Folic acid and NTDs
Other birth defects
    Heart defects
    Orofacial defects
    Urinary tract defects
    Limb defects
Other pregnancy outcomes
    Preterm birth
    Multiple births
    Miscarriage
Chronic diseases
    Vascular diseases
    Cancer
    Cognitive diseases and depression

Folic acid and NTDs

In 1965, Hibbard and Smithells suggested that nutritional factors, particularly folic acid, could be related to the occurrence of congenital malformations, including NTDs. This and the observation that NTDs occur more frequently in areas with low socioeconomic levels and, presumably, poor diets, led Smithells and other investigators to use periconceptional multivitamin supplementation to prevent the recurrence of NTDs (Laurence et al. 1981, Smithells et al. 1983, Smithells et al. 1981). Their findings, however, were not universally accepted because of the lack of adequate randomization.

In 1991, the Medical Research Council (MRC) reported the results of a multicenter, prospective, randomized study of folic acid supplementation for the prevention of NTDs in 1817 women who had a previous child with a NTD (MRC Vitamin Study Research Group). The data conclusively demonstrated a 71% reduction in the recurrence of NTDs in the group that took daily 4-milligram doses of folic acid with or without other vitamins before and during early pregnancy. Use of multivitamins without folic acid did not result in a reduction of NTDs.

In a nonrandomized study conducted in Cuba, a 5-milligram daily dose of folic acid given periconceptionally to 101 women who had a previous NTD birth resulted in no recurrences. In contrast, four recurrences were noted among 118 women who received no supplementation (Vergel et al. 1990).

In 1989, Milunsky et al reported the results of an observational study of first-occurrence of NTD in a cohort of women undergoing maternal serum alpha-fetoprotein screening or amniocentesis around 16 weeks of gestation. The prevalence of NTDs was 3.5 per 1000 among non-supplemented subjects (women who never used multivitamins before or after conception or who used multivitamins before conception only) vs. 0.9 per 1000 among women who took a multivitamin containing folic acid during the first six weeks of pregnancy. For women who used multivitamins without folic acid during the first 6 weeks of pregnancy and women who used multivitamins containing folic acid beginning after 7 or more weeks of pregnancy, the prevalences were similar to that of the non-supplemented group.

Three other observational case-control studies also identified a reduced risk for NTDs among women who reported taking adequate amounts of folic acid periconceptionally (Mulinare et al. 1988, Werler et al. 1993, Bower & Stanley 1989). Only one study failed to demonstrate this protective effect (Mills et al. 1989).

To assess the effect of folic acid on first occurrence of NTDs, Czeizel and Dudas, in Hungary, studied a large number of women planning a pregnancy (Czeizel & Dudas 1992). The investigators randomized the women to receive either a multivitamin supplement containing 0.8 milligrams of folic acid or a trace element supplement daily for at least a month before conception and until the date of the second missed menstrual period or later. Pregnancy was confirmed in 4753 women. The outcome of pregnancy was known in 2104 women who received the vitamin supplement and in 2052 who received the trace-element supplement. There were no cases of NTD among the offspring of women who received periconceptional multivitamin supplements, as compared with 6 cases of NTDs among those of women supplemented with the trace-mineral supplement. (P =0.029).

The preventive effect of folic acid has been further corroborated by the recently published results of an intervention study conducted by the Centers for Disease Control and Prevention (CDC) in two areas of China, one with high prevalence and the other with low prevalence of NTDs (Berry et al. 1999). The investigators found that among women who took 400 micrograms of folic acid from the time of their premarital examination until the end of the first trimester of pregnancy, the risk of NTDs was reduced by 85 percent in the region with high risk for NTDs and by 40 percent in the low risk region. The results of this study were important because it established that the observed reduction in NTD-affected births could be attributed solely to consumption of folic acid and also substantiated that a daily intake amount of 400 micrograms was effective for occurrence prevention.

Other birth defects

Several studies have associated folic acid or multivitamin use with reduced risk for other birth defects.

