In this particular study, constipation was defined as “a bowel movement frequency of less than daily.”

Constipation, laxative use and risk of colorectal cancer: The Miyagi Cohort Study, European Journal of Cancer, September 2004

41,670 subjects, 7 year follow-up:

In conclusion, the findings in this population-based, prospective cohort study in middle-aged Japanese men and women support the hypothesis that constipation and laxative use increase the risk of colon cancer.

It was a modest association. The equation is apparently more complex. Harvard’s (NIH grant) Nurses Health Study (women) and the Health Professionals Follow-up Study (men), 2013 didn’t find an association.

Still, there’s a biologically plausible mechanism:

The hypothesis that constipation increases the risk of colorectal cancer is based on evidence suggesting several possible mechanisms. First, bile acids [28,29], fecapentaene-12 [30] and ammonium acetate [31], present in faeces are reported to have cancer-promoting effects. Second, constipation causes bowel epithelial cells to come into contact with faeces for longer.

This study didn’t find a link either:

Chronic constipation as a risk factor for colorectal cancer: results from a nationwide, case-control study, Clinical Gastroenterology and Hepatology, 2021

But this response letter mentioned several things they could have done to improve their findings. It’s almost as if they were fine in reporting a null association.

Comment to “Chronic Constipation as a Risk Factor for Colorectal Cancer: Results From a Nationwide, Case-Control Study”, Clinical Gastroenterology and Hepatology, 2021

I am reminded of this post I wrote on my old blog 14 years ago. A former editor-in-chief of the New England Journal of Medicine, Dr. Marcia Angell said:

“It is simply no longer possible to believe much of the clinical research that is published, or to rely on the judgment of trusted physicians or authoritative medical guidelines. I take no pleasure in this conclusion, which I reached slowly and reluctantly over my two decades as an editor of The New England Journal of Medicine.”

Colon cancer is big business, from the high-priced screenings to the lengthy and involved treatments. One would not want to threaten such a lucrative business by, I mean, suggesting people eat more fiber (or drink less alcohol).