ReviewThe obesity paradox in the surgical population
Introduction
With advancement of medical care in modern societies, two distinct growing phenomena are observed, which pose new challenges to the surgeon. These are the overweight and obesity epidemic on the one hand, and the growing elderly population on the other hand.1, 2, 3 These two categories of patients share a number of risk factors and associated comorbidities that predispose them to cardiovascular and other life-threatening complications.4, 5
Body mass index (BMI), formerly known as Quetelet's index, has been introduced to public health science as a proxy of overall body fat content. It is calculated by dividing weight in kilograms by the square of height in meters. In late and even in upcoming years, much attention has been paid to this index and to other measures of total or abdominal fat, due to the increasing prevalence of overweight and obesity. Because of its simplicity, BMI has gained widespread acceptance and application in daily clinical practice. The World Health Organization (WHO) has defined different BMI categories (Table 1).6, 7
Clinical research in the surgical population frequently focused on the prognostic value of certain clinical variables obtained from the preoperative assessment and the perioperative course.8, 9, 10, 11 Some of these variables are incorporated in guidelines regarding preoperative cardiovascular management in non-cardiac surgery,12 which have been shown to reduce postoperative cardiac events and improve long-term outcomes. Furthermore, recognition and optimization of other, non-cardiac, chronic ailment conditions prior to surgery can also be beneficial, both in the perioperative stage as well as for the long-term.13 Although several preoperative risk-scoring systems exist,14 BMI has not been included, since it was not considered as an independent (preoperative) risk factor or predictor for postoperative and long-term outcomes.
The purpose of this article is to give an overview of the relationship between BMI and outcome in the surgical population, reporting both postoperative and long-term outcomes. Furthermore, the literature regarding the inverse relationship between BMI and outcome, known as the obesity paradox, as well as the theories explaining its causation, are reviewed.
Section snippets
Methods
We performed a PubMed search to identify available literature up to January 1, 2012. Search criteria included obesity paradox and BMI paradox, each of which was subsequently combined with additional search criteria including surgery, general surgery, cardiac surgery, outcome, and survival to narrow search results. Search criteria were restricted to English language, humans, and adults (age > 19 years). Original articles (observational, cohort, case–control, cross-sectional, longitudinal and
The risks of obesity in the surgical patient
The worldwide broadening of the obesity epidemic has also affected surgery, not only because more surgical patients are obese, but also because of an increase in obesity-related diseases that require surgery.1, 4 Substantial data from literature showed the preponderance of cardiovascular risk factors in the overweight and obese population.1, 4, 15 Moreover, increased body mass was found to be a predictor of increased cardiac risk, independent of cardiovascular risk factors.16 Obesity is also
The obesity paradox
Recent epidemiological studies in the general population have shown a longer life expectancy in modern societies with prevalent overweight and obesity, compared to those that did not join the obesity epidemic.39, 40 The inverse relationship between body fat composition, particularly defined by the BMI, and all-cause mortality, is frequently referred to as the obesity paradox. The more comprehensive term reverse epidemiology also comprises the obesity paradox. It represents the unexplained
The paradox theories
Since the first observation of the obesity paradox, several suggestions were made to overcome the unexpected survival benefit of the overweight and obese. One suggestion was that the values of BMI cut-offs representing the categories defined by the WHO should be revised, so that overweight patients showing survival improvement should merge into the control group, i.e. the normal BMI population.60 However, it is important to consider that BMI does not discriminate between fat mass and lean mass,
The benefits of obesity
Adipose tissue is a potential endocrine organ capable of secreting a variety of cytokines with opposing actions.4 Tumor necrosis factor-α (TNF-α) is a pro-inflammatory and atherogenic macrophage-derived cytokine, and is known to promote cardiac and endothelial injury through its apoptotic and negative inotropic effects.68 Adipocytes release soluble TNF-α receptors, which can neutralize TNF-α in various inflammatory wasting states.69 Moreover, adipocytes secrete adipokines, of which adiponectin
The hazards of underweight
The association of increased mortality in the underweight population might, at least in part, be attributable to reverse causation, which means that lower weight is not a cause but a result of chronic diseases that are related to poor outcome.86 Chronic diseases that cause weight loss may remain unnoticed for months or even years, for example, in the case of cancer, chronic respiratory or cardiac diseases. Smoking is another potential confounding factor, because it is associated with both a
Implications for the surgical population
As previously described, the obesity paradox has also been shown in the surgical population. The mechanisms explaining the survival benefit of the obese in the general population might also be applicable to the obese surgical patient. Moreover, it is speculated that overweight and mild obese patients have a more appropriate inflammatory and immune response to the stress of surgery than their leaner and morbid obese counterparts.26, 27 There is a close relationship between the immune and
Conclusion
Despite the feeling that obese patients requiring surgery are at increased risk for adverse postoperative outcomes, surgery can be relatively safely performed in the higher BMI categories. However, patients at the extremes of BMI, both the underweight and the obese class III, seem to have the highest postoperative morbidity and mortality hazard, which even persists at long-term. The inverse relationship between BMI and mortality is referred to as the obesity paradox, and has been observed both
Funding
Dr. Valentijn is supported by an unrestricted research grant from ‘Lijf en Leven’, Rotterdam, the Netherlands.
Conflicts of interest
The authors declare no conflicts of interest for the current manuscript.
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These authors equally contributed to the manuscript.