma folate, plasma arsenic, and urinary arsenic were assessed in 294 adults in Araihazar, Bangladesh. Genomic PBL DNA methylation was measured by using a [(3)H]-methyl incorporation assay. RESULTS: Urinary arsenic, plasma arsenic, and plasma folate were positively associated with the methylation of PBL DNA (P = 0.009, 0.03, and 0.03, respectively). Stratification of participants by folate nutritional status [<9 nmol/L (n = 190) or >or=9 nmol/L (n = 104)] showed that the associations between arsenic exposure and methylation of PBL DNA were restricted to persons with folate concentrations >or= 9 nmol/L. CONCLUSIONS: Contrary to our a priori hypothesis, arsenic exposure was positively associated with genomic PBL DNA methylation in a dose-dependent manner. This effect is modified by folate, which suggests that arsenic-induced increases in DNA methylation cannot occur in the absence of adequate folate. The underlying mechanisms and physiologic implications of increased genomic DNA methylation are unclear, and they warrant further study.     

Methylenetetrahydrofolate reductase 677C->T polymorphism and folate status affect one-carbon incorporation into human DNA deoxynucleosides

The methylenetetrahydrofolate reductase (MTHFR) 677C-->T polymorphism is thought to influence the partitioning of 1-carbon units between methylation and other components of 1-carbon metabolism and to influence the risk and etiology of several major cancers and cardiovascular disease. Our objective was to determine the effect of the MTHFR 677C-->T polymorphism and folate status on the relative rate and extent of in vivo synthesis of DNA precursors. Adequately nourished, healthy women (9 CC, 9 TT) were infused with [3-(13)C]serine and [(13)C(5)]methionine for 9 h before and after 7 wk of consumption of a low-folate diet. Blood was drawn over 5 d for monocyte DNA isolation. Isotopic enrichment of the nucleosides in DNA digests was determined by LC-MS/MS. Maximum thymidine enrichment tended to be higher (P = 0.07) in TT than in CC subjects, suggestive of marginally higher mean thymidylate synthesis. However, the subset of TT subjects who exhibited formyltetrahydrofolate in erythrocytes (an indicator of 1-carbon partitioning) had greater (P = 0.036) thymidine enrichment than CC subjects, who had no erythrocyte formyltetrahydrofolate. Purine enrichment was not affected by genotype or folate depletion. However, the deoxyadenosine to deoxyguanosine enrichment ratio was significantly higher in TT subjects, suggesting a greater relative rate of adenine synthesis. The approximately 40% greater (P = 0.012) labeling of the methyl group of methyldeoxycytidine during folate depletion suggests a change in the origin of this 1-carbon unit. This is the first time that 1-carbon incorporation into human DNA has been measured in vivo after infusion of (13)C-labeled 1-carbon precursors. These findings support the feasibility of further assessment of factors affecting deoxynucleotide synthesis and DNA methylation in human 1-carbon metabolism.     

Red blood cell folate concentrations increase more after supplementation with [6S]-5-methyltetrahydrofolate than with folic acid in women of childbearing age

BACKGROUND: For the primary prevention of neural tube defects (NTDs), public health authorities recommend women of childbearing age to take 400 mug folic acid/d 4 wk before conception and during the first trimester. The biologically active derivate [6S]-5-methyltetrahydrofolate ([6S]-5-MTHF) could be an alternative to folic acid. OBJECTIVE: We investigated the effect of supplementation with [6S]-5-MTHF compared with that of folic acid on red blood cell folate concentration, an indicator of folate status. DESIGN: The study was designed as a double-blind, randomized, placebo-controlled intervention trial. Healthy women (n = 144) aged 19-33 y received 400 microg folic acid, the equimolar amount of [6S]-5-MTHF (416 microg), 208 microg [6S]-5-MTHF, or placebo as a daily supplement for 24 wk. Red blood cell and plasma folate concentrations were measured at baseline and at 4-wk intervals. RESULTS: The increase in red blood cell folate over time was significantly higher in the group receiving 416 microg [6S]-5-MTHF/d than in the groups receiving 400 microg folic acid/d or 208 microg [6S]-5-MTHF/d (P < 0.001). No plateau was reached in red blood cell folate concentration in the 3 treatment groups during 24 wk of intervention; however, plasma folate plateaued after 12 wk. CONCLUSIONS: We showed that administration of [6S]-5-MTHF is more effective than is folic acid supplementation at improving folate status. In addition, the study indicates that the recommended period for preconceptional folic acid supplementation should be extended to >4 wk for maximal prevention of NTDs based on folate concentrations. [6S]-5-MTHF might be an efficient and safe alternative to folic acid.     

