An Overview and Update on Obesity and the Obesity Paradox in Cardiovascular Diseases☆
Introduction
The prevalence of overweight and obesity has reached epidemic proportions in Western countries,1 and is the second leading cause of preventable death following tobacco use.2 Although the etiology of the obesity epidemic has been intensely debated, it is widely accepted that increased body weight and overall adiposity are the result of a chronic positive energy balance, with energy intake exceeding energy expenditure.3 Obesity is a major independent risk factor for cardiovascular (CV) disease (CVD), such as hypertension (HTN), coronary heart disease (CHD), atrial fibrillation (AF) and heart failure (HF).3,4
While obesity increases a number of the established CVD risk factors, it has been shown that many types of CVD may have a better prognosis in the overweight or obese population compared to their leaner counterparts,3 and this phenomenon is referred to as the “obesity paradox”. An obesity paradox is seen in many forms of CVD,5 in addition to other diseases, such as end-stage renal disease, human immunodeficiency virus and various pulmonary diseases.6,7 Although the obesity paradox has been observed for most CVD, it most likely applies to the overweight and class I obesity, and less for class II and greater (Table 1).4
The focus of this review is to evaluate the current evidence regarding the obesity paradox in CVD and to understand its clinical implications. Concepts such as the “fat but fit” phenomenon, “normal weight obesity” (NWO) and “metabolically healthy obesity” (MHO) will also be discussed, as well as the critical importance of physical activity (PA), cardiorespiratory fitness (CRF) and metabolic derangements in the overweight and obese.
Section snippets
Utility of BMI
Overweight and obesity are generally defined by Body mass index (BMI) in clinical practice (Table 1).4 The modern definition of BMI comes originally from the Quetelet index in 1832, named after a Belgian astronomer and statistician who used height and weight to assess individuals in the French and Scottish armies.8 However, the World Health Organization defines obesity as excess body fat that impairs health, and further suggest that BMI is a rough guide of body composition since individuals
Physiologic impact of overweight/obesity
Increasing adiposity is associated with neurohormonal activation and metabolic abnormalities, including renin-angiotensin-aldosterone system activation, sympathetic system activation, hyperleptinemia and dysregulation of growth factors, such as insulin-like growth factor.2 These homeostatic aberrations induce increased sodium retention, vascular reactivity and hyperinsulinemia, and each contribute to the development of HTN in obesity.22 Adipose tissue, while previously viewed as a storage
CVD risk of obesity
Certainly, obesity is associated with worsening physiologic parameters that promote the development and progression of CVD, including dyslipidemia, high blood glucose, low-grade systemic inflammation and the MetS/DM.2,29 Overweight and obese patients consistently have a higher prevalence of CHD, and the Framingham study showed that 23% of CHD in men and 15% of CHD in women was attributable to excess adiposity.30 Another analysis of patients from the Framingham cohort study with follow-up of up
HF
The Framingham study32 was the largest to assess the risk of HF in obesity; every 1 kg/m2 increase in BMI increased the risk of HF by 7% in women and 5% in men. While obesity significantly increases the risk of HF,29 Pandey et al.33 have demonstrated that the increase in HF with reduced ejection fraction is lower compared to the markedly increased risk for HF with preserved ejection fraction. Furthermore, the risk of HF associated with obesity is independent of other risk factors as well as CRF
CHD
The major effect of obesity on CHD risk is attributable to atherogenic dyslipidemia and MetS/DM.2 This is supported by evidence from the INTERHEART study,46 which assessed 30,000 patients in 52 countries, finding that over 90% of the risk for acute myocardial infarction (MI) was attributable to nine modifiable risk factors; dyslipidemia being the leading factor, which could account for approximately 50% of the risk of developing acute MI. Despite having relatively normal total low-density
Following revascularization
The prevalence of patients undergoing percutaneous coronary intervention (PCI) or coronary artery bypass graft (CABG) surgery who are overweight or obese may be as high as 70%.2 A review of 26 PCI studies found that underweight patients had the highest rates of all-cause death, CVD death and MI with mean follow-up of approximately 1.7 years compared with normal BMI.51 In the same study, overweight patients had the lowest risk of these outcomes, with significant reductions in all-cause and CVD