Heart defects

Several studies have examined the association between maternal use of multivitamins or folic acid and the reduced risk for heart defects in infants. A randomized controlled trial conducted in Hungary found that women taking multivitamins containing 800 micrograms folic acid during the periconceptional period had a 58% reduced risk of having a child with a cardiovascular defect (Czeizel et al. 1998). A population-based case-control study conducted in Atlanta reported that use of multivitamin supplements periconceptionally reduced the risk for heart defects by 24% (Botto et al. 2000). A population-based case-control study in California reported a 47% risk reduction for women consuming folic acid containing supplements after controlling for confounders including maternal race/ethnicity, age, and education (Shaw et al. 1995). Risk reduction appears to be strongest for ventricular septal defects and some conotruncal defects (Botto et al. 2003). However, several studies have found no association between periconceptional folic acid (Scanlon et al. 1998) or multivitamin (Werler et al. 1999) use and reduced risk for heart defects. Because multivitamins contain vitamins and minerals other than folic acid there is no direct evidence that folic acid is responsible for the risk reduction seen in the before-mentioned studies. However, a large case-control study reported that women taking folate antagonist medications (dihydrofolate reductase inhibitors) and not taking a multivitamin containing folic acid had an almost 8-fold increased risk of having a baby with a heart defect. However, this elevated risk was reduced to 1.5-fold (not statistically significant) in women taking a multivitamin supplement with folic acid and these medications concurrently (Hernandez-Diaz et al. 2000).

Orofacial defects

The orofacial defects discussed here include cleft lip with and without cleft palate (CLP) and cleft palate alone (CP). Several case-control studies report a significant association between periconceptional intake of multivitamins and isolated CLP (Shaw et al. 1995) and CLP (Itikalaet al. 2001). Risk reduction in both studies was approximately 50% when multivitamins were taken periconceptionally. A multicenter U.S. case-control study reported a 60% lower risk for CP in women taking multivitamins before and during early pregnancy (Werler et al. 1999). In this study, no risk reduction was observed for CLP. A case-control study from the Netherlands reported that folic acid intake as part of a multivitamin or single tablet supplement was associated with a 47% reduced risk for CLP in offspring when the supplement was taken during 4 weeks before and 8 weeks after conception (van Rooij et al. 2004). In this study, highest risk reduction was observed in mothers who concurrently took supplemental folic acid and had dietary folate intakes greater than 200 micrograms/day. A comparison of two Hungarian datasets reported that a higher intake of folic acid (generally 6 milligrams/day during the critical palate formation period) was associated with reducing the risk for CLP by 18% and CP by 24%, but no significant risk reduction was associated with a lower dose of folic acid (800 micrograms/day) (Czeizel et al. 1999). A randomized controlled trial in Hungary found no association between periconceptional use of multivitamins containing 800 milligrams of folic acid and risk for orofacial clefts (Czeizel 1993). In a non-randomized intervention study, women supplemented with multivitamins plus 10 milligrams of folic acid on a daily basis had a significantly reduced risk for a recurrence of CLP (Tolarova 1982). These studies suggest that the protective effect of folic acid may be dose dependent. A retrospective population-based study in Canada reported no change in the prevalence of orofacial clefts following folic acid fortification of cereal grain products (Ray et al. 2003).

Urinary tract defects

Case-control studies have reported that women taking multivitamins periconceptionally had a 40-50% 18 and 85% (Li et al. 1995) reduced risk for having a baby with a urinary tract defect. A randomized controlled trial conducted in Hungary found an almost 80% reduced risk for urinary tract defects in the infants of women taking a multivitamin with 800 micrograms folic acid during the periconceptional period compared with women consuming a trace element supplement (Czeizel 1996). However, a subsequent cohort-controlled trial in Hungary failed to find a significant risk reduction for infants of women who took a multivitamin compared to unsupplemented women (Czeizel et al. 2004).                               

Limb defects

Studies are less supportive of a role for folic acid in reducing the risk for limb deficiencies. The Hungarian randomized controlled trial reported an 80% risk reduction with periconceptional multivitamin use, but the results were not statistically significant (Czeizel 1998). A Hungarian cohort-controlled trial failed to find an association between multivitamin use and reduced risk for limb deficiencies (Czeizel et al. 2004). Similarly, three case-control studies reported null findings (Shaw et al. 1995, Werler et al. 1999, Czeizel 1995). Only a single study (the Atlanta Birth Defects Case-Control Study) reported a significant 53% reduced risk for limb deficiencies in women taking a multivitamin 3 months prior to and 3 months following conception (Yang et al. 1997).

