WEDNESDAY, Aug. 23, 2023 (HealthDay News) -- It's long been known that obesity is tied to increased cancer risk, but can weight loss after bariatric surgery help lower a person's odds for the disease?

The surgeries have now been around long enough for researchers to finally study the link. And a study involving 40 years of follow-up now confirms that, for women at least, weight-loss surgery helps lower long-term risks for cancer.

“Certainly, there's evidence that bariatric surgery, which does result in significant weight loss and sustained weight loss, does reduce the risk for cancer,” said lead researcher Ted Adams, from the Intermountain Surgical Specialties/Digestive Health Clinical Program and adjunct professor in internal medicine at the University of Utah School of Medicine. This is a continuation of studies looking at the impact of bariatric surgery and its resulting weight loss on health.

The reasons why bariatric surgery may reduce cancer risk can’t be confirmed from this study, but Adams has some ideas.

“There's a possible hormonal mechanism. At least the cancers that were significantly lower for the bariatric surgery patients compared to the non-surgery patients were mostly female-related cancers -- uterine, ovarian and breast cancer,” Adams said. “Although we didn't look at mechanism, this would certainly suggest that that's part of the reason.”

Estrogen in particular may be a driver, suggested Dr. Christina Annunziata, senior vice president for extramural discovery science for the American Cancer Society (ACS). Annunziata was not involved in the study.

While a primary source of estrogen is the ovaries, in people who are obese there is extra estrogen because of the fat tissue, she said.

“So, getting rid of the fat tissue essentially could potentially lower that hormonal conversion of estrogen,” Annunziata said.

When people lose weight there is also a decrease in insulin, another hormone.

The study also found a reduction in colon cancer. While that one isn’t hormone-driven, that finding may be related to reduction of body fat, Adams said.

To study all of this, researchers compared cancer incidence and death by stratifying obesity- and non-obesity-related cancers, as well as a patients’ gender, cancer stage and the procedure.

They compared more than 22,000 bariatric surgery patients retrospectively with obese patients who did not have bariatric surgery using the Utah Population Database, the Utah Cancer Registry, birth and death certificates and driver license information. They also used three Utah bariatric surgery registries that included patients who had gastric bypass, gastric banding, sleeve gastrectomy and duodenal switch procedures.

The group that had surgery had a 25% lower risk of developing any cancers. Female surgery patients had a 41% lower risk for developing obesity-related cancers.

Male bariatric surgery patients did not show a lower cancer risk.

The cancers for which researchers saw significant reduction in risk were uterine, ovarian and colon, as well as both pre- and post-menopausal breast cancer.

Adams also noted that these cancers were detected at earlier stages in the bariatric surgery patients.

The message for patients and providers is that they should sit down together to explore the pros and cons of bariatric surgery given their personal health and well-being, he said.

“I think a unique point to this study is that not only did we show cancer prevention that has reduced the risk of developing cancer, but also appears to be cancer being detected at earlier stages,” Adams said. “I think that's an important point, too.”

Adams called for more research on the reasons why bariatric surgery appears to make a difference in cancer prevention.

He noted that it may not only be overall weight loss that has an impact on cancer prevention, but something else that happens because of the surgery, such as altering the anatomy of the intestinal tract and how that impacts the release of hormones.

The numbers of patients and the long-term follow-up that were part of the study are really meaningful and have the potential for impact, Annunziata said.

She also noted that the study is retrospective, which can mean that the populations on the surgery and non-surgery side may not be balanced, even when efforts are made to match the two groups.

“My biggest concern would be that the patients who are healthy enough to get surgery and undergo surgery and survive the surgery may possibly have different health status going in than the people who didn't get surgery,” Annunziata said.

A randomized study could help answer this question, she said. That trial could also compare the impact of new weight-loss drugs, such as Ozempic, which haven’t been in use long enough for this long-term research, she said.

“I think the stance [from the ACS] would be that this is a very promising direction to go in, but we probably need more data in order to make a formal recommendation,” Annunziata said.

The study findings were published Aug. 22 in Obesity, the flagship journal of The Obesity Society.

More information

The U.S. National Institutes of Health has more on weight-loss surgery.

SOURCES: Ted Adams, PhD, MPH, Intermountain Surgical Specialties/Digestive Health Clinical Program, adjunct professor, internal medicine, and adjunct associate professor, nutrition and integrative physiology, University of Utah School of Medicine, Salt Lake City; Christina Annunziata, MD, PhD, senior vice president, extramural discovery science, American Cancer Society; Obesity, Aug. 22, 2023