New research suggests that the presence of a certain kind of gut bacteria can increase the risk of bowel cancer by as much as 15%. The research is important in that it actually determines a causal effect – not merely an association – meaning that it suggests that certain bacteria in our guts may cause colorectal cancer. Lead author of the study Kaitlin Wade, Ph.D., from the UK’s University of Bristol, presented the research at the National Cancer Research Institute Cancer Conference in Glasgow.
In conducting the study, Wade and fellow researchers analyzed data from several large datasets – collected from the Flemish Gut Flora Project, the German Food Chain Plus study, the PopGen study, and the international Genetics and Epidemiology of Colorectal Cancer Consortium. In total, they compared the genomes of 124,218 individuals to attempt to determine causality between gut bacteria and colorectal cancer.
"Lots of studies in mice and humans have shown an association between the gut microbiome and bowel cancer," explains Wade, "but very few have provided convincing evidence for causality. In other words, it's really difficult to discern whether components of the gut microbiome can cause bowel cancer, whether the disease itself leads to variation in the gut microbiome, or whether the association is due to some other factors that cause variation in both."
The other factors that Wade is referring to include obesity, lack of physical exercise, smoking, high consumption of red meat and fried foods, as well as age, family history, and IBS. Along with gut bacteria, these factors will result in the diagnosis of more than 100,000 new cases of colon cancer and more than 44,000 new cases of rectal cancer in the United States in 2019, making the combined category of these cancers (called colorectal cancer), the third leading cause of cancer death in both men and women.
To confirm causality, the research team used a statistical method called Mendelian randomization that allowed them to pinpoint how certain bacteria influence health conditions, without having to involve actual individuals’ lives.
Wade explains, "With Mendelian randomization, we use people's natural, randomly inherited genetic variations, which alter levels of bacteria within the gut microbiome, in a way that mimics a randomized trial. In this way, we don't have to edit anyone's gut microbiome directly by giving antibiotics or probiotics in a randomized trial or waste time waiting to see whether people within the population get colorectal cancer. We just need studies that have already got this information measured," she explains.
This analysis determined that a specific kind of bacteria called Bacteroidales increased the risk of bowel cancer by between 2–15%, showing that the presence of Bacteroidales is greater in those people with colorectal cancer. Although this finding is promising, the researchers hope to continue their investigations, stating that more work is needed to broaden our understanding of gut bacteria’s influence on bowel cancer.