Biochemical and molecular aberrations in the rat colon due to folate depletion are age-specific

Elder adulthood and diminished folate status are each associated with an enhanced risk of colorectal carcinogenesis. We therefore examined whether these two factors are mechanistically related. Weanling male Sprague-Dawley rats (n = 44) and 1-y-old rats (n = 44) were each divided into three groups and fed diets containing 0, 4.5 or 18 micro mol folic acid/kg (deplete, replete and supplemented groups, respectively). Rats were killed at 0, 8 and 20 wk. The folate concentrations, the distribution of the different coenzymatic forms of folate, uracil incorporation into DNA and genomic DNA methylation were measured in the colonic mucosa. Folate-deplete and folate-replete elder rats had 30-45% lower colonic folate concentrations than young rats. Furthermore, 5-methyltetrahydrofolate was uniformly depleted in colons of the elder, folate-deplete rats, whereas this depletion occurred in only a minority of the younger rats. By the end of the experiment, the folate-deplete and folate-replete elder rats had approximately 50% more uracil incorporated into their colonic DNA than the corresponding young groups (P < 0.05). In elder rats, this uracil misincorporation was incremental across the three diet groups (P-test for trend < 0.05), whereas no excess uracil incorporation was observed in young rats. Neither age nor dietary folate affected genomic DNA methylation in the colon. In conclusion, the colon of elder rats is more susceptible to biochemical and molecular consequences of folate depletion than that of young rats. However, folate supplementation is as effective at sustaining adequate colonic folate status in elder rats as it is in the young.     

Folate supplementation increases genomic DNA methylation in the liver of elder rats

The availability of folate is implicated as a determinant of DNA methylation, a functionally important feature of DNA. Nevertheless, when this phenomenon has been examined in the rodent model, the effect has not always been observed. Several reasons have been postulated for the inconsistency between studies: the rodent is less dependent on folate as a methyl source than man; juvenile animals, which most studies use, are more resistant to folate depletion than old animals; methods to measure genomic DNA methylation might not be sensitive enough to detect differences. We therefore examined the relationship between folate and genomic DNA methylation in an elder rat model with a newly developed method that can measure genomic DNA methylation sensitively and precisely. Thirty-nine 1-year-old rats were divided into three groups and fed a diet containing 0, 4.5 or 18 mumol folate/kg (folate-deplete, -replete and -supplemented groups, respectively). Rats were killed at 8 and 20 weeks. At both time points, mean liver folate concentrations increased incrementally between the folate-deplete, -replete and -supplemented rats (P for trend <0.001) and by 20 weeks hepatic DNA methylation also increased incrementally between the folate-deplete, -replete and -supplemented rats (P for trend=0.025). At both time points folate-supplemented rats had significantly increased levels of DNA methylation compared with folate-deplete rats (P<0.05). There was a strong correlation between hepatic folate concentration and genomic DNA methylation in the liver (r 0.48, P=0.004). In the liver of this animal model, dietary folate over a wide range of intakes modulates genomic DNA methylation.     

Chronic alcohol consumption induces genomic but not p53-specific DNA hypomethylation in rat colon.