Other pregnancy outcomes

Preterm birth

Several studies have reported associations between folic acid intake and other pregnancy outcomes. Lower folate intake ( ≤ 240 micrograms/day) and lower serum folate at week 28 of gestation was associated with a higher risk for preterm delivery and low birth weight (Scholl et al. 1996). A recent study reported that dietary folate intake ≤ 500 micrograms/day or low serum or red blood cell folate concentrations during the second trimester resulted in a significantly higher risk for pre-term delivery (Relative Risk 1.7-1.8) (Siega-Riz et al. 2004). An observational study including almost 6 million California infants compared adverse pregnancy outcomes prior to and after folic acid fortification of cereal grain foods. Fortification coincided with small but statistically significant reductions in babies born with low or very low birth weights (6-9% reduction) or preterm delivery (4% reduction) (Shaw et al. 2004). However, some studies failed to confirm an association between folate status and risk for preterm birth (Ronnenberg et al. 2002), including a large randomized controlled trial (Czeizel et al. 1994b).

Multiple births

There was speculation that folic acid supplementation during the periconceptional period increased the risk for multiple births. A Hungarian randomized controlled trial described a significant increase in multiple births in women taking a multivitamin containing 800 micrograms of folic acid compared to women taking trace elements (Czeizel et al. 1994a). A subsequent study in Sweden found >70% increased risk for twin births with the use of folic acid periconceptionally (Kallen 2004). However, most studies do not support such an association. An analysis of the Medical Research Council's intervention cohort combined with an observational study in the United Kingdom reported no increase in multiple births with folic acid supplementation (Mathews et al. 1999). The strongest data comes from a non-randomized intervention study in >240,000 Chinese women. Women taking 400 micrograms of folic acid on a daily basis before and during pregnancy did not have a higher risk for twin pregnancies compared to women not taking folic acid (Li et al. 2003). Observational studies that evaluated multiple birth rates before and after folic acid fortification include three studies finding no difference in the number of multiple births before and after fortification (Lawrence et al. 2004, Shaw et al. 2003, Waller et al. 2003) and one study reporting an increase in multiple births but only in women > 30 years of age, which could possibly be explained by the use of assisted reproductive technologies (ART) in this age group (Kucik 2004). Studies of multiple births can be highly confounded by the use of ART and the use of these technologies should be accounted for in studies (Berry et al. 2005). A recent study in Norway reported no association between folate use and twin pregnancies after exclusion of pregnancies associated with ART (Vollset et al. 2005).

Miscarriage

The Hungarian randomized controlled trial observed a small but significant increase in miscarriages in women taking a daily multivitamin with 800 micrograms of folic acid compared to women taking a trace element tablet (Hook & Czeizel 1997). However, the Medical Research Council's randomized study (Wald & Hackshaw 2001) and the China folic acid intervention study (Gindler et al. 2001) failed to confirm such an association. An observational study in California reported a small increase in miscarriages by women taking folic acid, but this increase was not statistically significant (Windham et al. 2000). On the contrary, other studies report an association between low folate status and increased risk for miscarriage (Nelen et al. 2000, George et al. 2002).

Chronic diseases

Vascular diseases

Elevated blood homocysteine concentrations have been identified as an independent risk factor for cardiovascular disease, including diseases of the coronary, cerebral and peripheral vessels, and thromboembolism (Refsum et al. 1998). Folate serves as a coenzyme in the conversion of homocysteine to methionine and folate intake or status is inversely related to blood homocysteine concentrations (Riddell et al. 2000, Selhub et al. 1993, Verhoef et al. 1996). Elevated blood homocysteine concentrations have been associated with an increased risk for coronary heart disease in cross-sectional and case-control studies, although less consistently in prospective studies (Christen et al. 2000). Supplementation with folic acid at doses ranging from 500 micrograms to 5 milligrams appear to uniformly lower homocysteine concentrations by approximately 25% (Homocysteine Lowering Trialists' Collaboration 1998). Those with higher homocysteine concentrations or lower blood folate status appear to benefit the most from folic acid supplementation. A meta-analysis suggests that lowering homocysteine concentrations by 3 micromoles/liter would result in a decrease in the risk of ischemic heart disease, deep vein thrombosis, and stroke of 16%, 25% and 24%, respectively (Wald et al. 2002).

