An abundance of previous research has indicated a possible link between antibiotics and a heightened risk of developing bowel cancer. Now, a new study by Boston researchers suggests that long-term use of antibiotics can increase the risk of developing colon polyps, as a precursor to bowel cancer.

In order to learn more about the link, a research team from the Massachusetts General Hospital, Boston, led by Andrew Chan drew on data from the Nurses Health Study, an American programme that monitored the health of 121,700 nurses since 1976.

Strong link to proximal colon adenomas

Of this cohort of women, Chan combed through the health records of 16,642 women who were 60 years old or over in the year 2004. The women examined in the new study had had at least one colonoscopy between 2004 and 2010. During this period, 1,195 adenomas were newly diagnosed in the group.

They found that nurses who had taken antibiotics for two months or more, during their 20s and 30s, were 36% more likely to be diagnosed with adenomas in later life, compared with those who had not. The risk jumped to 69% for those in their 40s and 50s.

This association held true, irrespective of whether the adenoma was a high or low risk for bowel cancer, and was more strongly linked to proximal colon adenomas.

Women within their 20s to 50s who previously did not take antibiotics and then took the drugs for more than 15 days were of 73% at risk to be diagnosed with adenoma.

Gut microbiome a “plausible” factor

As this is an observational study and not a controlled experiment, no firm conclusions can be drawn about the cause and effect. Also, the researchers did not gather information on the type or formulation of antibiotic taken, and some adenomas might have been present before antibiotics were used.

Additionally, the study does not look at how many polyps had developed to become cancerous.

The authors acknowledged that the bacterial infections that the nurses suffered from might also play an important role. However, they say there is a “plausible biological explanation” for the patterns observed.

The findings add to emerging evidence that the type and diversity of bacteria in the microbiome may have a key role in the development of cancer.

As antibiotics fundamentally alter the gut microbiome, researchers wrote in the journal that this might have a “crucial role in the development of bowel cancer, added to which the bacteria that require antibiotics may induce inflammation, which is a known risk for the development of bowel cancer”.

“The findings, if confirmed by other studies, suggest the potential need to limit the use of antibiotics and sources of inflammation that may drive tumour formation,” the researchers concluded.

There is a general call for a limit to the use of antibiotics because of antibiotic resistance, which appears to stem from the over-prescription of antibiotics.

Antibiotics still do more good than harm

Sheena Cruickshank, a lecturer in immunology at the University of Manchester in Britain who was not involved in the study, said that it did not adequately investigate whether the diet of the women who took a lot of antibiotics contributed to the development of pre-cancerous lesions.

The study also did not consider the possibility that antibiotics in farm animals whose meat is consumed by people could play a role as well, according to the lecturer.

“Whilst the data adds to our growing knowledge of the importance of the gut bacteria to our health, I would be concerned about advising people to avoid using antibiotics,” she commented. MIMS

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