T he prevalence of obesity has increased starkly during the last decades, and this trend includes every age, sex, race and socioeconomic group. 1,2 According to the most recent epidemiologic data, 1,2 approximately two thirds of the US population are either overweight or obese, of whom approximately 30% are obese and more than 5% are morbidly obese. Although some reports suggest that the trend of obesity may have begun to stabilize within some segments of the US population, 3 other studies project that obesity prevalence will continue to worsen, with as many as 50% of Americans potentially being obese by the year 2030. 4 Obesity has been linked to increased mortality resulting from acute and chronic comorbidities including diabetes, stroke, and cardiovascular diseases (CVDs). 5 The epidemic of obesity also has reached the intensive care unit (ICU), such that 33% of ICU patients are obese and 7% are morbidly obese. 6 Consequently, obesity complicates all aspects of health care in the ICU by increasing the complexity of management, nutritional support, and changes in the pattern of comorbidities. 7 Furthermore, obesity induces anatomic and physiologic changes that may interfere with the body response to injury and complicate any hospitalization. 7 In this article, we will review recent studies that have examined the impact of obesity in critical illness. PATIENTS METHODSA MEDLINE/PubMed search from 1990 TO 2014 was conducted using the National Library of Medicine MeSH search terms obesity, abdominal obesity, body mass index, critical illness, mortality, and organ failure. All publication types were searched, and relevant English-language studies, case reports, meta-analysis review articles, and short communications were then extracted and manually cross-referenced. Only clinical studies using explicit definitions of obesity, morbid obesity, or body mass index (BMI) were included. Throughout the review, obesity is defined as a BMI of 30 kg/m 2 or greater, morbid obesity as a BMI of 40 kg/m 2 or greater, and overweight as a BMI of 25 kg/m 2 to 29.9 kg/m 2 , unless specified otherwise. Underweight and healthy BMIs were defined as BMI less than 18.5 kg/m 2 and 18.5 kg/m 2 to 24.9 kg/m 2 , respectively. Studies pertaining to critical illness included an ICU length of stay (LOS) of 24 hours or longer. Although the main focus of this review is the impact of obesity on critical illness, an appreciation of fundamental physiologic derangements associated with obesity is essential to evaluate the response of obese patients to critical illness. We have thus structured our review as follows. We begin by discussing the development of pathologic conditions-associated with obesity. We then review available data on the impact of obesity on critical illness outcomes. When used for discussion, the term obesity refers to BMI of 30 kg/m 2 or greater. In many instances, data are available for one demographic group but not the other, and care is taken to make this distinction clear throughout the review.

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