According to a recent study published in the journal Gut, long-term use of antibiotics during adulthood increases the likelihood of developing precursors to bowel cancer. The research, once again, underlines the vital role of gut bacteria.

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New research points to links between antibiotics, the microbiome, and bowel cancer risk.

In 2017, there will be an estimated 95,520 new cases of colon cancer and 39,910 new cases of rectal cancer in the United States.

With the exception of skin cancers, bowel cancer is the third most common cancer in the U.S.

Risk factors include a lack of physical activity, a low intake of fruits and vegetables, being overweight or obese, and alcohol consumption. A new study, published this week, may add long-term antibiotic usage to this list.

Links between antibiotics and a range of conditions have come to light over recent years, including irritable bowel disease, celiac disease, and even obesity.

This connection between antibiotics and disease is thought to be due to the effect of antibiotics on the makeup of gut bacteria (the microbiome); by altering the numbers and types of bacteria present in the gut, metabolic or pathological processes may be triggered.

Some studies have also hinted that antibiotic usage could be linked with bowel cancer, but previous studies have only consisted of relatively short monitoring periods.

A group of researchers recently set out to look at this potential interaction in more detail. They used data from the Nurses Health Study, a project that has monitored 121,700 U.S. nurses since 1976. The women were aged between 30 and 55 when the study began.

Every 2 years, the participants fill out detailed questionnaires documenting general demographic information, lifestyle factors (such as smoking and exercise), medical history, and disease development. Every 4 years, they also complete a questionnaire regarding their dietary habits.

In the present study, the researchers used data from 16,642 of these women, who were aged 60 or older in 2004. This group of women were able to provide information about antibiotic use between the ages of 20 and 59 and had undergone at least one colonoscopy (a type of bowel examination) between 2004 and 2010.

Across the investigation period, 1,195 adenomas were diagnosed in the group. Adenomas, also known as polyps, are benign tumors that precede most cases of bowel cancer.

Once the data were analyzed, the team saw that antibiotic use within the last 4 years was not associated with bowel cancer, “but long-term use in the past was.” Individuals who had taken antibiotics for 2 months or longer in their 20s or 30s were 36 percent more likely to be diagnosed with adenoma when compared with those who had not taken an extended course of antibiotics.

This link remained significant regardless of whether the adenoma was classed as high- or low-risk for bowel cancer. However, the association was stronger for growths located in the proximal rather than the distal colon.

The proximal colon is the first section of the colon, connected to the small intestine, and it consists of the caecum, ascending colon, hepatic flexure, transverse colon, and splenic figure. The distal colon is the section that connects to the rectum and comprises the descending and sigmoid colon.

The data also showed that women who had taken a course of antibiotics for at least 2 months during their 40s and 50s were 69 percent more likely to receive an adenoma diagnosis, compared with individuals who had not taken antibiotics for an extended period of time.

Once again, this association was present irrespective of whether the adenoma was considered high- or low-risk and were more strongly linked with adenomas in the proximal colon.

Similarly, when women who had not taken antibiotics in their 20s to 50s were compared with individuals who had taken them for more than 15 days between the ages of 20 and 59, there was a 73 percent increased risk of adenoma diagnosis.

Of course, further studies will be necessary to confirm the findings; although the study was large-scale, there are some shortcomings. The study is observational, making firm conclusions about cause and effect difficult to draw. Furthermore, some adenomas may have been present before antibiotics were used. It is also important to note that bugs that require antibiotics often cause inflammation in the gut, which in itself is a known risk factor for bowel cancer.

Although more work will need to be done, there is a plausible biological explanation for the potential link between antibiotics and bowel cancer. Antibiotics significantly alter the microbiome by depleting certain types of bacteria and changing the overall makeup of the gut flora.

This is backed up by earlier research that found lower levels of certain bacteria and higher levels of others in bowel cancer patients.

As the authors conclude: “The findings, if confirmed by other studies, suggest the potential need to limit the use of antibiotics and sources of inflammation that may drive tumor formation.” Because usage of antibiotics in the U.S. is on the rise, this area of study is all the more vital. The negative consequences of these drugs should be thoroughly probed.

Learn more about the link between gut bacteria and colorectal cancer.