Importance of faecal sample collection in prospective cohort studies to evaluate the effect of the gut microbiome on cancer development.

The gut microbiome has been linked to cancer in many cross-sectional studies. These studies compared individuals with and without cancer, but they do not establish causation. Prospective cohort studies, with faecal samples collected before a cancer diagnosis, will be instrumental in assessing the role of the gut microbiome in human cancer risk. We identified 18 prospective cohort studies, each of which collected faecal samples from at least 5000 individuals with expected future cancer linkages, and calculated the projected numbers of colorectal, stomach, pancreatic, and female breast cancers. Larger numbers of incident cancer cases need to be evaluated to identify robust associations between the gut microbiome and cancer with sufficient power. To achieve this goal, we propose that additional cohorts collect faecal samples, particularly those with participants from various backgrounds and from under-represented world regions. Furthermore, sufficient data sharing and improved methods are needed to pool and meta-analyse microbiome data. These data can then be used to gain a better understanding of how the gut microbiome is involved in cancer development and other health outcomes at multiple body sites.