The universal methyl donor in the body is S-adenosylmethionine (SAM) that donates its labile methyl group to more than 80 biological methylation reactions, including the methylation of DNA, RNA and protein. To generate the universal methyl donor SAM in the cell, homocysteine is methylated to methionine via
the one-carbon pathway by transferring a methyl group from 5-methyl-tetrahydofolate or betaine. The major external source of methyl groups comes from folate, methionine, and choline in diet (Niculescu, 2002
). It has been shown that folate depletion alone is a sufficient perturbing force to diminish the methyl pool (Miller et al., 1994
). Other vitamins B, such as vitamins B2
, are also key cofactors for one-carbon metabolism. Alcohol is a folate antagonist; excess consumption of alcohol impairs folate absorption by inhibiting expression of the reduced folate carrier and decreasing the hepatic uptake and renal conservation of circulating folate (Halsted et al., 2002
). Associations of folate intake and breast cancer risk have been extensively studied. However, studies on methionine, choline and betaine are limited (Cho et al., 2007
; Xu et al., 2008a
). In this review, we will focus on epidemiologic evidence of folate consumption and breast cancer.
Association studies using both cohort and case-control study designs have been conducted and subsequently pooled and meta-analyses combining data of individual studies have also been performed. While meta-analyses combine the published results of summary effects such as relative risk (RR) or odds ratio (OR), pooled analyses combine individual-level data that permit a full examination of effect modification within the data. It is important to note that these pooled or meta-analyses are performed retrospectively and subject to inherited limitations such as study heterogeneity and publication bias. Nevertheless, these analyses offer increased power to detect associations, especially as the number of included studies increases.
In epidemiologic studies, folate as an exposure of interest is often assessed from food frequency questionnaires (FFQ) or circulating biomarkers (i.e., plasma folate or red blood cell folate level). Folate intake can be referred to as dietary intake (from food) or total intake (from food and supplements). Folate naturally found in foods are predominantly in the form of 5-methyl-tetrahydrofolate (THF); meanwhile the fully unreduced (e.g. folic acid) and partially reduced forms (e.g. dihydrofolate (DHF)) are also found (Combs, 1992
). In contrast, folate in supplements and food fortification is the synthetic form, folic acid, which needs to be reduced before it can participate in cellular reactions (Machlin, 1991
). So the term “total folate intake” was used by adding up the folate in diet and in supplements (synthetic folic acid). This approach has limitation because folic acid has 1.7-fold greater bioavailability. Using the crude approach of simply summing up could result in misclassification and bias risk estimates toward null effects. Other approach, such as “Dietary Folate Equivalents (DFE)”, was recommended by the Institute of Medicine of USA (Sauberlich et al., 1987
; Hannon-Fletcher et al., 2004
), which allows for a combination of dietary and synthetic folic acid into a variable that accounts for this differential bioavailability.
Although a number of cohort and case-control studies have suggested an inverse association between folate status and the risk of breast cancer, these results are far from conclusive. In a meta-analysis summarizing studies published between 1966 and 2006 (Larsson et al., 2007
), folate intake (both dietary and total) with 200 μg/day increments was not associated with the risk of breast cancer in 8 prospective studies; however, an inverse association with dietary folate was observed in 13 case-control studies (OR: 0.80; 95% CI: 0.72–0.89). Data from several cohort studies, e.g. Nurses’ Health Study, the Canadian National Breast Screening Study and the Iowa Women’s Health Study, also indicate that adequate folate intake could attenuate the elevated risk associated with moderate alcohol consumption (Zhang et al., 1999
; Rohan et al., 2000
; Sellers et al., 2001
). In addition, there was an indication of inverse associations between blood folate concentrations and breast cancer risk, especially in case-control studies, although these associations failed to reach statistical significance (shown in ).
Summary of meta- and pooled analyses on folate and breast cancer risk
Similar results were observed from another meta-analysis by Lewis et al. (2006)
. A total of 13 case-control studies and 9 cohort studies were included; some overlapped with the study of Larsson et al. (2007)
. Summary odds ratios for dietary folate were 0.91 (95% CI: 0.87–0.96) for the case—control studies and 0.99 (95% CI: 0.98–1.01) for the cohort studies with a 100 µg/day increase in folate intake. This study lends additional support that dietary folate (not total folate) may be moderately protective against breast cancer (shown in ).
Results from subsequent cohort studies have been inconsistent or even conflicting. In a French cohort study (1,812 cases among 62,739 postmenopausal women with 9-year follow-up), an inverse association was observed (RR: 0.78; 95% CI: 0.67—0.90) (Lajous et al., 2006
). In a report from the Prostate, Lung, Colorectal, and Ovarian Cancer Screening Trial (691 cases among 25,400 women with 10-year follow-up), an increased risk of breast cancer was observed in postmenopausal women with folic acid supplemental use ≥400 µg/d (Stolzenberg-Solomon et al., 2006
). No apparent association between folate status and risk of breast cancer was observed in the Nurses’ Health Study II, in which 90,663 premenopausal women were followed for 12 years with 1,032 documented breast cancer cases (Cho et al., 2007
). In the Malmö Diet and Cancer cohort study (392 cases among 11,699 women with 9.5-year follow-up), a higher folate intake (both dietary and total) was associated with a ~40% lower incidence of postmenopausal breast cancer (Ericson et al., 2007
Alcohol is known to be a folate antagonist. There is quite consistent evidence that lower folate intakes in combination with high alcohol intakes are associated with an increased risk of breast cancer (Larsson et al., 2007
). This synergistic effect lends additional support to the notion that one-carbon metabolism is an important process in breast cancer etiology.
There is emerging evidence that indicates the relationship between one-carbon metabolism and breast cancer is complex and non-linear (Ulrich, 2007
). While folate- or one-carbon deficiency is thought to increase breast cancer risk, there is also evidence that very high intakes from diet and supplements may result in elevated risk (Stolzenberg-Solomon et al., 2006
). This inverted U-shaped relationship is alarming and requires further evaluation (Ulrich, 2007
In summary, the association of folate intake and breast cancer risk is complex and interactions exist (i.e., with alcohol intake). Further studies especially well-designed clinical trials and mechanistic investigations are needed to clarify the issue.