Can folic acid supplementation reduce the risk for heart disease? Observational data from the Nurses' Health Study indicate that total folate intakes of ≥ 393 micrograms/day from diet and supplements was associated with a 20-30% decreased risk for coronary heart disease in women (Rimm et al. 1998). A recent longitudinal study suggests that men in the highest quintile of folate intake (median 821 micrograms/day) had a ~30% reduced risk for ischemic stroke compared to men in the lowest quintile of intake (262 micrograms/day) (He et al. 2004). A similar observational study found no significant effect of higher folate intakes on stroke risk in women (Al-Delaimy et al. 2004). Several randomized controlled trials are ongoing in the US, Canada, Europe and Australia to assess the impact of supplemental folic acid alone or in combination with one or more other B vitamins (e.g., vitamins B6 and B12) on cardiovascular disease occurrence or recurrence (Eikelboom et al. 1999). To date (early 2005), the results of only one study have been published and reported no benefit to men or women taking a multiple B vitamin supplement (folic acid, vitamin B6 and vitamin B12) for 2 years in reducing the risk for recurring stroke (Toole et al. 2004).

Folic acid or homocysteine also have been associated with other diseases or chronic conditions. Recently, two studies indicate that elevated homocysteine concentrations were associated with an increased risk for bone fractures in elderly individuals (van Meurs et al. 2004, McLean et al. 2004). The proposed mechanism may be the potential for homocysteine to interfere with collagen synthesis, thereby affecting bone structure independent of mineral content. Higher folate intake from diet and supplements also has been associated with a decreased risk for hypertension in women (Forman et al. 2005).

Cancer

A relationship between folate status or intake and several types of cancer, including cancers of the colon, breast, and cervix, has been reported. There are two mechanisms by which folate may modulate cancer risk. First, folate is needed to form one of the four deoxynucleotide components of DNA, thymine. If folate is in short supply, the base uracil will inappropriately be incorporated into DNA in place of thymine. A DNA correction enzyme will attempt to correct the error by extracting the uracil and inserting a thymine moiety. However, this action can potentially leave nicks in the DNA strand. If two nicks occur adjacent to each other on DNA strands the DNA could break and become unstable, which could promote carcinogenesis. Folate depletion has been associated with an increased rate of uracil misincorporation into the DNA of humans (Blount et al. 1997). The second mechanism involves the process of DNA methylation. Folate is instrumental in producing methyl groups needed to modify DNA. Methylation patterns in DNA help control gene expression. When folate is in short supply, DNA can be undermethylated (hypomethylated) and genes that are normally silenced may be erroneously expressed. Erroneous expression of a proto-oncogene could initiate a process that leads to carcinogenesis. DNA hypomethylation or other abnormal methylation patterns have commonly been observed in cancerous tissue and DNA hypomethylation has been observed in humans consuming a low folate diet (Jacob et al. 1998, Rampersaud et al. 2000).

The relationship between folate and cancer is most clearly defined for colon cancer and precancerous colorectal adenomas. In a study of two large cohorts (Nurses' Health Study and the Health Professionals Follow-up study), highest folate intake (>700 micrograms/day) from diet and supplements was associated with a 30-40% decreased risk for colorectal adenomas compared to lower intakes (166-241 micrograms/day for women and men, respectively) (Giovannucci et al. 1993). In a large cohort of women, total folate intakes ≥ 400 micrograms/day was associated with a 30% reduced risk for colon cancer compared to folate intake < 200 micrograms/day (Giovannucci et al. 1998). In this study, a high percentage of women in the highest folate intake group were taking multivitamins. An evaluation based on duration of vitamin use reported a 75% reduced risk for colon cancer in women who had used a multivitamin for at least 15 years, while there was no benefit in women taking multivitamins for shorter periods of time. A prospective study of the Swedish Mammography Cohort reported ~ 30% reduced risk of colon cancer in women with each 100 microgram/day increase in dietary folate above 130 micrograms/day (Larsson et al. 2005). The significant results persisted even when controlling for fruit and vegetable intake and use of vitamin supplements. The potential benefits of folic acid with regard to colon cancer risk appear to be especially dramatic in those with a family history of colon cancer (Fuchs et al. 2002). Several on-going intervention trials are studying the effects of folic acid supplementation on the risk of colon cancer (Kim 1999). 

