Defining Obesity

Obesity might generally be defined as the condition of having excess weight, particularly excess weight from fat. It is the evolutionary response to two forces: the intense metabolic requirements of the brain and the historic unreliability of human food sources, long subject to drought, flooding, war and other disasters. While the concept of fatness dates back to prehistory, extending from Stone Age sculpture to Shakespeare, obesity as a medical concept is a modern one. Though some of the medical consequences of obesity, such as respiratory difficulties and gout, were recognised as early as the eighteenth century, it was not until the post‐World War II era and the advent of population health as a science that the medical community began to build a paradigm of obesity.

In the modern age, obesity is most commonly defined by means of body mass index (BMI), which is itself defined by the equation BMI = weight/(height). This relationship was first described more than a century ago by Adolphe Quetelet, a Belgian mathematician and social scientist, who noted that, across the population, body weight generally increases proportionally to the height squared. This equation, properly rearranged, was initially termed the Quetelet index. While it gained popularity amongst insurance agencies after World War II as a marker for obesity, it is most strictly a measure of the degree to which a person deviates from the height/weight curve first noted by Quetelet.

The usefulness of BMI to define obesity comes from its clinical utility, without which it would be an arbitrary classification of body type. BMI has been noted to correlate with disease burden and outcomes across a number of body systems, including gastrointestinal, cardiovascular, respiratory and endocrine, as well as all‐cause and disease‐specific mortality. While it can serve as a prognostic tool, its weakness lies in its simplicity: simply measuring height and weight fails to examine the sickness or health of a patient. Numerous other factors contribute to obesity as a cause of disease, including the proportion of weight contributable to fat, muscle and bone as well as sex, ethnicity, activity level and genetics.

This complicated understanding of obesity has driven the search for more nuanced ways to define and measure obesity, either by directly or indirectly measuring body fat content. The simplest methods are anthropomorphic measurements, such as waist circumference, waist‐to‐hip ratio and skinfold measurements. These methods, while useful and easy to implement in a clinical setting, suffer from the same varying correlation with actual body fat that afflicts BMI. At the same time, they typically require more training and effort than what is needed to measure BMI. More accurate methods to measure body fat percentage include dual‐energy X‐ray absorptiometry, densitometry and computed tomography scan. While the specifics of these modalities fall outside the spectrum of this discussion, each is a highly accurate method for measuring body fat percentage. However, they require time, expense and exposure to radiation that simple anthropometry does not involve.

While these other methods have shown utility with regard to both diagnosis and prognosis, BMI is still the most widely used measurement in the diagnosis of obesity. In addition, in the United States, its use in the field of bariatric and metabolic surgery is nearly universal due to reimbursement requirements. The current definition of obesity is BMI at or above 30, with further gradations to delineate severity (Tables 3.1 and 3.2). The grading system favoured by the World Health Organization (WHO) does not further differentiate amongst patients with class III (or severe) obesity (BMI ≥ 40). However, bariatric surgeons have found it useful to differentiate amongst patients with varying degrees of severe obesity, as these patients represent unique technical challenges that may not be present amongst patients with less severe disease. Another challenge represented by BMI as a diagnostic tool is the varying correlation with outcomes across race. In particular, East Asian populations have been shown to have increased disease burden at lower BMI, and so alternate BMI cut‐offs for obesity have been suggested for this population. Overall, however, BMI remains a useful clinical tool for diagnosis and prognostication.

The Pathophysiology of Obesity

The concept of obesity as a disease‐causing or disease‐associated patient factor usually exists within a larger paradigm of systemic metabolic dysfunction. The most common paradigm is that of the ‘metabolic syndrome’ (Figure 3.1). Though definitions of the metabolic syndrome differ from author to author, it is usually defined as some constellation of obesity, dyslipidaemia, hypertension, and insulin resistance. These disordered aspects of physiology are interlinked, contributing to both one another and to diseases such as diabetes, atherosclerosis, thromboembolism, and cancer. Obesity in this context is not monolithic: central obesity, or abdominal obesity, has different metabolic, endocrine and inflammatory effects than subcutaneous fat. Fat stored in the mesentery or around organs in the abdomen increases the risk for disease and mortality more than fat in subcutaneous tissues. This understanding has contributed to the description of differing phenotypes, including those patients who might have obesity but are metabolically healthy, or patients who, while at a normal weight, have a high body fat percentage and are metabolically deranged. However, patients with obesity who are metabolically healthy are still at an increased risk for metabolic disease down the road.

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