Chemical and nutritional aspects of folate research: analytical procedures, methods of folate synthesis, stability, and bioavailability of dietary folates

Analytical recovery of folate degradation products formed in human serum and plasma at room temperature

Folate is not stable in serum and plasma. This may impair laboratory diagnostics and distort the outcome of epidemiological studies on folate and chronic diseases. The present study was designed to determine the kinetics of folate loss in human serum and plasma (collected into tubes containing EDTA, heparin, or citrate) at room temperature and the recovery of folate as 4-alpha-hydroxy-5-methyltetrahydrofolate (hmTHF) or p-aminobenzoylglutamate (pABG) equivalents. Different folate species and pABG were determined by liquid chromatography-tandem MS and microbiologically active folate was measured by a Lactobacillus rhamnosus assay. Concentrations of 5mTHF and microbiologically active folate had a parallel and rapid decrease in EDTA plasma to approximately 60% of the initial concentration after 24 h. In serum, heparin plasma, and citrate plasma, folate decreased more slowly to approximately 50% after 192 h. The loss of 5mTHF that occurred within 48 h was totally recovered as hmTHF. Folate measured as pABG equivalents decreased slowly to approximately 80% in 192 h and the decline was essentially matrix independent. In conclusion, the degradation of 5mTHF and microbiologically active folate in serum and plasma at room temperature can largely be corrected for by determining hmTHF or measuring folate as pABG equivalents. Moreover, results obtained using conventional folate assays may be biased by improper sample handling or if samples contained high concentrations of hmTHF.

Liquid chromatography-tandem mass spectrometry analysis of folate and folate catabolites in human serum

BACKGROUND

Folate status is associated with several chronic diseases; thus accurate assessment of folate status has become important in the clinical setting and in epidemiological studies. The diversity of folate forms complicates the task of assaying endogenous folate. We developed and validated an assay that measures various forms of folate in addition to folate catabolites in human serum.

METHODS

We added ascorbic acid to serum samples from 168 healthy blood donors and 39 patients with renal failure, and precipitated the proteins with acetonitrile containing (13)C-labeled folate forms as internal standards. The supernatant was evaporated and the analytes redissolved in water. We then used liquid chromatography-tandem mass spectrometry to quantify 5-methyltetrahydrofolate (5mTHF), 4-alpha-hydroxy-5-methyltetrahydrofolate (hmTHF), folic acid (FA), 5-formyltrahydrofolate (5fTHF), p-aminobenzoylglutamate (pABG), and p-acetamidobenzoylglutamate (apABG).

RESULTS

Detection limits were 0.07-0.52 nmol/L, and the assay was linear to 140 nmol/L for all analytes. The mean serum folate concentration from 168 blood donors was 22.7 nmol/L, of which 85.8% was 5mTHF, 12.1% hmTHF, 2.1% FA, and 0.0% 5fTHF. In the same individuals, the mean concentrations of pABG and apABG were 0.07 nmol/L and 0.47 nmol/L, respectively. The concentrations of folate catabolites were 22-30 times higher in 39 patients with renal failure. This folate assay correlated well with the microbiologic assay (r(2) = 0.92) and with measurement of serum folate as pABG equivalents (r(2) = 0.93).

CONCLUSIONS

This method based on liquid chromatography-tandem mass spectrometry measures the most abundant folate species and 2 folate catabolites in human serum.

Folate in wheat genotypes in the HEALTHGRAIN Diversity Screen

As part of the diversity screen of the HEALTHGRAIN project, the total folate contents of bread wheat (130 winter and 20 spring wheat genotypes), durum wheat (10 genotypes), earlier cultivated diploid einkorn and tetraploid emmer wheat (5 genotypes of each), and spelt (5 genotypes), grown in the same location in a controlled manner, were determined by a microbiological assay. The total folate contents ranged from 364 to 774 ng/g of dm in winter wheat and from 323 to 741 ng/g of dm in spring wheat, thus showing a marked variation. The highest mean for total folate content was measured in the durum wheat genotypes, whereas the earlier cultivated diploid and tetraploid wheat genotypes and spelt were shown to possess comparable or even higher folate contents than bread wheat. HPLC analysis of selected genotypes showed that 5-formyltetrahydrofolate was the major vitamer. The data provide a basis for breeding wheat genotypes with improved folate content.

