Most colorectal cancers develop from adenomas, which are precancerous growths that can take years to progress. New research shows that even years after colorectal adenomas are removed, people may retain gut microbial and metabolic changes that resemble colorectal cancer. 

The findings, published in Cell Host & Microbe, suggest that monitoring or reshaping the gut microbiota after adenoma removal could become part of future strategies to reduce colorectal cancer risk.

Scientists have known that removing colorectal adenomas lowers cancer risk, but people with a history of adenomas remain at higher risk than people who never had them. However, it’s unclear whether long-lasting changes in the gut microbiota and its byproducts might help explain this increased risk.

So, Ana Nogal at Harvard T.H. Chan School of Public Health in Boston, Massachusetts, and her colleagues analyzed stool samples from 354 women who previously had colorectal adenomas removed and compared them with 354 women who had no history of adenomas.

Cancer patterns

Women with a past adenoma had gut microbial patterns that differed from women who had no adenomas and resembled patterns seen in colorectal cancer. About 30 microbial species changed in the same direction in both women with adenoma history and those with colorectal cancer.

Microbes often associated with the tumor surface, such as certain Escherichia coli and Clostridium species, were detectable in women with past adenomas but not present at increased levels. Mouth-associated microbes often linked to colorectal cancer, including Fusobacterium nucleatum, Parvimonas micra, and Gemella morbillorum, were rarely detected in stool from women with past adenomas. 

The researchers also found 30 stool metabolites that differed between women with past adenomas and those with no adenomas. Several altered metabolites belonged to lipid-related pathways, especially sphingolipids, which are involved in cell signaling, inflammation, and gut barrier function. 

Higher risk

The researchers found that some microbes and metabolites were connected differently in women with past adenomas than in those with no adenomas. For example, Sellimonas intestinalis was linked with sphingolipids and Bilophila wadsworthia and Parabacteroides distasonis were linked with small protein fragments called dipeptides only in adenoma cases. 

The findings suggest that adenoma removal may not fully reset the gut ecosystem. Instead, some microbial and metabolic features linked to colorectal cancer may persist for years. 

Although the study cannot prove cause and effect, the authors say, “these insights help elucidate the persistently higher [colorectal cancer] risk in individuals with a history of adenoma and inform microbiome-based preventive strategies.”