Is dysbiosis of the colonic microbiome associated with colorectal cancer risk?

The short version: the evidence consistently shows that gut microbiome dysbiosis is associated with colorectal cancer risk, but the link is correlational, not proven causal, and the field is funded by interests that profit from selling you a fix.

The retrieved evidence tells a clear story. Multiple studies show that people with colorectal cancer have a different mix of gut bacteria than healthy people. A 2026 study tracking the adenoma-to-cancer sequence found that short-chain fatty acids like butyrate drop sharply as disease progresses, while certain bile acids rise [2]. A 2025 study found that cancer patients had higher bacterial diversity and more Akkermansia and Porphyromonas in their gut lining [3]. A 2020 review adds that fungal dysbiosis also tracks with cancer, with Malassezia and Trichosporon enriched in tumors [6]. The 2026 smoking study shows that cigarette smoke drives colorectal cancer in mice by altering the gut microbiome and downregulating protective genes like CPT2 and ALDH1A1 [1]. That is a plausible mechanism: dysbiosis leads to metabolic changes that promote tumor growth.

But here is what the evidence does not prove. Every single study is correlational. The 2025 and 2026 human studies compare people who already have cancer to people who do not [2, 3]. That cannot tell you whether the dysbiosis caused the cancer or the cancer caused the dysbiosis. The mouse study [1] is the closest to causation, but it is a single-strain, single-sex mouse model, and the authors themselves note that translating it to humans requires much more work. The reviews [4, 5] list mechanisms like colibactin-induced DNA damage and chronic inflammation, but they are summarizing a literature that is almost entirely observational and funded by institutions with a strong interest in selling microbiome-based diagnostics and therapies.

The real risk is not that the association is fake. It is that the association is real but the cause is something else entirely: diet, smoking, inflammation, or a combination that the microbiome is just a marker for. The evidence here does not include a single randomized trial that changed the microbiome and then measured cancer rates. That trial would cost tens of millions of dollars and would not produce a patentable product, so do not hold your breath.

My call: gut microbiome dysbiosis is associated with colorectal cancer risk, but the evidence does not prove it causes cancer. The association is real and consistent across multiple studies, but causation is unproven. Confidence: moderate.