Folic acid in ruminant nutrition: a review

Folic acid plays an essential role in DNA and methionine metabolism. Micro-organisms in the rumen can synthesise folates, but it has not been verified that these amounts are sufficient to achieve the best efficiency of dairy cows. However, the amount of folates synthesised in the rumen could possibly, to some extent, be affected by the forage:concentrate ratio. Degradation of orally supplemented folic acid in the rumen seems to be very high (about 97 %), as supplementation of folic acid hardly increases folate concentrations in the digesta at the duodenum. However, it must be considered that dietary supplements of folic acid higher than 0·5 mg/kg body weight increased serum folate concentrations in all available studies and milk folate concentrations in most studies. Additionally, milk production tended to be increased in some studies. Therefore, degradation of folic acid in the rumen may be overestimated as folates can be absorbed at the proximal duodenum. For future research it is necessary to consider the whole flow and the metabolic pathways of folates from the rumen to duodenum, blood, tissue, milk and transfer to calf to declare requirement values for cows. Consequently, the present review discusses current knowledge and emphasises areas for future research.

An assay for uracil in human DNA at baseline: effect of marginal vitamin B6 deficiency

Improvements are made to our gas-chromatography-mass-spectrometry-based assay for quantifying low levels of DNA-uracil. Folate deficiency leads to increased deoxyuridine monophosphate/thymidylate (dUMP/dTMP) ratios and uracil misincorporation into DNA, which may increase cancer risk. Vitamin B6 (B6) deficiency might also result in increased DNA-uracil because B6 is a cofactor for serine hydroxymethyltransferase, which catalyzes the methylation of tetrahydrofolate (THF) to methylene-THF, the folate form that is required to convert dUMP to dTMP. However, the low baseline levels of DNA-uracil in healthy human lymphocytes are difficult to measure accurately. This version of the assay (Uracil assay V3) has an approximately 10-fold increase in signal strength over the previous method and a 10-fold lower detection limit (0.2 pg uracil). Five micrograms of DNA, the amount in about 1 ml of human blood, is a suitable amount for this assay. Using this improved assay, DNA-uracil was measured in lymphocytes from 12 healthy smoking or nonsmoking young men and women who consumed a B6-restricted diet (0.7 mg B6/day, or approximately half the recommended dietary allowance) for 28 days. DNA-uracil concentration was not significantly related to B6 status or smoking. More severe and/or prolonged B6 deficiency may be necessary to detect significant changes in DNA-uracil in humans. The average concentration of DNA-uracil in these subjects was found to be approximately 3,000 uracils per diploid lymphocyte, which is comparable to steady state levels of one of the oxidative adducts of DNA, 8-oxoguanine.

Folate salvage in plants: pterin aldehyde reduction is mediated by multiple non-specific aldehyde reductases

Folates undergo oxidative cleavage in vivo, releasing a pterin aldehyde fragment that can be re-used in folate synthesis if the aldehyde group is reduced. High levels of NADPH-dependent reductase activity against pterin-6-aldehyde and its dihydro form were detected in Arabidopsis, pea and other plants; modeling predicted that the activity would maintain in vivo pterin aldehyde pools at extremely low levels (<0.2 pmol g(-1) FW). Subcellular fractionation showed that the pea leaf activity is mainly cytosolic, and anion exchange chromatography revealed multiple isoforms, all of which catalyzed reduction of other aldehydes. Arabidopsis seed activity likewise comprised various isoforms. An Arabidopsis gene (At1g10310) encoding a pterin aldehyde reductase was identified by searching the short-chain dehydrogenase/reductase family for proteins predicted to be NADPH-linked, and sharing conserved residues with reductases that mediate analogous reactions. The recombinant protein behaved as a dimer in size exclusion chromatography. In addition to pterin aldehydes, it catalyzed the reduction of diverse aromatic and aliphatic aldehydes: Vmax values varied <5-fold, but Km values ranged from 3.6 microm to 1.7 mm, those for pterin-6-aldehyde and dihydropterin-6-aldehyde being 36 and 56 microm, respectively. Activity with dihydropterin-6-aldehyde was unusually high at 0 degrees C. The At1g10310 transcript was most abundant in seeds, but, as expected for multiple isoforms, inactivating the At1g10310 gene caused only a minor change in seed pterin aldehyde reductase activity. We conclude that pterin aldehyde salvage in plants involves multiple, generalist NADPH-linked reductases, and that the At1g10310 enzyme is typical of these and hence suitable for use in engineering studies of folate turnover.

