Normal, underweight people may face higher death risk after surgery

Weight may be a risk factor for a higher risk of death soon after major surgery, but it may not be obesity that’s the problem. A study finds that people with a normal body mass index or who are underweight may be more likely to die after an operation.

The study, released online Monday in the journal Archives of Surgery, examined data on 189,533 surgeries of people whose likelihood of death was known. Among those patients, 3,245 (1.7%) died in the month following surgery. Researchers divided the patients into five groups based on their body mass index for comparison. One-fifth of all patients had a BMI of less than 23.1.

A BMI of 18.5 to 24.9 is considered normal weight, 25 to 29.9 is overweight, under 18.5 is underweight, and 30 and higher is considered obese.

The percentage of people who died who had a BMI of under 23.1 was 2.8%, but among those with a BMI of 35.3 or higher the rate was far less -- 1% -- even after adjusting for the risk of death associated with various types of surgery and for other health factors.

Those who had a BMI of less than 23.1 also had higher odds of dying compared with those who had a BMI ranging from 26.3 to 29.6. The study did not offer any theories about why normal and underweight people may be more at risk for dying following surgery.