Bariatric Surgery Linked to Greater Colon-Cancer Risk

By Anne Harding

December 17, 2019

NEW YORK (Reuters Health) - Obese adults who undergo bariatric surgery may face a greater risk of colon cancer than their peers who don't have weight-loss surgery, a new study in a large Nordic cohort suggests.

"Individuals with obesity have an increased risk of colon cancer. While bariatric surgery results in substantial weight loss in a majority of patients with obesity, our study suggests that their risk of colon cancer remains increased and might even be higher than the risk of individuals with obesity who have not undergone bariatric surgery," Dr. Wenjing Tao of Karolinska Institutet at Karolinska University Hospital in Solna, Sweden, told Reuters Health by phone.

"Taking available scientific evidence into account, we believe this might justify heightened awareness among doctors of colon cancer in this patient group," she added. "For example, bariatric surgery patients presenting with new-onset bowel symptoms should probably undergo colonoscopy promptly to rule out cancer, and individuals with additional risk factors for colorectal cancer might be considered for screening interventions."

The increased risk of colorectal cancer (CRC) associated with obesity is well established, but some "surprising" evidence has linked obesity surgery to increased CRC risk, Dr. Tao and colleagues note in the International Journal of Cancer.

They studied more than 500,000 adults - virtually everyone diagnosed with obesity in 1980-2015 in Denmark, Finland, Iceland, Norway, and Sweden - including nearly 50,000 (9.9%) patients who had bariatric surgery. Most underwent gastric bypass.

During follow-up, 155 people (0.3%) in the bariatric-surgery group and 3,158 (0.6%) in the non-operated group developed CRC.

Overall colon-cancer risk was significantly increased in the patients who underwent bariatric surgery compared with the general population (standardized incidence ratio, 1.56).

Operated patients had a 13% higher risk of colon cancer than the non-operated obese group, a difference that did not reach significance, but had become significant by 10-14 years after surgery (hazard ratio, 1.55).

Rectal cancer alone was not significantly associated with bariatric surgery, but the risk appeared to grow over time.

Colon-cancer risk in obese people who did not have bariatric surgery was significantly higher than that of the general population (SIR, 1.31). However this risk did not increase over time, and was generally lower for non-operated patients than for bariatric surgery patients.

"A number of mechanisms that might explain an increased risk of colorectal cancer after bariatric surgery have been proposed by the literature," Dr. Tao said. "The two main mechanisms might be changes to the gut microbiome and increased exposure of the distal colon to secondary bile acids as a result of the anatomical alterations induced by common bariatric procedures, such as gastric bypass."

She and her colleagues plan to investigate whether bariatric surgery is associated with other types of cancer. They are also interested in studying "the impact of bariatric surgery on cancer survival in patients that have received a cancer diagnosis."

"Bariatric surgery results in drastic dietary changes with an increased risk of malnutrition and we are interested to know whether this can have an impact on cancer prognosis, especially of cancers in the gastrointestinal tract," the researcher noted.

While the new study is epidemiological, other research has supported the findings, Dr. Tao said. "For example, samples taken from the bowels after gastric bypass have indicated hyperproliferation of the mucosa cells," she explained.

"However, the scientific evidence is still limited at this stage and it is too early to say whether there should be any change in clinical practice for the bariatric surgery patient group as a whole, but we believe (as explained above) that it justifies heightened awareness," she concluded.

SOURCE: International Journal of Cancer, online December 3, 2019.