Evidence for folate-salvage reactions in plants

Folates in vivo undergo oxidative cleavage, giving pterin and p-aminobenzoylglutamate (pABAGlu) moieties. These breakdown products are excreted in animals, but their fate is unclear in microorganisms and unknown in plants. As indirect evidence from this and previous studies strongly suggests that plants can have high folate-breakdown rates (approximately 10% per day), salvage of the cleavage products seems likely. Four sets of observations support this possibility. First, cleavage products do not normally accumulate: pools of pABAGlu (including its polyglutamyl forms) are equivalent to, at most, 4-14% of typical total folate pools in Arabidopsis thaliana, Lycopersicon esculentum and Pisum sativum tissues. Pools of the pterin oxidation end-product pterin-6-carboxylate are, likewise, fairly small (3-37%) relative to total folate pools. Second, little pABAGlu built up in A. thaliana plantlets when net folate breakdown was induced by blocking folate synthesis with sulfanilamide. Third, A. thaliana and L. esculentum tissues readily converted supplied breakdown products to folate synthesis precursors: pABAGlu was hydrolysed to p-aminobenzoate and glutamate, and dihydropterin-6-aldehyde was reduced to 6-hydroxymethyldihydropterin. Fourth, both these reactions were detected in vitro; the reduction used NADPH as cofactor. An alternative salvage route for pABAGlu, direct reincorporation into dihydrofolate via the action of dihydropteroate synthase, appears implausible from the properties of this enzyme. We conclude that plants are excellent organisms in which to explore the biochemistry of folate salvage.

Effects of folic acid on epithelial apoptosis and expression of Bcl-2 and p53 in premalignant gastric lesions

AIM: To evaluate the effects of folic acid on epithelial apoptosis and expression of Bcl-2 and p53 in the tissues of premalignant gastric lesions.

METHODS: Thirty-eight patients, with premalignant gastric lesions including 18 colonic-type intestinal metaplasia (IM) and 20 mild or moderate dysplasia, were randomly divided into a treatment group (n = 19) receiving folic acid 10 mg thrice daily and a control group (n = 19) receiving sucralfate 1000 mg thrice daily for 3 mo. All patients underwent endoscopies and four biopsies were taken prior to treatment and repeated after concluding therapy. Folate concentrations in gastric mucosa were measured with chemiluminescent enzyme immunoassay. Epithelial apoptosis and the expression of Bcl-2 and p53 protein in gastric mucosa were detected with flow cytometric assay.

RESULTS: The mean of folate concentration in gastric mucosa was 9.03±3.37 μg/g wet wt in the folic acid treatment group, which was significantly higher than 6.83±3.02 μg/g wet wt in the control group. Both the epithelial apoptosis rate and the tumor suppressor p53 expression in gastric mucosa significantly increased after folic acid treatment. In contrast, the expression of Bcl-2 oncogene protein decreased after folic acid therapy.

CONCLUSION: These data indicate that folic acid may play an important role in the chemoprevention of gastric carcinogenesis by enhancing gastric epithelial apoptosis in the patients with premalignant lesions.

