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As the new coronavirus pandemic hovers over our continents, it becomes clear that obesity is a risk factor for being hospitalized with COVID-19. Would this mean that being obese predisposes a person to get the virus, or to be at a higher risk of dying from the infection? The potential connection between the two has garnered much attention, including a question about the relationship raised during a recent webinar held by the International Food Policy Research Institute (IFPRI) on food systems and COVID-19. In this blog we explain possible reasons for the association between obesity and severe COVID-19 disease.
Obesity is defined by the body mass index, a simple measure based on a formula that divides one’s weight in kilograms by the square of one’s height in meters. A person with a BMI of 18 to 25 is considered normal weight, 25 to 30 is considered overweight, and over 30 is obese.
There are four potential explanations for the increased risk of COVID-19 infections in obesity. First, the association could be explained in part by confounding by age. Overweight or obese people tend to be older than their peers with normal weight. Old age is a known risk factor for severe COVID-19. Recent data from England, for example, found that the prevalence of obesity was highest among men aged 45-64 years (36 percent) and among women aged 45-54 years (37 percent) (See Figure 1). This relationship could at least partly explain the association between obesity and hospitalization for COVID-19.
Figure 1: Prevalence of being obese and overweight, by age and sex, in England (National Health Survey 2018)
Secondly, based on epidemiological studies, it has been well established that individuals with obesity (and diabetes) are more susceptible to infections and their complications. For example, various studies report on a higher incidence of respiratory tract infections in obese people compared to lean individuals (e.g., Huttunen and Syrjӓnen 2012, Milner and Beck 2012). However, mechanistic studies to explain these observations are scarce. Nonetheless, the presence of obesity is known to cause impaired innate and adaptive immune responses. It is known that obesity drives a subtle yet chronic activation of our immune system, promoting a state of low-grade inflammation. This constant state of activation alters the response of our immune cells towards infections. Once an immune cell is in a chronic state of activation, even at a low level, its capacity to mount an acute and robust response to an infection is hampered, which can eventually lead to more pathogenic complications during an infection. For example, the presence of obesity leads to a delayed and blunted response to an influenza virus infection leading to poor recovery from the disease (Honce and Schultz-Cherry 2019).
Third, obesity can cause marked changes to the mechanics of the lung and chest wall, which may lead to an impaired respiratory function and a predisposition to respiratory disease such as asthma and asthma-like symptoms. However, it is not known whether these changes also predispose to COVID-19, or whether COVID-19 can exacerbate pre-existing respiratory disease in people with obesity. In the latter case, it would be a combination of obesity and COVID-19 that predisposes to severe disease and death.
Lastly, obesity also causes diabetes and high blood pressure, which are associated with an increased risk that COVID-19 infection will lead to death. Thus, the relationship between obesity and severe COVID-19 is probably mediated in part by chronic diseases.
Further studies are needed to confirm these hypotheses. In our opinion, it is premature to use obesity as a screening indicator to decide whether or not to be admitted to the Intensive Care when IC beds are scarce, as suggested already in the popular media.
Rinke Stienstra is Assistant Professor, Sander Kersten is Professor and Division Chair, and Hans Verhoef is Associate Professor in the Division of Human Nutrition and Health at Wageningen University & Research. The analysis and opinions expressed in this piece are solely those of the authors.
Photo:Image taken by an electron microscope shows the virus that causes COVID-19. The spikes on the outer edges of the virus particles give coronaviruses their name: crown-like (Photo: NIAID-RML via Flickr, copyright free)
References
Honce R, Schultz-Cherry S. Impact of obesity on influenza A virus pathogenesis, immune response, and evolution. Front Immunol 2019;10:1071.
Huttunen R, Syrjӓnen J. Obesity and the risk and outcome of infection. Int J Obes 2013;37:333-40.
Milner JJ, Beck MA. The impact of obesity on the immune response to infection. Proc Nutr Soc 2012;71:298-306.
National Health Service. Health Survey for England 2017, 2018. Available at: https://files.digital.nhs.uk/EF/AB0F0C/HSE17-Adult-Child-BMI-rep-v2.pdf (accessed 17 April 2020).
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