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AUSTRALIAN experts say the risks of surgery for obese people are often overstated, with new research showing that postsurgery mortality rates are much higher among low-to-normal weight patients than among the obese.
A large study, published in the Archives of Surgery, found that a low body mass index (BMI), not a high one, was the biggest predictor of mortality within 30 days of major surgery. Patients with a BMI of less than 23.1 had a 40% higher risk of death than overweight patients with a BMI of between 26.3 and 29.7. (1)
The study, which looked at almost 190 000 surgical cases in the US, found the 30-day mortality rate among patients with a BMI of less than 23.1 (2.8%) was almost three times that of obese patients with a BMI of 35.3 or greater (1%).
In healthy people, a BMI of less than 18.5 is classed as underweight, 18.5–25 is classed as normal weight, 25–30 is considered overweight and over 30 is considered obese.
Associate Professor John Dixon, an obesity researcher at the Baker IDI Heart and Diabetes Institute and head of the Obesity Research Unit in the department of general practice at Monash University, said the results came as no surprise to him.
“It’s exactly as we’d expect. People have been saying surgery is highly risky for obese people for a long time and we’ve been trying to tell them it’s not”, he said.
“We’re not saying obese people aren’t at risk of premature death — they are. But when it comes to putting them through a surgical procedure, there just aren’t those horrendous risks people talk about. We’ve got to be careful that we don’t discriminate against obese people because of perceived risk.”
Professor Paul O’Brien, head of the Centre for Bariatric Surgery at Monash University, said, contrary to popular opinion, obese people tended to be quite fit, due to the amount of extra weight they had to carry.
“Obesity is a health risk in the medium term — their bodies will wear out, but this study is telling us that maybe we shouldn’t overstate the immediate, short-term risk”, he said.
Professor O’Brien said the study did not necessarily show that underweight people in general were at higher risk of death.
“This doesn’t imply that being relatively thin or normal is bad, but that unintentional weight loss is a sign of illness. In the lower weight group, those who lost weight recently would be suffering from protein malnutrition which is associated with postoperative mortality”, he said.
However, an analysis of individual types of procedures in the study did show that obesity was related to a higher risk of mortality after certain procedures, including colorectal resection, colostomy formation, cholecystectomy, hernia repair, mastectomy and wound debridement.
Another study by Australian researchers has found that obesity did not adversely affect patients’ emergency department experience, assistance required or length of stay.
The study of 700 patients attending a Melbourne hospital emergency department found that obese patients had a lower mortality rate than non-obese patients, but they were also significantly younger: on average 63 years old compared to 70 years old for the non-obese.
The author of the study, Professor David Taylor, director of emergency and general medicine research at the Austin Hospital in Melbourne, presented the research last week at the annual scientific meeting of the Australasian College for Emergency Medicine held in Sydney. He said previous research showed obese patients did place extra pressure on emergency department staff, but this study showed this did not affect the patient experience.
“With this study we looked at things from the patient perspective. We thought maybe they were suffering in the emergency department from increased waiting time and overinvestigations, and generally what we thought might be happening, wasn’t.”
– Amanda Bryan
Posted 28 November 2011
This was a difficult study to encapsulate in a brief news story but it’s great to see that it is generating a robust discussion. In respect to two of the questions raised I will quote directly from the study’s methods.
The problem of healthy vs obese study population definitions was addressed in the following way:
“Standard thresholds of BMI, obtained from analysis of data collected from large populations of generally healthy adults, classify individuals into the following groups: underweight (under 18.5), normal (18.5 to 25), overweight (25 to 30), and obese (30 or over). However, these threshold values are not directly relevant to our population of patients who underwent major surgery because the overall distribution of values is different from that of the general population. Instead of these standard thresholds, our study identified the actual quintiles of the distribution of BMI values in the study population”. These are the values you see quoted in our news story.
Comorbidities were accounted for in the following way:
“Baseline differences in mortality risk were measured for each patient in the study population using the ACS NSQIP probability of 30-day mortality risk score. The score is calculated using values for more than 30 demographic characteristics, comorbidities, and preoperative laboratory values measured for each patient.”
I would encourage anyone who is interested in the detail of this study to access the full version.
Within the limits imposed by having read only this MJA article and the study abstract online, it remains concerning that there seems little or inadequate consideration of the many confounders that would plague a study such as this: firstly, that healthy slender persons do not so often find themselves candidates for major procedures until they are advanced in age, and, secondly, that many chronic disease processes are associated with weight loss pathological in origin.
From the abstract we see little clue as to how the authors have managed such concerns. For results to be meaningful the study groups would have to be age-matched and matched in terms of preoperative morbidity – but there’s no suggestion here of this. The abstract concludes only:
“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.”
But what adjustments? How valid? Surely if a study appears to demonstrate that a patient of BMI 23 has 4 times the mortality risk of someone of BMI 35+, one’s first response must be to reassess the design of the study.
(Your correspondent’s BMI: 22)
This is extremely misleading as there is an unmeasurable selection bias as many of the obese may have been refused surgery in the first place — no surgery = no complications of surgery. Intention to treat is also important.
Tightening of the “normal” BMI range has produced misleading results – need to relax the range to allow for a wider range of heights as well as weights – there seems to be a problem with the measuring stick
This headline is really misleading, as is the classification of obesity. Most of my clinically healthy, normal weight patients have a BMI 25-30. 30-35 seem overweight. It is not till a patient has a BMI over 35 that we start to worry about obesity-related surgical risks. Let’s not confuse BMI over 35-40 with “overweight” patients with a BMI 25-30!
From my experience this is correct. Well conducted surgery, anaesthesia and medical management has made the risks low for these people, but it has not been without effort. In the past many of these patients came unstuck because they were treated without adequate care. The real risk is to the thin young nurses who are left to manage them and to the hospitals that have to spend enormous amounts providing special chairs, operating tables, large CT machines and special lifting devices. Transporting these patients, particularly the industrial-sized obese patients is a major logistic exercise. I don’t find too many volunteers to take the very sick obese, ventilated patient for the casual CT scan that occasionally is ordered to rule out an abdominal cause for sepsis in large patients whose elective operation has gone badly. This study though good doesn’t measure the complete effort throughout the hospital experience and certainly does not examine some subgroups of obese patients.