Determination of folate vitamers in human serum by stable-isotope-dilution tandem mass spectrometry and comparison with radioassay and microbiologic assay

BACKGROUND

Current clinical methods for folate give different results and cannot measure the various forms of folate. We developed an isotope-dilution tandem mass spectrometric method coupled to liquid chromatography (LC/MS/MS) as a candidate reference method for 5-methyltetrahydrofolic acid (5MeTHF), 5-formyltetrahydrofolic acid (5FoTHF), and folic acid (FA) in human serum.

METHODS

We quantitatively isolated folates from 275 microL of serum with a phenyl solid-phase extraction cartridge, then detected and quantified them in stabilized serum extracts by positive-ion electrospray ionization LC/MS/MS. We used an isocratic mobile phase of acetic acid in organic solvent on a C(8) analytical column. (13)C-labeled folates were used as internal standards.

RESULTS

Limits of detection in serum were 0.13 (5MeTHF), 0.05 (5FoTHF), and 0.07 (FA) nmol/L. Within- and between-run imprecision (CV) was <7% for 5MeTHF and <10% for 5FoTHF at concentrations >0.5 nmol/L, and <10% for FA at concentrations >2.0 nmol/L. Total folate (TFOL) concentrations determined by competitive protein binding radioassay were approximately 9% lower than results obtained with LC/MS/MS. The microbiologic assay gave approximately 15% higher TFOL results with FA calibrator and no difference with 5MeTHF calibrator. The mean (SD) [range] TFOL in 42 sera was 35.5 (17.8) [6.5-75.6] nmol/L. Thirty-two samples with TFOL <50 nmol/L had, on average, 93.3% 5MeTHF, 2.3% FA, and 4.4% 5FoTHF. Ten samples with TFOL >50 nmol/L had, on average, 81.7% 5MeTHF, 15.7% FA, and 2.5% 5FoTHF.

CONCLUSIONS

This stable-isotope-dilution LC/MS/MS method can quantify 5MeTHF, 5FoTHF, and FA in serum. Currently used clinical assays agree with this candidate reference method.

Protocol for the production of concentrated extracts of food folate for use in human bioavailability studies

To provide a tool to study folate bioavailability under controlled conditions, a methodology was developed to produce extracts representative of natural food folates but removed from their matrix and sufficiently concentrated so as to elicit a response in biomarkers of folate status without distorting usual dietary intake patterns. Egg, spinach, and yeast were selected to represent the wide range in extent of folate conjugation found in foods (0, 60, and 100% polyglutamyl folate, respectively). The protocol, which was based on extracting food folates using only reagents safe for human consumption, was optimized in the laboratory (thermal extraction for 10 min in a 2% ascorbate solution at pH 5) and then adapted for industrial scale production in a food-processing facility. Results showed that the extracts were 2.3-12 times more concentrated in folate compared with their corresponding food sources. Neither the mono- to polyglutamate ratio nor the distribution of the main folate derivatives was altered during processing, making these extracts suitable for use in human bioavailability studies.

Increased production of folate by metabolic engineering of Lactococcus lactis

The dairy starter bacterium Lactococcus lactis is able to synthesize folate and accumulates large amounts of folate, predominantly in the polyglutamyl form. Only small amounts of the produced folate are released in the extracellular medium. Five genes involved in folate biosynthesis were identified in a folate gene cluster in L. lactis MG1363: folA, folB, folKE, folP, and folC. The gene folKE encodes the biprotein 2-amino-4-hydroxy-6-hydroxymethyldihydropteridine pyrophosphokinase and GTP cyclohydrolase I. The overexpression of folKE in L. lactis was found to increase the extracellular folate production almost 10-fold, while the total folate production increased almost 3-fold. The controlled combined overexpression of folKE and folC, encoding polyglutamyl folate synthetase, increased the retention of folate in the cell. The cloning and overexpression of folA, encoding dihydrofolate reductase, decreased the folate production twofold, suggesting a feedback inhibition of reduced folates on folate biosynthesis.

