People who are under or on the slightly lighter side of normal weight appear to have a higher risk of death in the 30 days following surgery than people who are on the heavier side of normal or overweight, according to a new study published online this week in the Archives of Surgery journal.

First author Dr Florence E. Turrentine, of the department of surgery at the University of Virginia, in Charlottesville, USA, and colleagues, examined the link between patients' BMI and their risk of dying in the first 30 days following surgery.

(BMI stands for Body Mass Index, the ratio of a person's weight in kilos to the square of their height in metres. A BMI of 18.5 to 24.9 is classed as normal weight, 25 to 29.9 is overweight, under 18.5 is underweight, and 30 and higher is classed as obese.)

Their aim was to help inform the growing number of surgeons who are caring for an increasing population of obese patients in the US.

For the study they used participant data from the database of the American College of Surgeons National Surgical Quality Improvement Program. They included 189,533 cases from 183 sites of patients who had undergone major general and vascular surgery reported in 2005 and 2006 and where there was sufficient data to calculate overall probabilities of death. Of these, 3,245 patients (1.7%) died within 30 days of their surgery.

Turrentine and colleagues used established tools to test the statistical significance of links between BMI and rates of death among the participants, adjusting for other known influences such as differences between patients and operating procedures.

They then ranked the results according to BMI, using the middle 20% (BMI 26.3 to under 29.7) as the reference point. Their analysis found that patients with a BMI under 23.1 showed a significant increased risk of death, with 40% higher odds compared with patients in the middle 20% range.

The researchers concluded:

"Body mass index is a significant predictor of mortality within 30 days of surgery, even after adjusting for the contribution to mortality risk made by type of surgery and for a specific patient's overall expected risk of death."

They did not speculate on why being overweight or on the heavier side of normal weight appears to protect patients from a higher risk of death following surgery.

Written by Catharine Paddock PhD