Alcohol consumption has been implicated as an etiologic agent in colorectal carcinogenesis, but the mechanism by which alcohol enhances the development of colorectal cancer is not yet known. Recent reports indicate that alcohol consumption can diminish cellular S-adenosylmethionine levels, thus possibly altering normal patterns of DNA methylation, a phenomenon that is mediated by S-adenosylmethionine and whose abnormalities are observed in colonic neoplasia. This study investigated the effect of chronic alcohol consumption on genomic DNA methylation of rat colonic epithelium and methylation of the p53 tumor suppressor gene, abnormalities of which have been implicated in colonic carcinogenesis. Two groups of rats (n = 10/group) were pair-fed either an alcohol-containing or an isocaloric control Lieber-DeCarli diet for 4 wk. The extent of genomic DNA methylation was assessed by incubating the extracted DNA with [(3)H]S-adenosylmethionine and Sss1 methyltransferase. Gene-specific methylation was assessed by using semiquantitative polymerase chain reaction (PCR). Tritiated methyl uptake by colonic DNA (which is inversely correlated with genomic methylation) from alcohol-fed rats was 57% less than that in control DNA (P < 0.05). However, gene-specific DNA methylation, both in the p53 gene (exons 5-8) and in the beta-actin gene, a control gene, did not differ between the two groups. In conclusion, this study indicates that chronic alcohol consumption produces genomic DNA hypomethylation in the colonic mucosa. This may constitute a means by which carcinogenesis is enhanced, although further studies are required to establish causality.     

Homocysteine synthesis is elevated but total remethylation is unchanged by the methylenetetrahydrofolate reductase 677C->T polymorphism and by dietary folate restriction in young women

The effects of folate status and the methylenetetrahydrofolate reductase (MTHFR) 677C-->T polymorphism on the kinetics of homocysteine metabolism are unclear. We measured the effects of dietary folate restriction on the kinetics of homocysteine remethylation and synthesis in healthy women (20-30 y old) with the MTHFR 677 C/C or T/T genotypes (n = 9/genotype) using i.v. primed, constant infusions of [(13)C(5)]methionine, [3-(13)C]serine, and [(2)H(3)]leucine before and after 7 wk of dietary folate restriction (115 mug dietary folate equivalents/d). Dietary folate restriction significantly reduced folate status ( approximately 65% reduction in serum folate) in both genotypes. Total remethylation flux was not affected by dietary folate restriction, the MTHFR 677C-->T polymorphism, or their combination. However, the percentage of remethylation from serine was reduced approximately 15% (P = 0.031) by folate restriction in C/C subjects. Further, homocysteine synthesis rates of T/T subjects and folate-restricted C/C subjects were twice that of C/C subjects at baseline. In conclusion, elevated homocysteine synthesis is a cause of mild hyperhomocysteinemia in women with marginal folate status, particularly those with the MTHFR 677 T/T genotype.     

6S]-5-Methyltetrahydrofolate is at least as effective as folic acid in preventing a decline in blood folate concentrations during lactation

BACKGROUND: Studies in nonpregnant, nonlactating women suggest that folate supplementation in the form of 5-methyltetrahydrofolate ([6S]-5-methylTHF) is at least as effective as folic acid in increasing blood folate indexes. No data, however, are available on the effect of supplemental [6S]-5-methylTHF on blood folate concentrations during lactation. OBJECTIVE: We assessed the relative effectiveness of [6S]-5-methylTHF, a placebo, and folic acid in maintaining blood folate indexes during lactation in a sample of healthy Canadian women consuming folic acid-fortified foods. DESIGN: This study was designed as a 16-wk, randomized, placebo-controlled intervention. Pregnant women (n = 72) advised to consume a folic acid-containing prenatal supplement (1000 microg/d) during pregnancy were enrolled at 36 wk gestation. After delivery, the women were randomly assigned to receive [6S]-5-methylTHF (416 microg/d, 906 nmol/d) or a placebo or were assigned to a folic acid (400 microg/d, 906 nmol/d) reference group. RESULTS: At 16 wk of lactation, the mean red blood cell (RBC) folate concentration in women in the [6S]-5-methylTHF group (2178; 95% CI: 1854, 2559 nmol/L) was greater than that in the folic acid (1967; 1628, 2377 nmol/L; P < 0.05) and placebo (1390; 1198, 1613 nmol/L; P < 0.002) groups after adjustment for baseline concentrations (36 wk gestation). The distribution of folate forms in RBCs did not differ significantly between the [6S]-5-methylTHF and placebo groups. However, the folic acid group had greater amounts of 5-formylTHF (P < 0.03). CONCLUSION: [6S]-5-MethylTHF appeared to be as effective as, and perhaps more effective than, folic acid in preserving RBC folate concentrations during lactation.     

