Commentary

The tendency to develop colorectal cancer is largely acquired, although genes play a part in some people. Age adjusted incidence rates vary 10 to 15-fold throughout the world, and people who move from low to high incidence areas, such as North America and Europe, acquire higher rates during their lifetimes. Observational studies suggest that calcium and fibre are protective along with other factors,¹ but this has been difficult to confirm because of the limitations of observational studies, especially the lack of ability to measure long term dietary exposure.

The randomised trial by Bonithon-Kopp et al ought to settle the matter. But does it? The trial has limitations, which are common even in the best preventive trials. The results are imprecise because enrolment was lower than the investigators had planned. The main outcome event was recurrence of adenomas, but a different set of factors may govern their growth, evolution to cancer, and distant spread. Many participants did not cooperate with their assigned treatment, leaving room for systematic differences among compared groups despite initial randomisation. Finally, 3 years of follow up is a short time in the natural history of this cancer, which usually takes at least 10 years to develop. With this study, the benefits of calcium supplementation look more promising, whereas fibre supplementation looks less promising. But that is a matter for investigators in this difficult field to tackle. Clinicians should still advise patients to consume ≥5 vegetables and fruits, a multivitamin, and calcium supplements each day. This practice may not only help to prevent colorectal cancer but also is advisable for many other health reasons.