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Weight loss alone did not explain the cardiovascular and survival benefits of bariatric surgery in people with obesity and type 2 diabetes, according to an observational study.

Risk of major adverse cardiovascular events (MACE) over nearly 5 years -- a composite of all-cause mortality, coronary artery events, cerebrovascular events, heart failure, nephropathy, and atrial fibrillation -- decreased after approximately 10% of weight was lost among surgical patients and approximately 20% in nonsurgical controls.

Risk of all-cause mortality started to fall at around 5% weight loss after metabolic surgery and 20% weight loss without surgery, Ali Aminian, MD, of the Cleveland Clinic, and colleagues reported in .

"Furthermore, in our analysis, the effect of surgery was still present after accounting for weight loss, which may suggest the presence of weight-independent beneficial effects of metabolic surgery on MACE and survival," the investigators wrote.

These benefits may be related to post-surgical , such as changes in the secretion of gut-derived hormones, bile acid metabolism, intestinal nutrient sensing and glucose utilization, and the gut microbiome, Aminian and colleagues suggested.

The study adds evidence to the ongoing debate on the mechanisms of cardio protection after surgery, according to Dan Azagury, MD, of Stanford University in California, who was not involved with either study.

"I think this study nicely demonstrates that the benefits of surgery go way beyond just weight loss and that even with small to moderate amounts of weight loss after surgery, patients will benefit from cardiovascular improvements," Azagury told MedPage Today.

David Cummings, MD, of the University of Washington in Seattle, called the study "yet another piece of high-quality evidence" pointing to the health benefits of metabolic surgery going beyond weight loss alone.

"For example, a very large, high quality body of evidence indicates that the remarkable remission of type 2 diabetes that occurs in most cases after metabolic surgery results from numerous, powerful weight-independent anti-diabetes mechanisms that are engaged by these operations, in addition to the secondary consequences of weight loss," he said.

Bariatric surgery has also been linked to less need for hypertension medications.

Overall, the study seems to challenge another group's report that the metabolic benefits of gastric bypass surgery and diet can be attributed to weight loss alone.

"Quite a few senior investigators I'm aware of are currently writing rebuttals to the conclusion of that publication, pointing out its limitations, and highlighting the huge evidence base that it contradicts," Cummings said.

Aminian's team previously reported that metabolic surgery was associated with improved cardiometabolic health compared with usual care in an observational matched-cohort study of people with obesity and type 2 diabetes.

For this secondary analysis of that study, the authors included 7,201 people (median age 54.6 years, 65.5% women, median BMI 42.7). Each of 1,223 patients who underwent metabolic surgery in the Cleveland Clinic Health System was propensity-matched to five nonsurgical patients. Median length of follow-up was 4.9 years. Minimum weight loss thresholds were modeled using weight loss data in the first 18 months after usual care or bariatric surgery.

"I don't think these thresholds are ones we would use in clinical practice, but rather, this confirms the emphasis on health improvements yielded by bariatric surgery," commented Azagury.

The retrospective nature of the study left room for potential coding errors, and the authors could not account for causes of death, they acknowledged.

"More importantly, we do not know whether patients in the nonsurgical group, unlike the surgical patients, were exposed to any intentional weight loss intervention," according to Aminian's group.

"Therefore, the observed weight loss in the nonsurgical group 8 to 18 months after the index date could be unintentional and secondary to development of a disease condition. This may be an alternative explanation for not observing any CV [cardiovascular] and survival benefits in our nonsurgical patients ... until losing >20% of body weight," the authors cautioned.

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