Older age and dietary folate are determinants of genomic and p16-specific DNA methylation in mouse colon

Older age and inadequate folate intake are strongly implicated as important risk factors for colon cancer and each is associated with altered DNA methylation. This study was designed to determine the effects of aging and dietary folate on select features of DNA methylation in the colon that are relevant to carcinogenesis. Old (18 mo; n = 34) and young (4 mo; n = 32) male C57BL/6 mice were randomly divided into 3 groups and fed diets containing 0, 4.5, or 18 mumol folate/kg (deplete, replete, and supplemented groups, respectively) for 20 wk. Genomic DNA methylation and p16 promoter methylation in the colonic mucosa were analyzed by liquid chromatography/electrospray ionization/MS and methylation-specific PCR, respectively. p16 gene expression was determined by real-time RT-PCR. Old mice had significantly lower genomic DNA methylation compared with young mice at each level of dietary folate (4.5 +/- 0.2, 4.8 +/- 0.1, and 4.9 +/- 0.1 vs. 6.0 +/- 0.1, 5.3 +/- 0.2, and 5.9 +/- 0.2%, in folate-deplete, -replete, and -supplemented groups, respectively, P < 0.05) and markedly higher p16 promoter methylation (61.0 +/- 2.7, 69.7 +/- 6.9, and 87.1 +/- 13.4 vs. 10.8 +/- 3.6, 8.4 +/- 1.8, and 4.9 +/- 1.7%, respectively, P < 0.05). In old mice, genomic and p16 promoter DNA methylation each increased in a manner that was directly related to dietary folate (P(trend) = 0.009). Age-related enhancement of p16 expression occurred in folate-replete (P = 0.001) and folate-supplemented groups (P = 0.041), but not in the folate-deplete group. In conclusion, aging decreases genomic DNA methylation and increases promoter methylation and expression of p16 in mouse colons. This effect is dependent on the level of dietary folate.     

Folate status and age affect the accumulation of L-isoaspartyl residues in rat liver proteins

Formation of atypical L-isoaspartyl residues in proteins and peptides is a common, spontaneous and nonenzymatic modification of aspartyl and asparaginyl sites. The enzyme protein-L-isoaspartyl methyltransferase (PIMT) catalyzes the transfer of the methyl group of S-adenosyl-L-methionine (SAM) to these L-isoaspartyl sites, thereby allowing reisomerization and restoration of the original alpha peptide linkage. Because SAM is in part a product of folate metabolism, the present study was undertaken to determine the effects of folate deficiency on the presence of L-isoaspartyl residues in hepatic proteins. Young (weanling) and older (12 mo) Sprague-Dawley rats were fed a folate-sufficient (2 mg folate/kg diet) or folate-deficient (0 mg folate/kg diet) diet for 20 wk. Liver proteins were analyzed for L-isoaspartyl residues. This analysis was based on the PIMT-dependent incorporation of [(3)H]-methyl groups from [(3)H]-SAM and the subsequent (nonenzymatic) sublimation of these methyl groups into a nonaqueous scintillant. The amount of L-isoaspartyl residues in hepatic proteins was higher in younger folate-deficient than in folate-sufficient rats (deficient: 187 +/- 71, sufficient: 64 +/- 43 pmol/mg protein, P < 0.025). This difference, however, was not seen among the older groups of rats who instead exhibited a much larger accumulation of L-isoaspartyl residues in their hepatic proteins (deficient: 528 +/- 151, sufficient: 470 +/- 204 pmol/mg protein, P = 0.568). The importance of these observations is discussed.     

