Obesity increases a number of cardiovascular disease (CVD) risk factors, but patients with many types of CVD may have a better prognosis if classified as overweight or obese, a phenomenon known as the "obesity paradox". This paradoxical benefit of a medically unfavorable phenotype is particularly strong in the overweight and class I obesity, and less pronounced in the more severe or morbidly obese populations (class II-III and greater). Rather than an obesity paradox, it is possible that this phenomenon may represent a "lean paradox", in which individuals classified as normal weight or underweight may have a poorer prognosis with respect to CVD, as a result of a progressive catabolic state and lean mass loss. Cardiorespiratory fitness (CRF) is a fundamental part of this discussion. A greater CRF is associated with lower CVD risk, regardless of body mass index (BMI). Also, the assessment of body composition compartments (i.e., fat mass, fat-free mass, lean mass) and the presence of metabolic derangements may be better indicators of CVD risk than BMI alone. The focus of this review is to summarize the current evidence of the obesity paradox. Moreover, we discuss the utility and limitations of BMI for cardiometabolic risk stratification, in addition to concepts such as "metabolically healthy obesity" (MHO) and the "fat but fit" phenomenon, which describe patients who are diagnosed with obesity using BMI, but without major metabolic derangements and with greater CRF, respectively. Finally, we propose that obese patients presenting with an excess body fat, yet without metabolic abnormalities, should still be viewed as an "at risk" population, and as such should receive advice to change their lifestyle to improve their CRF and to prevent the development of impaired fasting glucose, diabetes mellitus and other CVD risk factors as a form of primary prevention.
Keywords: Atrial fibrillation; Cardiorespiratory fitness; Cardiovascular; Cardiovascular disease; Coronary heart disease; Heart failure; Obesity; Obesity paradox; Physical activity.
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