O besity is a complex chronic disease in which abnormal or excess body fat (adiposity) impairs health, increases the risk of long-term medical complications and reduces lifespan. 1 Epidemiologic studies define obesity using the body mass index (BMI; weight/height 2), which can stratify obesity-related health risks at the population level. Obesity is operationally defined as a BMI exceeding 30 kg/m 2 and is subclassified into class 1 (30-34.9), class 2 (35-39.9) and class 3 (≥ 40). At the population level, health complications from excess body fat increase as BMI increases. 2 At the individual level, complications occur because of excess adiposity, location and distribution of adiposity and many other factors, including environmental, genetic, biologic and socioeconomic factors (Box 1). 11 Over the past 3 decades, the prevalence of obesity has steadily increased throughout the world, 12 and in Canada, it has increased threefold since 1985. 13 Importantly, severe obesity has increased more than fourfold and, in 2016, affected an estimated 1.9 million Canadian adults. 13 Obesity has become a major public health issue that increases health care costs 14,15 and negatively affects physical and psychological health. 16 People with obesity experience pervasive weight bias and stigma, which contributes (independent of weight or BMI) to increased morbidity and mortality. 17 Obesity is caused by the complex interplay of multiple genetic, metabolic, behavioural and environmental factors, with the latter thought to be the proximate cause of the substantial

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Many effective transitional intervention components are feasible and likely to be cost-effective. Future research can provide direction about the specific components necessary and/or sufficient for preventing early psychiatric readmission.

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