Methylenetetrahydrofolate reductase 677C-->T variant modulates folate status response to controlled folate intakes in young women

A common genetic variant in the methylenetetrahydrofolate reductase (MTHFR) gene involving a cytosine to thymidine (C-->T) transition at nucleotide 677 is associated with reduced enzyme activity, altered folate status and potentially higher folate requirements. The objectives of this study were to investigate the effect of the MTHFR 677 T allele on folate status variables in Mexican women (n = 43; 18-45 y) and to assess the adequacy of the 1998 folate U.S. Recommended Dietary Allowance (RDA), 400 micro g/d as dietary folate equivalents (DFE). Subjects (14 CC, 12 CT, 17 TT genotypes) consumed a low folate diet (135 micro g/d DFE) for 7 wk followed by repletion with 400 micro g/d DFE (7 CC, 6 CT, 9 TT) or 800 micro g/d DFE (7 CC, 6 CT, 8 TT) for 7 wk. Throughout repletion with 400 micro g/d DFE, the TT genotype had lower (P 0.05) in their response relative to the CC genotype. Throughout repletion with 800 micro g/d DFE, the CT genotype had lower (P 0.05) in the measured variables between the TT and CC genotypes. Repletion with 400 micro g/d DFE led to normal blood folate and desirable plasma tHcy concentrations, regardless of MTHFR C677T genotype. Collectively, these data demonstrate that the MTHFR C-->T variant modulates folate status response to controlled folate intakes and support the adequacy of the 1998 folate U.S. RDA for all three MTHFR C677T genotypes.     

The absorption of pteroylpolyglutamate and intestinal pteroylpolygammaglutamyl hydrolase activity in zinc-deficient rats

The effect of dietary zinc deficiency on pteroylpolygammaglutamyl hydrolase (folate hydrolase) activity and on pteroylpolyglutamate absorption was studied in rats. Three groups of male Sprague-Dawley rats (zinc-deficient, restricted-fed and ad libitum-fed controls) were fed a semipurified 25% egg white protein diet. The zinc-deficient group received 0.7 mg zinc/kg diet, whereas restricted-fed and ad libitum-fed control groups received 106 mg zinc/kg diet. After 6 wk of feeding, intestinal mucosal folate hydrolase activity was determined, and the absorption of pteryl-U[14C]glutamylhexaglutamic acid [(14C]PteGlu7) and [3H]pteroylglutamic acid [(3H]PteGlu) was measured after intragastric administration. The intestinal mucosal folate hydrolase activity of zinc-deficient rats was not significantly reduced compared with two control groups. No significant differences in the absorption of [14C]PteGlu7 and [3H]PteGlu were found among the three groups. These results indicate that intestinal folate hydrolase is not zinc dependent in rats and the intestinal absorption of pteroylpolyglutamate is not reduced in zinc-deficient rats.     

