The current meta-analysis showed that the prevalence of obesity was 33% among individuals with COVID-19. Not only was obesity associated with increased occurrence of COVID-19; but it was also associated with greater odds of developing critical conditions (e.g. hospitalization, mechanical ventilation, ICU admission), and mortality. Notably, these associations were consistently observed among Caucasians. To the best of our knowledge, this is the first study that has provided a comprehensive evaluation of both occurrence and prognosis of COVID-19 in relation to obesity.
The 33% prevalence of obesity among patients with COVID-19 corroborates a recent review that has shown obesity is the most prevalent comorbidity among patients with severe or fatal COVID-19 (42%) (12). Similar observations have been reported in other respiratory-related outbreaks including MERS-CoV (74), influenza (75), and SARSCoV (76). Moreover, the positive association of obesity with the occurrence and severity of COVID-19 is in line with similar reviews that only included initial studies from China (14).
In the current study obesity was associated with poor prognosis of COVID-19 by increasing the need for hospitalization, mechanical ventilation, ICU admission, and even mortality. This finding was in the line with a current systematic review that included mostly case-report, case-series, letter to editor, and comments (77). That review found that obesity was associated with the increasing prevalence of hospitalization (average of 20.4%) and greater lethality (average of 20.4%) in the patient with COVID-19 (77). The contribution of obesity to diseases severity and the requirement of advanced medical care in COVID-19 has been also stated in another initial review that only included 3 studies (78). Our study also further added to a meta-analysis that showed obesity is a risk factor for both ICU admission and mechanical ventilation requirement in COVID-19 patients (11). This is of particular importance since restricted IUC capacity has created great concern across the world. So, knowledge of relevant risk factors can help clinicians better identify and guide the high-risk populations for making the most use of available facilities to reduce morbidity and mortality outcomes of COVID-19 infection.
Although the underlying mechanism linking obesity to COVID-19 has remained to be elucidated, several potential pathways may justify this association through chronic inflammation, higher Angiotensin-Converting Enzyme-2 (ACE-2) concentration, and functional restrictive capacity of the lung. Chronic inflammation is accompanied by the increased level of C-reactive protein, interleukin 6, and adipokines, all of which can suppress the immune system and put the body at greater risk for the COVID-19 infection (67, 79). Moreover, ACE-2 receptors -responsible for facilitating COVD-19 entry into cells- can be expressed in different parts of the body including adipose tissue (80). That is, greater adiposity is equal to having more ACE-2 receptors and subsequently be more susceptible to catch COVID-19. Finally, individuals with obesity have physiological respiratory dysfunction which can increase the risk for hypoventilation (81), and thus may contribute to a worse prognosis of COVID-19.
The limitations of this study should be reported. Studies used different criteria to define obesity such that some studies define obesity based on national cut-points (BMI > 25 kg/m2), while others used the WHO definition of obesity (BMI > 30 kg/m2). Also, the included studies did not mention the detailed comorbidities of obese patients, such as diabetes and hypertension. Additionally, although we divided disease severity based on clinical symptoms, ICU care and death, the included studies still varied in their differentiation of patients’ disease severity in clinical definition, with classifications of “mild, moderate, severe, and critical”, “ordinary and severe/critical”, “common and severe”, and “non-severe and severe” disease. A high heterogeneity existed between studies; however, subgroup analyses were conducted to trace potential sources. Nonetheless, this is the first study that provided an extensive evaluation of health literature to assess the association of obesity with odds of occurrence and prognosis of COVID-19. This study brings attention to obesity as an important risk factor for COVID-19, which has dire consequences in relation to morbidity, mortality and the financial burden generated by the current pandemic. We followed a rigorous methodology as all stages were conducted by two reviewers independently including study selection, data extraction, and quality appraisal. The large sample size and extensive coverage of different regions around the world will increase the power and representativeness of the results to the whole patient population worldwide.