Bariatric surgery was associated with a significant reduction in the risk of colorectal cancer among obese adults in a retrospective study of more than 1 million individuals.

Although some studies have suggested that bariatric surgery may reduce the risk of obesity-associated cancers, such as colorectal cancer, other studies have shown an increased colorectal cancer risk after surgery, according to Laurent Bailly, MD, of Université Côte d’Azur in Nice, France, and colleagues.

In a study published in JAMA Surgery, Dr. Bailly and colleagues compared the incidence of colorectal cancer in obese patients who underwent bariatric surgery with the incidence in obese patients who did not have surgery and the incidence in the general population.

Using the French National Health Insurance Information System database, the researchers identified 1,045,348 obese adults aged 50-75 years who had no colorectal cancer at baseline. Of these patients, 74,131 underwent bariatric surgery and 971,217 did not. The mean age was 57.3 years in the surgery group and 63.4 years in the nonsurgery group.

The mean follow-up period was 6.2 years for patients who underwent adjustable gastric banding, 5.5 years for those with sleeve gastrectomy, 5.7 years for those who underwent gastric bypass, and 5.3 years for the nonsurgery group.

Results

Overall, the colorectal cancer rate was 0.6% in the surgery group and 1.3% in the nonsurgery group (P < .001).

The researchers calculated standardized incidence ratios (SIRs) to compare the risk of colorectal cancer in the study population with the risk among the French general population; in other words, the number of observed colorectal cancer cases divided by the number of expected cases.

In the surgery group, 423 cases of colorectal cancer were observed and 428 cases were expected, which leads to an SIR of 1.0. In the nonsurgery group, 12,629 cases were observed and 9,417 cases were expected, leading to an SIR of 1.34.

These results suggest patients in the nonsurgery group had a 34% higher risk of colorectal cancer compared with the general population, whereas the risk in the surgery group was similar to that in the general population.

Patients who underwent either gastric bypass or sleeve gastrectomy had fewer new colorectal cancer diagnoses (0.5% for both) compared with patients who had adjustable gastric banding (0.7%).

The researchers noted that this study was limited by several factors, including the retrospective, observational design and potential selection bias among surgery patients. However, the results were strengthened by the large study population and long-term follow-up.

Putting results into context

The authors of an invited commentary noted that this study is supported by results from a retrospective, U.S.-based study, which indicated that bariatric surgery has a “protective effect” against colorectal cancer (Ann Surg. 2019 Jan;269[1]:95-101).

However, these results conflict with other retrospective studies. A study of Nordic patients suggested that bariatric surgery is associated with an increased risk of colon cancer but perhaps not rectal cancer (Int J Cancer. 2019. doi: 10.1002/ijc.32770).

And a study of English patients showed an increased risk of colorectal cancer in patients who underwent gastric bypass but not in those who underwent gastric banding or sleeve gastrectomy (Br J Surg. 2018;105(12):1650-7).

These conflicting results “imply that the jury is still out on whether bariatric surgery increases or decreases” the risk of colorectal cancer, the commentators wrote. They added that future studies “must account for differences in study population (i.e., race/ethnicity and national origin), mechanistic variation in bariatric surgical type, and length of follow-up, while also distinguishing between rectal and colon cancer before the case is settled.”

This study had no outside sponsorship, and the researchers and commentators had no financial conflicts to disclose.

SOURCE: Bailly L et al. JAMA Surg. 2020 Mar 11. doi:10.1001/jamasurg.2020.0089; Davidson LE et al. JAMA Surg. 2020 Mar. 11. doi:10.1001/jamasurg.2020.0090.