Folate intake and colorectal cancer risk: A meta-analytical approach
Miguel A. Sanjoaquin*, Naomi Allen, Elisabeth Couto, Andrew W. Roddam and Timothy J. Key
Cancer Research U.K., Epidemiology Unit, University of Oxford, Oxford, United Kingdom
Adequate consumption of folate may reduce the risk of colorectal
cancer. We performed a meta-analysis of 7 cohort and 9 case-
control studies that examined the association between folate con-
sumption and colorectal cancer risk. In cohort studies, the asso-
ciation between folate consumption and colorectal cancer risk was
stronger for dietary folate (folate from foods alone; relative risk
for high vs. low intake ? 0.75; 95% CI ? 0.64–0.89) than for total
folate (folate from foods and supplements; relative risk for high vs.
low intake ? 0.95; 95% CI ? 0.81–1.11) and there was no signif-
icant heterogeneity between studies. There was significant heter-
ogeneity between case-control studies. These results offer some
support for the hypothesis that folate has a small protective effect
against colorectal cancer but confounding by other dietary factors
cannot be ruled out.
© 2004 Wiley-Liss, Inc.
Key words: colorectal cancer; folate; folic acid; meta-analysis
In Europe and most of the industrialized world, colorectal can-
cer (CRC) is the third most common cancer in men after lung and
prostate cancer and the second most common in women after
breast cancer.1There is experimental evidence suggesting that
folate, a vitamin found mostly in vegetables and cereals, may
reduce the risk of CRC.2An inadequate intake of folate may
increase the risk of CRC by various mechanisms, including hy-
pomethylation of DNA, which interferes with gene expression and
impairs DNA repair, or by uracil misincorporation leading to DNA
breaks and instability. The epidemiologic data, however, are not
consistent; to date, 53–7out of 73–9prospective investigations and
710–16out of 11 case-control studies10–20have found a relatively
low risk for CRC in association with a relatively high intake of
folate. No intervention studies have investigated the effect of folate
supplementation on CRC risk, although the one trial with adeno-
matous polyps as an endpoint found a nonsignificant reduction in
colorectal adenoma recurrence after 2 years in those receiving a
folate supplement compared to placebo.21
The aim of this study is to conduct a metaanalysis of published
data to provide an overall estimate of the association of dietary and
total folate (folate from food and supplements) with CRC risk.
Material and methods
We conducted electronic searches of the PubMed database,
which includes articles back to the 1950s and up to January 2004,
using both MeSH headings and text words including the terms
“folate,” “folic acid,” “colorectal,” “colon,” “rectum,” “bowel,”
“cancer” and exploded variants. All full articles that matched the
inclusion criteria were retrieved and the reference lists in those
articles were hand-searched for other relevant publications.
We sought to identify all published English-language articles
that provided estimates of risk (with 95% confidence intervals) of
cancer of the colon and/or rectum in relation to dietary or total
folate intake. Two studies that did not show confidence intervals
for the estimates were excluded from the meta-analysis but the
results of those studies were qualitatively assessed.10,16
Data extraction and classification
We abstracted relative risks (RRs; odds ratios reported in case-
control studies are described as RRs in this article) for the highest
intake of folate relative to the lowest, together with 95% confi-
dence intervals (CIs). When several estimates were presented
using different adjustments,3,14,22,23the fully adjusted ones were
included. Where studies presented information for dietary and total
folate intake separately, the estimates for both were abstracted.
Statistical methods and analyses
The studies mostly reported relative risks according to quintiles
(cohort studies) or quartiles (case-control studies) of folate intake;
therefore, for this analysis, they were transformed using an estab-
lished method24so that all the meta-analyses are based on the
relative risk in the top vs. bottom quintiles of intake for cohort
studies and top vs. bottom quartiles for case-control studies.
Summary estimates of the standardized RRs were derived using
random- and fixed-effect models; both yielded similar results and
only estimates from random-effect models are presented. The 95%
CIs from the original publications were used to calculate the
standard errors of the standardized RRs, and the weighted average
of the RRs was calculated by giving each study a weight propor-
tional to its precision (i.e., the inverse of the variance). Thus, larger
studies, with more precise estimates and narrower confidence
intervals, were given greater weight than smaller ones. We tested
for heterogeneity using Cochran’s Q-test to evaluate the consis-
tency of findings. Separate analyses were performed for cohort and
A metaregression analysis25was performed to investigate
whether the association between folate intake and risk differed
according to sex, endpoint (colon or rectum) and source of folate
(from foods only or foods plus supplements).
The results are presented graphically, whereby squares represent
study-specific estimates and diamonds represent pooled estimates.
