This is the first prospective study to investigate the association of iron exposure with colorectal neoplasia using a heme iron database for the assessment of dietary iron, serum iron indices and variation in multiple genes involved in iron homeostasis. Serum TIBC and UIBC, indicators of low iron stores, were both inversely associated with colorectal adenoma, suggesting that low body iron stores are protective against colorectal adenoma and that iron may play a role in the early stages of colorectal tumorigenesis.
TIBC is commonly assessed in clinical settings as an indicator of iron load and is an indicator of levels of transferrin. Transferrin is the major iron binding and transfer protein in circulation and levels increase as serum iron availability decreases. The association between serum TIBC level and colorectal adenoma incidence was in the expected direction, indicating that as the concentration of transferrin increases (indicative of reduced iron levels), the risk of colorectal neoplasia decreases. Unlike other serum iron indices, such as ferritin, TIBC is not affected by the inflammatory response, and does not exhibit day-to-day variation in measurements (32
). UIBC is also a measure of the availability of iron binding sites on transferrin, and also increases as iron availability decreases. Prospective data on TIBC and UIBC in relation to colorectal neoplasia are limited. In contrast to our study, a nested case-control study of women reported no association between TIBC and colorectal adenoma (19
), although this was a study of pre-menopausal women, whose serum iron indices could be affected by menstrual cycle phase. The female participants of the current study were almost all post-menopausal, limiting the potential issues of iron loss through menstruation. Another study conducted in men reported a positive association between UIBC and colon cancer (33
In agreement with our investigation, a nested case-control study of women found no association between serum iron, ferritin, or transferrin saturation in relation to colorectal adenoma (19
); whereas a nested case-control study of men found these three serum markers to be inversely related to colon, but not rectal, cancer (33
). A potential limiting factor in the assessment of serum iron parameters in relation to colorectal cancer is that circulating iron levels may be affected by bleeding from sub-clinical lesions in the colorectum, which can deplete iron stores (34
). Although up to 91% of colorectal cancers bleed, only 6-14% of small adenomas (<10 mm) bleed, increasing to 11-47% of 10-20 mm adenomas, and 20-84% of >20 mm adenomas (34
). If iron is positively associated with colorectal neoplasia, such bleeding could bias the association toward the null or even reverse the direction. Given that most colorectal tumors can take more than 10 years to develop (38
), it is possible that even prospectively collected blood specimens could be subject to such bias if pre-malignant sub-clinical lesions, such as adenomas, were present. In the current study we sought to address this issue by investigating whether the development of colorectal adenoma is associated with significant temporal changes in iron levels. Although we observed small decreases in median levels of ferritin and TIBC over a 5-year time period, these changes were not statistically significant and were evident in both incident adenoma cases and in the controls who remained polyp-free. This finding suggests that bleeding and consequent iron losses in individuals with sub-clinical colorectal lesions may not be of concern.
The positive association we observed for red meat intake and colorectal adenoma is in agreement with previous studies (1
); in addition, calculations of population attributable risks in the large NIH-AARP Diet and Health study cohort estimated that if individuals in the highest quintile of red meat intake adopted intake levels equivalent to those in the first quintile of intake, the colorectal cancer burden would be lowered by 9% (39
). One of the hypothesized mechanisms relating red meat to colorectal carcinogenesis is the endogenous formation of NOCs, which is related to heme iron intake (10
). Previous studies have attempted to estimate heme iron intake by applying a standard factor to total iron intake from meat. Using such estimations, two studies reported no association between heme iron intake and colorectal adenoma (19
) nor cancer (13
); whereas two other studies found positive associations within the subgroups of males (12
) and proximal colon cancers (14
). The heme iron content of meat can not only vary by meat type, but also by cooking method and doneness level (40
); therefore, these previous estimations of heme iron likely lead to a substantial degree of error in risk estimates. Our study utilized a unique, newly developed, heme iron database to assign specific heme iron values according to meat type, cooking method and doneness level (24
). Despite this improved assessment tool, we did not identify a statistically significant association between heme iron intake and colorectal adenoma incidence. We note, however, that the serologic sample was relatively small and the power to detect modest associations may be limited.
Our study found no evidence for an association between variation in iron metabolism genes and colorectal adenoma, though we did observe significant relationships between several SNPs and serum iron indices-suggesting that circulating iron levels may be influenced by genetic variation in iron uptake and regulatory pathways. Most prior studies that have investigated iron homeostasis genes and colorectal neoplasia have focused on the HFE
gene, which is related to the iron overload condition hereditary hemochromatosis. Although a previous study found a relation between SNPs in HFE
and colorectal cancer risk, none of the investigations of colorectal adenoma have found such associations, including a study within the PLCO Cancer Screening Trial (18
). A limitation of the current study was possible low coverage of variation across the gene regions of interest and hence potential variants associated with colorectal adenoma may have been missed. However, when we subsequently investigated the extent to which our selected SNPs accounted for the total variation within these gene regions in HapMap, the coverage was greater than 75% for all genes except for HP01
. In addition, we note that recent genome-wide association studies of colorectal cancer did not reveal any statistically significant relationships between SNPs in iron metabolism gene regions and colorectal cancer, suggesting that germline variation within these genes does not contribute to colorectal tumorigenesis (42
The advantages of our study included the study of screening detected adenomas, which meant that the cases and controls were asymptomatically selected during a standardized procedure, and being precursor lesions, they were less likely to have been bleeding at the time of the baseline serum collection, which could bias the risk estimates. Furthermore, we used prospectively collected serum samples and dietary data, which included a unique heme iron database to more accurately assess intake of heme iron. We also conducted a small sub-study to evaluate the reliability of a one-time measurement to capture iron parameter levels by comparing serum iron indices in repeat specimens several years apart from the same individual. We found very good agreement for TIBC, UIBC and ferritin; however, the ICCs for iron and transferrin saturation were much lower, indicating that a one-time measurement is not likely to reflect average exposure levels and future investigations may wish to collect data from multiple time points to more accurately assess these factors. Our study also had several caveats, including limited power for detecting associations within genetic subgroups and the use of flexible sigmoidoscopy as the screening method meant that some of the controls could have had proximal adenomas out of reach of detection of the 60cm sigmoidoscope.
To conclude, both TIBC and UIBC were inversely related to colorectal adenoma. These findings are consistent with laboratory data that support a role for iron in the early stages of colorectal tumorigenesis. If real, the iron metabolic pathway may offer a potential route for chemopreventive strategies against colorectal cancer.