Determination of 5-methyltetrahydrofolate (13C-labeled and unlabeled) in human plasma and urine by combined liquid chromatography mass spectrometry

The association of folates with the prevention of neural tube defects and reduced risk of other chronic diseases has stimulated interest in the development of techniques for the study of their bioavailability in humans. Stable isotope protocols differentiate between oral and/or intravenous test doses of folate and natural levels of folate already present in the body. An liquid chromatography/mass spectrometry (LC/MS) procedure is described that has been validated for the determination of [13C]5-methyltetrahydropteroyl monoglutamic acid ([13C]5-CH3H4PteGlu) in plasma and urine, following oral dosing of volunteers with different labeled folates. Folate binding protein affinity columns were used for sample purification prior to LC/MS determination. Chromatographic separation was achieved using a Superspher 100RP18 (4 microm) column and mobile phase of 0.1 mol/L acetic acid (pH 3.3):acetonitrile (90:10; 250 microL/min). Selected ion monitoring was conducted on the [M-H](-) ion: m/z 458 and 459 for analyzing 5-CH3H4PteGlu; m/z 464 [M+6-H](-) to determine 5-CH3H4PteGlu derived from the label dose; m/z 444 for analysis of 2H4PteGlu internal standard, and m/z 446 and 478 to confirm that there was no direct absorption of unmetabolized compounds. Calibration was linear over the range 0-9 x 10(-9) mol/L; the limits of detection and quantification were 0.2 x 10(-9) and 0.55 x 10(-9) mol/L, respectively. The mean coefficient of variation of the ratios (m/z 463/458) was 7.4%. The method has potential applications for other key folates involved in one-carbon metabolism.

Prevention of pathologies associated with oxidative stress and dietary intake deficiencies: folate deficiency and requirements

Folate (folic acid, folacin) is an essential vitamin that is found in nature. Folates contain the core chemical structure of pteroylglutamic acid, but vary in their state of reduction, the single carbon moiety they bear and/or the length of the glutamate chain. At least 50% of whole body folate is stored in the liver. The influence of intracellular folate concentration depends largely on dietary intake. The supply of folate depends primarily on the quantity and bioavailability of ingested folate and the rate of loss by urinary and fecal routes and through catabolism.

Low serum and red blood cell folate are moderately, but nonsignificantly associated with increased risk of invasive cervical cancer in U.S. women

Previous observational epidemiologic studies of folate and cervical cancer, as well as folate supplementation trials for cervical dysplasia, have produced mixed results. We examined the relationship between serum and RBC folate and incident invasive cervical cancer in a large, multicenter, community-based case-control study. Detailed in-person interviews were conducted, and blood was drawn at least 6 mo after completion of cancer treatment from 51% of cases and 68% of controls who were interviewed. Blood folate was measured with both microbiologic and radiobinding assays. Included in the final analyses were 183 cases and 540 controls. Logistic regression was used to control for all accepted risk factors, including age, sexual behavior, smoking, oral contraceptive use, Papanicolaou smear history and human papillomavirus (HPV)-16 serology. For all four folate measures, the geometric mean in cases was lower than in controls (e.g., 11.6 vs. 13.0 nmol/L, P < 0.01 for the serum radiobinding assay). Folate measures using microbiologic and radiobinding assays were correlated (serum: r = 0.90; RBC: r = 0.77). For serum folate, multivariate-adjusted odds ratios (OR) in the lowest vs. highest quartile were 1.3 [95% confidence interval (CI) = 0.8--2.9] and 1.6 (0.9--2.9), using the microbiologic and radiobinding assays, respectively. For RBC folate, comparable OR were 1.2 (0.6--2.2) and 1.5 (0.8--2.7). Similar risks were obtained when restricting analyses to subjects with a history of HPV infection. Thus, low serum and RBC folate were each moderately, but nonsignificantly, associated with increased invasive cervical cancer risk. These findings support a role for one-carbon metabolism in the etiology of cervical cancer.