Data from large cohort studies suggest that the increased risk for breast cancer observed at moderate to high intakes of alcohol consumption may be attenuated by adequate folate intakes (Rohan et al. 2000, Zhang et al. 1999). Folate status can be negatively impacted by chronic alcohol intake either through reduced intake of dietary folate, decreased absorption, or altered metabolism (Halsted et al. 2002). Higher folate intakes may compensate for this effect and translate into reduced risks for disease. Folate also has been associated with decreasing the risk for breast cancer in women with a family history of breast cancer (Sellers et al. 2004). Collectively, these data suggest that there may be no direct association between folate and breast cancer risk, but that folate may be beneficial when other risk factors for disease are present.

Compared with colon and breast cancer, data are less supportive of a protective effect of folate on cervical cancer or dysplasia. However, associations have been reported between folate status and the presence of human Papillomavirus (HPV), an important risk factor for cervical dysplasia. Women with low red blood cell folate concentrations and HPV had an increased risk for cervical dysplasia (Butterworth et al. 1992) and lower serum folate concentrations coexisting with HPV infection have been associated with a 7-fold increase in the risk for cervical dysplasia and carcinoma (Kwasniewska et al. 1997). If folate is important to the onset of cervical cancer, it appears to be most important during the early stages of carcinogenesis.

Cognitive diseases and depression

Low folate status has been associated with mild cognitive impairment (Quadri et al. 2004), vascular dementia (Quadri et al. 2004, Malaguarnera et al. 2004), and Alzheimer's disease (Malaguarnera et al. 2004, Postiglione et al. 2001, Renvall et al. 1989). Elevated homocysteine concentrations also have been observed in patients with vascular dementia or Alzheimer's disease (Malaguarnera et al. 2004). In a longitudinal study conducted in Sweden, 370 non-demented persons were followed for 3 years. Those with low serum folate concentrations and good cognition at baseline were over 3 times more likely to develop Alzheimer's disease during the follow-up period (Wang et al. 2001). A longitudinal study of the Framingham cohort reported a relative risk of dementia and Alzheimer's disease of 1.4 and 1.8, respectively, for each 1 standard deviation increase in serum homocysteine at baseline compared to follow-up (Seshadri et al. 2002). A longitudinal study of over 670 elderly subjects without dementia at baseline reported that those in the highest quartile of homocysteine had a slightly higher risk for Alzheimer's, although this result was not statistically significant (Luchsinger et al. 2004). A recent cross-sectional analysis of over 1,100 older adults reported that those with higher homocysteine concentrations were more likely to score lower on tests for various cognitive skills compared to those with lower homocysteine concentrations (Schafer et al. 2005). Data are inconsistent but suggest that folate or homocysteine may play a role in dementia diseases and cognitive function, although it is not known whether the relationship reflects a cause or consequence of the disease.  

Folate has been widely studied with regard to its association with depressive disorders. Several cross-sectional studies have associated markers of depression with low folate status (Rosche et al. 2003, Ramos et al. 2004, Morris et al. 2003). Elevated blood homocysteine concentrations were associated with an almost 2-fold increased risk for depression in two cohort studies (Bjelland et al. 2003, Tiemeier et al. 2002). A prospective study indicated that Finnish men with folate intakes below the median had 2.5 to 3 times the risk for a diagnosis of depression during a 13-year follow up period compared to men with folate intakes above the median (Tolmunen et al. 2004).

A meta-analysis of two randomized controlled trials testing the effectiveness of folic acid supplementation either in conjunction with psychotropic medication or as a replacement for it in treating depressive disorders concluded that adding folate to a treatment regimen appeared to be effective in reducing depressive symptoms as measured by the Hamilton Depressive Rating Scale (Taylor et al. 2004). However, there was no difference in outcome when comparing folate treatment alone with the psychotropic medication trazodone. These limited data suggest that folate may be of benefit when coupled with the use of antidepressant medications. However, it remains unclear whether folate treatment alone could benefit depressive disorders.     

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