The area of each square is proportional to the inverse of the
variance of the natural logarithm of the RR; 95% CIs for individual
RRs are represented by horizontal lines and for the pooled esti-
mates by the width of the diamonds. All the analyses were per-
formed using the statistical package Stata 8.0 (StataCorp, College
Seven cohort studies3–9and 9 case-control studies11–15,17–20
were included in this meta-analysis. Four cohort studies7–9and
3 case-control studies13,15,19presented information on total folate
intake as a measure of exposure (Table I). Two cohort studies3,4
and 4 case-control studies14,15,17,20presented RRs adjusted for
fiber while 4 cohort studies4–6,9and 1 case-control study13pre-
sented RRs adjusted for other vitamins. Folate intake ranged from
less than 103 to more than 422 ?g/day for dietary folate and from
less than 188 to more than 2,430 ?g/day for total folate. All but
2 studies3,17categorized folate intake in quartiles or quintiles.
Figure 1 shows the RRs, after standardization, of CRC for the
highest vs. lowest category of dietary and total folate intake in
Grant sponsor: Cancer Research U.K.
*Correspondence to: Cancer Research U.K., Epidemiology Unit, Uni-
versity of Oxford, Gibson Building, Radcliffe Infirmary, Oxford OX2 6HE,
United Kingdom. Fax: ?44-1865-310-545.
Received 14 June 2004; Accepted after revision 9 August 2004
Published online 21 October 2004 in Wiley InterScience (www.
Int. J. Cancer: 113, 825–828 (2005)
© 2004 Wiley-Liss, Inc.
Publication of the International Union Against Cancer
cohort and case-control studies. Very similar results were found
using the nonstandardized RRs as reported in the individual studies
(results not shown).
For cohort studies, the summary estimate of risk for high dietary
folate intake was 0.75 (95% CI ? 0.64–0.89), with no significant
heterogeneity between studies (chi-square ? 4.96; 7 df; p ? 0.67).
The association of total folate intake with CRC risk in cohort
studies was close to null (RR ? 0.95; 95% CI ? 0.81–1.11), with
no significant heterogeneity between studies (chi-square ? 4.57;
4 df; p ? 0.33).
Among case-control studies, the overall RR for high vs. low
dietary folate intake was 0.76 (95% CI ? 0.60–0.96), although the
test for heterogeneity was highly significant (chi-square ? 23.10;
9 df; p ? 0.01); for total folate from foods and supplements, the
RR was 0.81 (95% CI ? 0.62–1.05), with no heterogeneity be-
tween the 4 estimates from 3 studies (chi-square ? 2.39; 3 df; p ?
The estimates from the 2 case-control studies excluded from the
meta-analysis were compatible with these results. RRs in the
highest vs. the lowest intakes of dietary folate were 0.56 (ptrend?
0.05) for both sexes in a study in Spain10and 2.08 (ptrend? 0.05)
and 0.73 (ptrend? 0.05) for men and women, respectively, in a
study in Washington State.16
The metaregression analysis revealed that, within cohort studies,
the association between folate intake and risk differed significantly
to the endpoint considered, folate intake having a slightly stronger
association with risk for colon compared to rectal cancer (p ?
0.03). The association between folate intake and risk for CRC did
not differ significantly according to source of folate intake (dietary
or total, p ? 0.06) or according to sex (p ? 0.74). Within
case-control studies, there were no significant differences in the
association of folate intake and risk according to endpoint, source
of folate, or sex.
Overall, this meta-analysis shows that, among cohort studies,
there is a significant 25% lower risk of CRC among those in the
highest category of dietary folate intake compared with those in
the lowest category, with no evidence of heterogeneity between the
study estimates. For total folate intake, there was a nonsignificant
5% lower risk of CRC. Although the pooled estimates for case-
control studies also showed a reduced risk, there was strong
evidence of heterogeneity between studies, which may not abso-
lutely be due to differences in the study characteristics examined
of date, endpoint, or sex, and thus the pooled estimates should be
interpreted with caution.
Folate intake is generally correlated with intake of fiber and
some vitamins. The apparent protective effect of folate may there-
fore have been confounded by other dietary factors that may
themselves be protective for CRC. Where possible, the RRs in-
cluded in this meta-analysis were adjusted for intake of other
vitamins and fiber. However, the effect of such adjustment is not
clear and in 23,20of the 3 studies3,4,20where results were shown
unadjusted and further adjusted for fiber, the RR did not change
appreciably. Giovannucci et al.3found also that a significant
TABLE I – CHARACTERISTICS OF THE COHORT AND CASE-CONTROL STUDIES INCLUDED IN THE METAANALYSIS
Giovannucci et al.3
Su and Arab5
? 300 vs. ? 200
? 249 vs. ? 103
Flood et al.8
? 272 vs. ? 142
? 633 vs. ? 188
? 634 vs. ? 230
NoHamack et al.9
? 463 vs. ? 282RCNo
Konings, et al.4
The NetherlandsWomen Dietary folate5
? 243 vs. ? 150 CCYes
Men Dietary folate5
? 266 vs. ? 168CC Yes
? 243 vs. ? 150RCYes
Men Dietary folate5
? 266 vs. ? 168 RC Yes
Terry et al.6
? 367 vs. ? 233CRC No
Wei et al.7
? 400 vs. ? 200
? 400 vs. ? 200
Freudenheim et al.12
? 340 vs. ? 210
? 380 vs. ? 240
? 310 vs. ? 220
? 385 vs. ? 250
? 261 vs. ? 162
? 320 vs. ? 184
? 388 vs. ? 268
Ferraroni et al.11
Boutron-Ruault et al.18
Glynn et al.13
? 388 vs. ? 268RC No
La Vecchia et al.14
Slattery et al.20
Kato et al.19
Levi et al.17
Le Marchand et al.15
? 422 vs. ? 301
? 626 vs. ? 224
? 403 vs. ? 173
? 406 vs. ? 252
? 2,430 vs. ? 297
1This article reports results for dietary and total folate intake; however, the results for total folate are cited, based on a longer follow-up, in
Wei et al.7
SANJOAQUIN ET AL.
