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Thin and normal-weight people may have a higher risk of dying soon after surgery than patients who are overweight, a new study finds.

The reason for the link isn’t clear, but the findings suggest that BMI, or body mass index, could help identify patients who might be at greater risk after some procedures.

The researchers looked at data on 189,533 patients who underwent various general and vascular surgeries in 2005 and 2006. Among these patients, 3,245 died within 30 days of surgery. When researchers compared the patients by BMI, they found that those in the lowest quintile, with a BMI of less than 23.1 — in the thin to normal range — were 40% more likely to die 30 days after surgery than those in the middle quintile of BMI (26.3 to 29.6). The increased risk of death persisted even after researchers accounted for the type of surgery and for patients’ baseline risk of death.

By standard cutoffs, a BMI of 18.5 to 24.9 is considered normal weight, 25 to 29.9 is overweight, 30 or higher is obese, and less than 18.5 is underweight.

Among the patients with a BMI of less than 23.1, about 2.8% died soon after surgery, compared with 1.5% of those in the middle quintile of BMI and only 1% of obese patients in the top quintile, with a BMI of 35.3 or higher, the study found. Overall, people who underwent exploratory abdominal surgery were most likely to die (14%) and those who had breast lumpectomies were least likely (0.1%).

When researchers looked more closely at individual surgeries, however, they found that some procedures showed the opposite association with weight: a higher risk of death for patients with higher BMI. Those included surgeries to resect the bowel or form a colostomy, gall bladder removal, hernia repair, mastectomy, and surgeries to clean out wounds. The authors noted that this association between mortality and obesity ran counter to some previous studies, which found no such risk.

Again, the current study, published in the Archives of Surgery, couldn’t say why weight may affect death risk after an operation. “This is an interesting question, though, and something we should think more about,” lead researcher George Stukenborg, an associate professor in the department of public health sciences at the University of Virginia School of Medicine, told HealthDay.