How haem intakes affect human health?
In support of the previous studies findings, a meta-analysis of prospective cohort studies of colon cancer analyzed haem intakes. The analysis determined that 3/5 of the studies displayed a link between a high haem iron intake and an increase in colon cancer. The relative risk of colon cancer was 1.18 (95% CI: 1.06–1.32) for the participants in the higher haem iron group compared to the participants in the lowest haem intake group. This shows a significant but modest association between haem iron intake and development of colorectal cancer. However, the meta-analysis only included five cohort studies, suggesting the scope of studies was not broad enough to come to a definitive conclusion. Only 3/5 of the studies showed a significant positive association questioning the reliability of the results when just under half showed no link. This highlights the disadvantages of cohort studies in that a differential loss to follow up can cause bias – as colorectal cancer may develop later than the study continues for. The method to decipher haem intake also differed for each study, suggesting data collection was not consistent and therefore errors may be present across the studies in determining haem content.
In contrast, a few studies have found there to be no link between dietary iron intake or body iron levels and the risk of developing colorectal carcinoma.
Estimations of iron intake were calculated by averaging together mean daily intake at baseline and at the end of the study. The patients who had one or more recurring polyps between years 1-4 were determined as cases. Interestingly the results showed dietary iron had a strong inverse association with recurring adenomas. Similarly, red meat intake also displayed little association between intake and recurrence of adenomas. This suggests dietary iron did not increase the risk of developing recurring adenomas. This is an important find, as colorectal carcinoma is a gradual process which includes the formation and progression of adenomatous polyps into cancer. However, the association between dietary iron and recurring adenomas in this study could have been confounded by other diet and lifestyle factors. As an example, a high correlation of fiber intake suggests iron bioavailability may have been interfered and thus provided a protective quality to the study population. Also, the study results cannot be generalized as the subjects could only participate if they already had an adenoma removed. This means the results can relate only to the risk of recurring adenomas. The risk of developing the first adenoma may be different.