Canadian Society of Plant Physiologists Gold Medal Review / Synthèse médaillée d'or de la Société canadienne physiologie végétaleThe fascinating world of folate and one-carbon metabolism

Folate was first isolated from spinach leaves in 1941 and characterized as pteroylglutamic acid. Although plants, fungi, and bacteria synthesize folate de novo, animal cells lack key enzymes of the folate biosynthetic pathway and a dietary source of folate is required for normal growth and development. Folates have importance in human nutrition, health, and disease, and antifolate drugs are commonly used in cancer chemotherapy. In the majority of living cells folates occur as one-carbon substituted tetrahydropteroylpolyglutamate derivatives. These folates donate one-carbon groups during the synthesis of purines, formylmethionyl-tRNA, thymidylate, serine, and methionine. In the last 30 years, research on the folate biochemistry of plant species has intensified and been aided by the development of improved methods for folate isolation and characterization. These studies have resulted in basic information on the nature of plant folylpolyglutamates, folate biosynthesis, the enzymology of several folate-dependent reactions, and the roles of chloroplasts, mitochondria, and the cytosol in the pathways of one-carbon metabolism.Key words: plants, folates, folate biosynthesis, folate-dependent enzymes, one-carbon metabolism.

Analysis of Folate Form Distribution by Affinity Followed by Reversed-Phase Chromatography with Electrochemical Detection

Background: Naturally occurring folates exist in multiple forms, differing in pteridine ring structure and number of glutamate residues. The ability to measure these folate coenzymes in tissues and cells gives important information about in vivo folate metabolism.

Methods: Folates were heat-extracted from biological samples. A two-column HPLC system with four-channel coulometric electrochemical detection was used for analysis. An affinity column was used first to purify folates from the extract. Purified folates were eluted from the affinity column onto a phenyl analytical column, utilizing a switching valve, and folate forms were separated using an acetonitrile gradient.

Results: Folate forms differing in pteridine ring structure and number of glutamate chain residues were identified by retention time and characteristic response across the channels of the detector. Folates were quantified by comparison to an external calibration mixture. Limits of detection for pentaglutamyl folates ranged from 0.21 pmol for tetrahydrofolate to 0.41 pmol for 5-methyltetrahydrofolate. CVs (n = 5) for peaks containing 9–67 pmol of folate were 0.6–6.4% (within day) and 5.2–8.4% (between days). CVs (n = 5) for peaks containing 0.9–3.5 pmol folate were 5.7–16% (within day) and 8.4–13% (between days).

Conclusions: This automated HPLC system allows the simultaneous determination of polyglutamyl forms of folates from biological samples, including tetrahydrofolate, 5-methyltetrahydrofolate, formylated folates, and pteroylglutamate. The low detection limits allow analysis of folate form distribution in human samples such as erythrocytes and lymphocytes.

Estimated folate intakes: data updated to reflect food fortification, increased bioavailability, and dietary supplement use

BACKGROUND

There is a critical need to estimate dietary folate intakes for nutrition monitoring and food safety evaluations, but available intake data are seriously limited by several factors.

OBJECTIVE

Our objective was to update 2 national food consumption surveys to reflect folate intakes as a result of the recently initiated food fortification program and to correct folate intakes for the apparently higher bioavailability of synthetic folic acid (SFA; ie, folate added to foods or from dietary supplements) than of naturally occurring folate so as to express intakes as dietary folate equivalents.

DESIGN

It was not possible to chemically analyze foods, so adjustments were made to food-composition data by using information about food ingredients and characteristics. Total folate intakes were estimated for several sex and age groups by using the modified data coupled with dietary supplement use.

RESULTS

Within the limitations of the data, our findings suggested that 67-95% of the population met or surpassed the new estimated average requirement, depending on the sex and age group and survey. Nonetheless, some subgroups had estimated intakes below these standards. Estimated SFA intakes suggested that approximately 15-25% of children aged 1-8 y, depending on the survey, had intakes above the newly established tolerable upper intake level. We estimated that 68-87% of females of childbearing age had SFA intakes below the recommended intake of 400 microgram/d, depending on the age group and survey.

CONCLUSION

There is a need to explore ways to improve folate intakes in targeted subgroups, including females of childbearing age, while not putting other population groups at risk of excessive intakes.

