Folic acid intake derived from fortified foods was significantly higher than originally anticipated in the early era of fortification probably due to the practice of overage [30
] of the mandated amount of folic acid. In combination with the use of multivitamin supplements, certain segments of the population probably far exceeded a folic acid intake of 1 mg/d, which is the tolerable upper intake level defined by the Dietary Reference Intakes [31
Naturally occurring folates differ from oxidized folic acid which is used in supplements and fortification. Natural folates are unstable and can readily lose their activity in foods depending on the food source and preparation method. Folic acid (pteroylmonoglutamic acid) is inexpensive, more stable and has greater bioavailability than the naturally occurring polyglutamate folates which need to be hydrolyzed to the monoglutamate form at the intestinal wall before absorption [32
The intake of folic acid from fortified foods and concurrent supplement use can result in very high chronic intake of folic acid, such as serum folate levels of 40 nmol/L [33
] and detectable levels of unmetabolized folic acid in the plasma [34*
]. It has been shown that the oral intake of folic acid at physiological levels of 400 mcg saturates the enzymatic reduction to 5-methyl THF in cells of the intestinal mucosa [34*
Although unproven, high intake of folic acid feasibly could exert antagonistic effects due to an accumulation of dihydrofolate (DHF, the co-enzymatic form of folate through which folic acid enters one-carbon metabolism) since it inhibits thymidylate synthase and hence the synthesis of thymidylate. DHF also inhibits methylenetetrahydrofolate reductase (MTHFR) by which high concentrations of folic acid could inhibit the formation of 5-methyl tetrahydrofolate (5-methyl THF) (see ).
A potential mechanism for the inhibitory effect of high folic acid intake on one-carbon metabolism that controls biological methylation and nucleotide synthesis
Although folate is generally regarded as safe, with the exception of those with compromised B12 status, comprehensive data about possible antagonistic effects of folic acid on metabolism are lacking. A summary of recent mechanistic studies on folic acid is shown in .
Recent studies on the impact of folic acid supplementation on metabolic and epigenetic endpoints
The observations of Troen et al. are consistent with the concerns about excess intake of the pharmaceutical form of folate, folic acid, although the study was cross-sectional in design and therefore could not provide definitive evidence of causality. These investigators reported detectable levels of folic acid in the blood of 78% of a study population of free-living women, of whom 54% were daily users of a folate supplement. Among the elder participants, the authors observed that increasing concentrations of plasma folic acid, but not total folate, were inversely associated with the cytotoxicity of circulating natural killer cells in those individuals [35
]. This population of lymphocytes plays a role in cell-mediated immunosurveillance and is thought to be instrumental in the destruction of arising clones of neoplastic cells.
Recent preclinical studies have pointed out some previously unexplored effects of chronic exposure to supraphysiological doses of folic acid. A recent study by Canistro et al. [38*
] reported that folic acid supplementation has a major impact on the activity of xenobiotic metabolizing enzymes in the liver of rats, underscoring that the spectrum of toxicological characteristics of supraphysiological doses of folic acid is not fully understood yet. An in vitro
study pointed out that long-term supplementation with high doses of folic acid might also affect intestinal and renal folate uptake processes by down-regulating folate carriers [37*
Pellis et al. (2008) observed in a colon cancer cell line that exposure to supplemental levels of folic acid decreased markers of cell differentiation and increased cell turnover, both of which would be consistent with a promoting effect of high doses of folate on neoplastic cells [39*
]. An in vivo
study examined the effects of supranormal folic acid supplementation on methionine metabolism and DNA methylation and reported that supplementation for 4 weeks with a supraphysiological dose of 40 mg/kg diet did not cause biochemical evidence of hepatic or renal damage in male rats [36
]. The authors concluded that under the studied conditions, extreme high folic acid intake did not have adverse effects on one-carbon metabolism in rodents, although the spectrum of endpoints here was very limited, providing only a narrow degree of reassurance [36*
Some human studies have also raised concern regarding the overly abundant intake of supplemental folic acid. Although two large prospective cohort studies found that total folate intake was not associated with pancreatic cancer risk, a comprehensive meta-analysis demonstrated a protective effect of dietary folates against this cancer [44
]. In contrast, multivitamin supplement users in the two prospective cohorts had a non-significant higher risk of pancreatic cancer [45
]. Moreover, the results of a large prospective study on postmenopausal women show a positive association between total folate and supplemental folic acid intake and breast cancer, but no association between dietary folate intake and cancer risk [11
]. In this study, there was a 19% higher risk of developing breast cancer in the subjects using folic acid supplements. This result contrasts with other studies that have generally demonstrated a protection against postmenopausal breast cancer with generous intake of folate [46
]. The apparent discrepancy is possibly explained by the fact that the high-risk group in the Stolzenberg-Solomon study [11
] was ingesting more than twice the level of total folate than the ‘high folate intake’ group in other studies.