HTN
The Physicians' Health Study demonstrated a strong association between higher BMI and the risk of HTN with approximately 8% increase per 1-unit increase in BMI.53 Interestingly, several studies have demonstrated that patients with HTN who are overweight or obese have a better prognosis than leaner patients with HTN, even in those with less optimal BP control and more LV geometric abnormalities.2 Patients with obesity and HTN typically respond favorably to diuretics and calcium channel blockers,
AF
Along with obesity, the prevalence of AF has been increasing and is expected to increase 3.5-fold during the next 30 years.59 Obesity is an independent risk factor for AF,60 and obese patients have been shown to have a 50% increased risk for developing AF.61 Another study showed that every 1-unit increase in BMI has been associated with an almost 4% increased risk of AF.62 Obesity may also be a risk factor for progression of paroxysmal to persistent AF, which carries higher morbidity and
Lean paradox
Some have argued that the observation of worse clinical outcomes in CVD in those with low BF% and low BMI may be suggestive of a “lean paradox” even more so than an obesity paradox.3,65,66 Low BF% and low BMI are independent predictors of worse outcomes, and those with both have demonstrated increased mortality rates.14
Unintentional weight loss carries an extremely high burden of morbidity and mortality for most medical conditions, especially for HF.4,67 None of the major HF societies recommend
CRF and the “fat but fit” phenomenon
The importance of CRF has often been neglected for CVD risk stratification, despite the fact that it correlates with overall health status and is a potent predictor of an individual's future risk of CVD.72 High levels of CRF largely neutralize the adverse effects of excess adiposity and other CVD risk factors, which has led to what is described as the “fat but fit” phenomenon.73,74 Substantial evidence suggests that CRF remains very predictive and largely negates the adverse effects of body
MHO
MHO is generally defined as BMI ≥ 30 kg/m2 without HTN, glucose abnormalities or dyslipidemia.7 Because there is no official definition for MHO, Ortega et al.65 have proposed a definition, in short, BMI ≥ 30 kg/m2 and meeting 0 of the MetS criteria (excluding WC). As discussed earlier, obesity is associated with multiple CVD risk factors, and some authors have argued that obesity should never be considered “healthy”. Supporting this notion, a meta-analysis by Kramer et al.93 found that in
Weight loss
Although an obesity paradox exists, it may not apply to more morbid obesity in which prognosis is adversely affected in CHD, cardiac revascularization (both PCI as well as CABG) and HF.14 While arguments have been made for the “fat but fit” phenomenon, CRF assessment of the morbidly obese may be technically challenging. Many changes in cardiac performance and morphology associated with obesity are reversible with purposeful weight loss,2 as purposeful weight loss reduces total and central blood
Confounders
Mechanisms for an obesity paradox are not well understood, and it has been argued that this observation may be due to confounding variables,18 or biases such as lead time, confounding or publication biases.110 Typically, the overweight/obese patients are significantly younger than patients with normal BMI in most observational cohorts, and this may be an age confounder.60 A lead time bias due to earlier detection of CVD in the overweight and obese population (due to higher pretest probability
Conclusion
There is much evidence to suggest that increased PA and CRF may be more important and effective treatment methods rather than weight loss alone for CVD, considering the fallibility of BMI. Although the use of BMI to assess adiposity is flawed, it is a good marker of excess body weight, and seems to be a better marker of CVD risk than more expensive, cumbersome and precise measurements of adiposity, as we recently described.17
Obesity is associated with multiple unfavorable physiologic and
Statement of Conflicts of Interest
Salvatore Carbone is supported by a Mentored Clinical & Population Research Award 16MCPRP31100003 from the American Heart Association, by the VCU. DOIM Pilot Project Grant Program 2017 and by the VCU Pauley Heart Center Pilot Project Grant Program 2017. All other authors report no conflicts of interest.
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Statement of Conflict of Interest: see page 148.