Properties of food folates determined by stability and susceptibility to intestinal pteroylpolyglutamate hydrolase action

The intestinal absorption of folate occurs at the monoglutamyl level, and an important measure of food folate bioavailability is how much folate from the food reaches the intestinal sites in forms that can readily be absorbed. In the absence of protecting agents, e.g., vitamin C and reduced thiols, many labile folates may be lost during cooking and during residence in the acid-peptic milieu of the stomach. On the other hand, the presence of polyglutamyl folate necessitates the action of intestinal hydrolases, which could be affected by food constituents. In this study, we developed an in vitro assay for the determination of an index of food folate availability. The index of folate availability in this study was defined as that proportion of folate that has been identified as monoglutamyl derivatives after tests for stability and susceptibility to an enzymatic hydrolysis. The index of folate availability varied widely among foods. The highest index was for egg yolk (72.2%), followed by cow's livers (55.7%), orange juice (21. 3%), cabbage (6.0%), lima beans (4.5%) and lettuce (2.9%). Yeast folate had the lowest index (0.3%). The availability indices generated by this study correlate with the indices of the bioavailability of the corresponding food folate observed in earlier studies, R2 = 0.529 (P = 0.068). Additional information is required on the bioavailability of other food products to test the usefulness of this in vitro approach for assessing food folate availability.

Folic acid fortification of grain: an economic analysis

OBJECTIVES

The purpose of this study was to compare the economic costs and benefits of fortifying grain with folic acid to prevent neural tube defects.

METHODS

A cost-benefit analysis based on the US population, using the human capital approach to estimate the costs associated with preventable neural tube defects, was conducted.

RESULTS

Under a range of assumptions about discount rates, baseline folate intake, the effectiveness of folate in preventing neural tube defects, the threshold dose that minimizes risk, and the cost of surveillance, fortification would likely yield a net economic benefit. The best estimate of this benefit is $94 million with low-level (140 micrograms [mcg] per 100 g grain) fortification and $252 million with high-level (350 mcg/100 g) fortification. The benefit-to-cost ratio is estimated at 4.3:1 for low-level and 6.1:1 for high-level fortification.

CONCLUSIONS

By averting costly birth defects, folic acid fortification of grain in the United States may yield a substantial economic benefit. We may have underestimated net benefits because of unmeasured costs of neural tube defects and unmeasured benefits of higher folate intake. We may have overestimated net benefits if the cost of neurologic sequelae related to delayed diagnosis of vitamin B12 deficiency exceeds our projection.

[Dietary folates--a timely review. Stability, physiological significance, bioavailability, analytical determination methods, effect of food handling]

Because of the unequal and, in some instances, low stability of different folate vitamers against extreme conditions the analytical determination of folate and the estimation of folate losses in food processing and preparation cause considerable difficulties. HPLC allows determination of the native folate derivative patterns. As the bioavailability of folates is influenced by a variety of factors and different methods were employed for assessing bioavailability there is a considerable inconsistency in the results of these studies. Folates labeled with radioactive or stable isotopes provide new approaches to metabolic and bioavailability studies.

Folate and cancer: A review of the literature

Folate, a water-soluble vitamin, part of the vitamin B complex, plays an important role in methylation reactions and DNA/RNA synthesis. This review examines the experimental and epidemiological evidence for the association between folate status and risk of cancer. Data have accumulated indicating that low folate status may promote carcinogenesis. Low folate levels are associated with cytogenetic abnormalities in vivo and in vitro. Findings from animal studies are conflicting and suggest that the effect of folate on neoplasia depends on factors such as the animal and tumor model, the type, timing, dose, and length of application of carcinogen, the stage of carcinogenesis, and the level and form of folate administered. Epidemiological studies examined the association between folate and cancer of the cervix, colorectum, lung, esophagus, and brain and suggest that low folate status may play an important role early in the neoplastic process. The potential for inhibition of precursor lesions in the cervix and colorectum, namely, cervical intraepithelial neoplasia and adenomatous polyps, respectively, is of particular interest. Additional research designed to clarify the role of folate in carcinogenesis is warranted.