Bioavailability of food folates is 80% of that of folic acid

BACKGROUND: The bioavailability of natural food folates is lower than that of synthetic folic acid, but no agreement exists as to the extent of the difference. OBJECTIVE: In a 4-wk dietary intervention study, we determined the aggregate bioavailability of food folates from fruit, vegetables, and liver relative to that of folic acid. DESIGN: Seventy-two healthy adults were randomly divided into 4 treatment groups. Group A (n = 29) received a high-folate diet with 369 mug food folate/d and a placebo capsule; groups B, C, and D (n = 14 or 15) received a low-folate diet with 73 microg food folate/d and folic acid capsules. These capsules contained 92 microg folic acid/d for group B, 191 microg for group C, and 289 microg for group D. In addition, all 72 subjects daily ingested a capsule with 58 microg [(13)C(11)]-labeled folic acid. We measured the percentage of [(13)C(11)]-labeled folate in plasma folate at the end of the intervention and ascertained the changes in serum folate concentrations over the 4 wk of the intervention. RESULTS: Bioavailability of food folate relative to that of folic acid was 78% (95% CI: 48%, 108%) according to [(13)C(11)]-labeled folate and 85% (52%, 118%) according to changes in serum folate concentrations. CONCLUSIONS: The aggregate bioavailability of folates from fruit, vegetables, and liver is approximately 80% of that of folic acid. The consumption of a diet rich in food folate can improve the folate status of a population more efficiently than is generally assumed.     

Folate, DNA methylation and colo-rectal cancer

Prospective cohort and case-control studies suggest an association between low folate intake and increased risk of colo-rectal adenoma and cancer. Some, but not all, animal studies indicate that folate supplementation protects against the development of colo-rectal neoplasms, although supraphysiological folate doses have been shown to enhance tumour growth. Folate is a methyl donor for nucleotide synthesis and biological methylation reactions, including DNA methylation. A low dietary folate intake may increase the risk of colo-rectal neoplasia by inducing genomic DNA hypomethylation, which can affect the expression of proto-oncogenes and tumour suppressor genes associated with the development of cancer. Common polymorphisms in genes involved in the methylation pathway, such as methylenetetrahydrofolate reductase and methionine synthase, have been shown to influence risk of colo-rectal neoplasia, with interactions dependent on folate status and/or alcohol intake, which is known to antagonise methyl group availability. There is some evidence to show that DNA from normal-appearing colo-rectal mucosa in individuals with colo-rectal cancer is hypomethylated. In a case-control study DNA methylation in normal-appearing colo-rectal mucosa has been shown to be lower in individuals with colo-rectal cancer (P = 0.08) and colo-rectal adenoma (P = 0.009) than in controls free of colo-rectal abnormalities. Human intervention trials to date suggest that supraphysiological doses of folate can reverse DNA hypomethylation in colo-rectal mucosa of individuals with colo-rectal neoplasia. In a double-blind randomised placebo-controlled study folate supplementation at physiological doses has been shown to increase DNA methylation in leucocytes (P = 0.05) and colonic mucosa (P = 0.09). Further studies are required to confirm these findings in larger populations and to define abnormal ranges of DNA methylation.     

Postweaning dietary folate deficiency provided through childhood to puberty permanently increases genomic DNA methylation in adult rat liver

Folate plays an important role in the pathogenesis of several chronic diseases by its potential ability to modulate DNA methylation. We hypothesized that the postweaning period might be a highly susceptible period to dietary folate intervention for DNA methylation patterning. We determined the effects of timing and duration of dietary folate intervention provided during the postweaning period on genomic DNA methylation in adult rat liver. In study 1, weanling rats were randomized to receive an amino acid-defined diet containing 0 (deficient), 2 (control), or 8 (supplemented) mg folic acid/kg until 8 wk of age, after which all the rats were fed the control diet until 30 wk of age. In study 2, weanling rats were fed the control diet until 8 wk of age and then randomized to receive the diet containing 0, 2, or 8 mg folic acid/kg until 30 wk of age. In study 3, weanling rats were randomized to receive these diets until 30 wk of age. Dietary folate deficiency, but not supplementation, provided during the postweaning period through childhood to puberty significantly increased genomic DNA methylation by 34-48% (P < 0.04) in rat liver that persisted into adulthood following a return to the control diet at puberty. In contrast, dietary folate deficiency or supplementation continually imposed at weaning or at puberty did not significantly affect genomic DNA methylation in adult rat liver. Our data suggest that early folate nutrition during postnatal development plays an important role in epigenetic programming that can have a permanent effect in adulthood.