association between folate and CRC remained after adjusting for
other vitamins and minerals (vitamins A, C, D, E and calcium);
conversely, after adjustment for folate intake, none of these dietary
factors was significantly associated with CRC risk.
Food frequency and recall questionnaires do not estimate folate
intake with great precision. In addition, estimated folate in food
may not accurately reflect actual intake and absorption because
folate is sensitive to heat, pH and oxidation; the availability of fo-
FIGURE 1 – Estimatedrelativerisksand95%confidenceintervalsofcolorectalcancerassociatedwithdietaryandtotalfolateintake(folatefromfoods
and folate from foods plus supplements, respectively) among cohort and case-control studies.3CRC, colorectal cancer; CC, colon cancer; RC, rectal
cancer.4Number of cases in cohort studies and cases/controls in case-control studies.5RR standardized into quintiles for cohort studies and quartiles
for case-control studies. Test for heterogeneity: between cohort studies with dietary folate, chi-square7? 4.96, p ? 0.67; between cohort studies with
total folate, chi-square4? 4.57, p ? 0.33; between case-control studies with dietary folate, chi-square9? 23.10, p ? 0.01; between case-control studies
with total folate, chi-square3? 2.39, p ? 0.50.
FOLATE AND COLORECTAL CANCER
late is also affected by the presence of certain inhibitors and folate
conjugates found in some foods.26Moreover, folate from supple-
ments (folic acid or pteroyl-monoglutamic acid) has a greater
bioavailability than polyglutamate folate from foods, which needs
to be hydrolysed to monoglutamate in the intestine before absorp-
tion.28All these factors can introduce some degree of measurement
error, thought to be largely random, in the assessment of folate
intake, which would be expected to bias the results toward the null;
this type of measurement error is thus unlikely to account for the
inverse association found between dietary folate intake and CRC
The larger reduction in risk associated with folate from dietary
sources alone compared with total folate intake was unexpected.
Only 3 studies included in this metaanalysis presented estimates
for both dietary and total folate intake separately. The Nurses’
Health Study3found a large reduction in risk among those taking
multivitamin supplements for 15 years or more compared with
those taking them for less than 15 years or never, but no significant
reduction in risk was observed for high intakes of dietary folate.
Another study15found similar RRs for high intakes of both dietary
and total folate and a third study8found a stronger, although
nonsignificant, association of risk with dietary folate compared to
total folate. More data are needed to clarify whether dietary or total
folate is more strongly related to risk, but a plausible explanation
is that confounding by other dietary factors might be playing a
bigger role when investigating the association between CRC and
dietary folate than when looking at total folate.
It is unclear whether the association between folate intake
and CRC risk is stronger for colon than rectum. From the
metaregression analysis and within cohort studies, there was an
apparent stronger association with colon cancer than with rectal
cancer. However, this result is dominated by 2 studies reported
in one publication7and became nonsignificant after their exclu-
sion. The most appropriate way to disentangle this would have
been to pool the estimates for the 2 endpoints from each study
separately; however, information on colon cancer (CC) and
rectal cancer (RC) separately was only available for 3 cohort
studies and 2 case-control studies. Therefore, the question re-
mains open and further studies looking at CC and RC will be
needed to clarify this.
It is possible that other factors, in combination with inadequate
folate intake, are needed to provoke changes such as disturbances
in DNA methylation and other processes related to CRC risk. It has
been shown that a high alcohol intake might have an adverse effect
on folate metabolism,28and at least one epidemiologic study has
observed that individuals with a diet low in folate (and methionine)
and high in alcohol have a relatively high risk of CRC.29Further-
more, it is possible that certain subgroups of the population, such
as those with specific polymorphisms2or with a family history of
CRC, may receive a greater benefit from a diet high in folate.22
In conclusion, the results of this meta-analysis offer some
support for the hypothesis that relatively high folate consump-
tion is associated with a decrease in the risk of CRC, but
confounding by other dietary factors cannot be ruled out. Fur-
ther studies are needed to determine whether folate from foods
alone or from supplements is associated with a greater reduction
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