New findings presented at UEG Week 2025 suggest that microplastics could alter gut acidity and microbial composition, triggering patterns linked to colorectal cancer and other health issues.

As concerns grow over plastic pollution invading every corner of the planet, scientists are now looking inward — into the human gut — to understand how microscopic plastic fragments might be quietly impacting our health.

A new study presented at United European Gastroenterology (UEG) Week 2025 in Vienna has revealed that microplastics can alter gut pH levels and microbial composition, creating patterns resembling those seen in colorectal cancer and depression.

Tiny Plastics, Big Impact

Microplastics — defined as plastic particles smaller than 5 millimeters — are now ubiquitous in water, food, and even the air we breathe. These particles can enter the body through ingestion or inhalation and are increasingly being detected in human stool, blood, and even placenta samples.

The new research, conducted under the microONE project led by CBmed GmbH in Graz, Austria, sought to investigate how microplastics interact with the human gut microbiome — the community of bacteria that plays a vital role in digestion, metabolism, and immune function.

Researchers used stool samples from healthy volunteers to create bioreactive cultures, which were then exposed to five commonly encountered microplastics — polystyrene, polypropylene, low-density polyethylene, polymethyl methacrylate, and polyethylene terephthalate — at realistic and elevated exposure levels.

The results were striking: microplastic exposure significantly lowered the pH of the cultures, indicating an increase in acidity and a disruption in normal microbial metabolism.

Gut Disruptions Resembling Disease Patterns

The study found that these pH changes coincided with alterations in bacterial composition, particularly within a group of gut bacteria called Bacillota. Such microbial shifts, researchers noted, mirrored those previously observed in people with colorectal cancer and depression, though they differed from changes linked to irritable bowel syndrome (IBS) or Parkinson’s disease.

“Microplastic particles were reported to increase acidity in stool microbial cultures and to shift the microbial community structure in polymer-dependent ways,” said Dr. Heather A. Leslie, an independent scientist specializing in microplastic risk assessment. “This supports the hypothesis that microplastics do not go unnoticed by microorganisms in the microbiome. We do not want people inadvertently microdosing microplastics if these are interfering with the complex balance of gut bacteria that their health depends on.”

The researchers also found changes in metabolic byproducts, such as valeric and lactic acids, which are linked to gut health and immune regulation. However, the total number of bacterial cells remained relatively stable.

Implications for Colorectal Cancer Research

Colorectal cancer (CRC) — one of the most common cancers globally — has been closely associated with microbial imbalances and altered gut acidity. The findings from the microONE project suggest that microplastics may play a subtle but concerning role in creating these disease-like microbial patterns.

“Microplastic particles show an impact on the microbiome, but further research is needed to specifically look into health implications, particularly in relation to human host factors such as the immune system,” said Christian Pacher-Deutsch from the University of Graz’s Department of Gastroenterology and Hepatology, one of the study authors. “Disruptions in the microbial balance by microplastics could have multiple implications. Continued research and biomonitoring are crucial steps forward.”

Limitations and the Road Ahead

Despite the compelling data, the researchers caution against drawing definitive conclusions. The study involved stool samples from only five individuals, limiting its generalizability. Moreover, the exposure period lasted just five days, leaving questions about the long-term effects of chronic microplastic ingestion unanswered.

The findings have not yet been peer-reviewed or published in a scientific journal, but the research team plans to expand its work to include more participants and explore the influence of diet and lifestyle on microplastic–microbiome interactions.

Why It Matters

While this study stops short of establishing a direct causal link between microplastics and colorectal cancer, it adds to growing evidence that plastic pollution is not just an environmental crisis but a potential public health threat.

Scientists worldwide are urging stricter regulation of microplastic emissions, better waste management, and continued public awareness to reduce exposure through food and water.

As researchers race to uncover the full picture, one thing is becoming increasingly clear — what we throw away as plastic waste might eventually come back to harm us from within.