In this prospective, population based study, we report a bell‐shaped association between plasma folate concentrations and CRC risk. This observation is in contrast with considerable evidence supporting a protective effect for folate in CRC development.1,7
Results also indicated a reduced CRC risk associated with the MTHFR 677C>T polymorphism that was present even in subjects with an inadequate folate status.
This is only the second prospective CRC study to incorporate data on plasma folate levels as well as both the MTHFR 677C>T and 1298A>C polymorphisms.27
It is also the largest prospective study of circulating levels of folate and risk of CRC. Greater power would, however, have been desirable for the subgroup analyses. Although we were unable to control for a number of covariates, including other dietary intake variables, aspirin or non‐steroidal anti‐inflammatory drugs, and hereditary CRC, factors such as folate degradation during storage and seasonal fluctuations in folate status were accounted for by the matching process. Levels of red blood cell folate provide a more stable measure of folate status than do plasma concentrations but in large epidemiological studies a single measurement of plasma folate per subject appears to be adequate.35
The main strength of this study was its prospective design, which minimised the problems of selection bias and reverse causation. Given the slow growth of CRC, undiagnosed tumours in the case group at baseline almost certainly existed. However, excluding subjects with follow up times less than one year did not materially affect risk estimates (data not shown).
Our results seem to suggest a reduced CRC risk for subjects with the lowest folate levels (table 2). Although studies have provided some evidence that supraphysiological folate levels may enhance colorectal tumorigenesis,36,37,38
a cancer preventative effect for folate deficiency seems less likely. However, the MTHFR 677C>T polymorphism, which limits the accessibility of folate for methylation reactions, has been reported to decrease the risk of C→T transitions in the p53 gene.39
These mutations occur particularly often at methylated CpG island “hotspots” and are a common event in colorectal tumorigenesis. A decrease in the number and size of induced colorectal tumours in folate deficient rats has also been observed40
but a review by Kim suggested that the quality of the test and control diets rather provides further evidence for a carcinogenic effect of supraphysiological folate intakes.7
Recently, the idea that folate might protect against CRC development but aid the progression of an established neoplasm has gained attention.7,41,42
In the present study, however, a strong positive association between folate and CRC risk was apparent in subjects with follow up times over the median of 4.2 years, who would likely have had a lower prevalence of undiagnosed CRC at baseline (table 3). Folate might be speculated to promote the transition from adenoma to carcinoma, a serious consideration given the high prevalence of adenoma in western populations, including Sweden.43
However, this should also have resulted in a positive risk association in the subgroup with shorter follow up times. Of course, the observation may simply be a chance finding. It is none the less interesting and may warrant further study.
Plasma folate concentrations were considerably lower in the present study than in the two previous prospective reports of an inverse association between serum or plasma folate status and CRC risk, despite blood samples in those studies having been collected before the implementation of mandatory folic acid fortification in the USA.11,13
For example, in the New York University Women's Health Study,13
the lowest quartile cut off was comparable with our highest quintile cut off. Although our study included a greater proportion of fasting blood samples, we observed no variation in plasma folate concentrations by fasting duration. However, in northern Sweden, consumption of fruits and vegetables, major sources of folate in populations without fortification of foods, has been reported to be among the lowest in Europe.44
Our study population may thus be better suited to investigating the effects of low plasma folate concentrations on CRC risk. The low folate status of the study group may also have precluded detection of a protective effect for high plasma folate concentrations. The observed bell‐shaped, rather than linear, relationship between plasma folate concentrations and CRC risk supports this interpretation.
In the prospective Alpha‐Tocopherol Beta‐Carotene Study of male smokers in Finland,12
both plasma folate concentrations and results were comparable with those of the present study. Thus an ethnic explanation for the discrepancy between our findings and the two previous American studies cannot be excluded.11,13
In contrast with previous reports suggesting a possible positive association between homocysteine and CRC risk,13,45
no clear relationship was observed in the present study. This may have been due to the relatively high homocysteine levels of the study subjects or possibly to confounding by folate, although including both plasma folate and homocysteine concentrations in the same multivariate model did not materially affect risk estimates (data not shown).
The MTHFR 677C>T polymorphisms were associated with a decreased CRC risk in this study, which is in line with several previous reports,11,17,18,19,20,21,22
However, as demonstrated in fig 1, we found no evidence that this risk reduction might be eliminated or reversed when folate status is inadequate. This contradicts some findings,11,21,22
including those of Ma et al
, in which a similar figure was presented, but not other large dietary studies.23,28
Although ours may be a chance finding due to the low number of TT homozygous subjects, it underscores the need for further investigation of the gene‐nutrient interaction.
Our observation of an increased CRC risk in subjects with the MTHFR 1298A>C polymorphism contradicts a number of previous reports of a decreased, albeit seldom statistically significant, CRC risk associated with 1298CC homozygosity,17,18,21,23,27,28
but supports one large study.20
Based on the interaction analysis in table 4, it seems the association between MTHFR 1298A>C and CRC risk may have been largely due to the strong linkage disequilibrium with the MTHFR 677C>T polymorphism.
In light of accumulating evidence that dosage and timing may be critical to the effect of folate in carcinogenesis, concerns have been raised about the consequences of folic acid fortification on cancer incidence.46
Supporting this are the results of the recent Aspirin‐Folate Polyp Prevention Trial, in which folic acid supplementation was associated with a statistically significant 44% increase in the number of recurrent adenomas.36
Our observations, which provide additional evidence for a detrimental component to the role of folate in colorectal tumorigenesis, may thus have implications in the ongoing debate in Europe concerning mandatory folic acid fortification of foods.
In conclusion, in this nested case referent study, plasma folate concentrations were associated with CRC risk in a bell‐shaped manner. In subjects with longer follow up times, plasma folate concentrations were strongly positively related to CRC risk. The MTHFR 677C>T polymorphism was associated with a reduced risk of CRC that was independent of folate status. We speculate that inadequate folate status in the background population allowed us to investigate associations at the lower end of the biological folate spectrum. Without negating the possibility of a protective effect for high folate levels, our findings suggest that low folate status may reduce the risk of CRC.