PERSPECTIVE https://doi.org/10.1038/s42255-020-0237-2 A metabolic handbook for the COVID-19 pandemic Janelle S. Ayres ✉ For infectious-disease outbreaks, clinical solutions typically focus on efficient pathogen destruction. However, the COVID-19 pandemic provides a reminder that infectious diseases are complex, multisystem conditions, and a holistic understanding will be necessary to maximize survival. For COVID-19 and all other infectious diseases, metabolic processes are intimately connected to the mechanisms of disease pathogenesis and the resulting pathology and pathophysiology, as well as the host defence response to the infection. Here, I examine the relationship between metabolism and COVID-19. I discuss why preexist- ing metabolic abnormalities, such as type 2 diabetes and hypertension, may be important risk factors for severe and critical cases of infection, highlighting parallels between the pathophysiology of these metabolic abnormalities and the disease course of COVID-19. I also discuss how metabolism at the cellular, tissue and organ levels might be harnessed to promote defence against the infection, with a focus on disease-tolerance mechanisms, and speculate on the long-term metabolic consequences for survivors of COVID-19. he COVID-19 outbreak, caused by the novel coronavirus Because COVID-19 is a multisystem condition, understanding the (CoV) called severe acute respiratory syndrome (SARS) recovery and rehabilitation process in infectious-disease survivors Tcoronavirus 2 (SARS-CoV-2), was first detected in late is also important; however, this aspect of infectious-disease treat- December of 2019 in Wuhan, China. The first cases outside China ment is highly neglected. Survivors of severe or critical COVID-19 were reported in January 2020, and in March, the World Health will exhibit an array of morbidities, possibly years after the infec- Organization (http://who.int/) declared the outbreak a pandemic. tion. We must understand how the damage resulting from the At the time of writing of this Perspective, more than 200 countries infection and any patient treatments influences the development of have reported COVID-19 cases, and nearly seven million cases and new conditions in the recovery phase. If we can step beyond our 400,000 deaths have been reported globally, according to the World focus on the virus, we will learn how to survive the infection and Health Organization. enable full recovery of patients, to maximize their healthspan and The immediate response to any infectious-disease outbreak is to quality of life. approach it from the pathogen perspective, because disease severity Metabolism will emerge as a critical regulator of susceptibility is assumed to be a direct function of pathogen burden1. However, to, recovery from and survival after COVID-19. Infectious diseases the complexities of SARS-CoV-2 infection serve as an important and host metabolic processes are intimately connected, and changes reminder that this perspective is not sufficient for understanding in host metabolism occur at all levels—cellular, tissue, organ and the survival of infectious diseases2. Like that of most infections, the physiological—during infection4–6. For COVID-19, these changes virulence of SARS-CoV-2 exists on a continuum: most individuals are clearest at the cellular level, at which the virus hijacks the host who acquire the infection experience mild disease, whereas a subset cell machinery to support viral replication and promote pathogen- of individuals progress to severe or critical disease3. These severe and esis. However, host metabolic responses at the tissue, organ and critical cases are driven by the host response to the infection, thus physiological levels also occur during viral infections, and some of resulting in multisystem dysfunction and pathology. The patholo- these responses are likely to reflect the adaptive mechanisms of the gies seen in patients with COVID-19 are not necessarily new among host to defend against the infection4,5. Although most attention has infectious diseases. For example, extreme clotting and multiorgan been paid to understanding how metabolism influences the host damage can be consequences of a diverse array of infectious con- resistance response that destroys pathogens, recent evidence from ditions that progress to critical stages. Instead, this pandemic has other infectious diseases has demonstrated that metabolic processes highlighted the necessity to change our perspective towards infec- are important mediators of host defence mechanisms that protect tious diseases so that we can understand how to survive infections2. against the physiological damage that occurs during infections and For COVID-19 and all other infectious diseases, this perspective consequently enable survival5,7–10. Furthermore, early clinical data will require an understanding of (1) the pathogenesis, pathology and on COVID-19 have demonstrated that people with type 2 diabe- pathophysiology of the infection; (2) how the pathology and result- tes (T2D) and other metabolic conditions that compromise over- ing pathophysiology of preexisting conditions (beyond changes in all metabolic health have greater risk of developing a more severe immune function) influence susceptibility to developing disease infection course than people who are metabolically healthy before once infection occurs; (3) the intrinsic defence mechanisms of the acquiring the infection11. Although this finding has been largely body that protect against damage and the resulting pathophysiol- attributed to these individuals’ being more permissive to viral ogy; and (4) how to develop treatments that alleviate the pathology infection and replication, the physiological complications caused and pathophysiology and complement antiviral-based approaches. by metabolic syndrome and T2D also probably render individuals Molecular and Systems Physiology Laboratory, Gene Expression Laboratory, NOMIS Center for Immunology and Microbial Pathogenesis, The Salk Institute for Biological Studies, La Jolla, CA, USA. ✉e-mail: [email protected] 572 NATURE METABOLISM | VOL 2 | JULY 2020 | 572–585 | www.nature.com/natmetab NATURE METABOLISM PERSPECTIVE Uninfected SicknessRecovery and they may lose their senses of taste and smell, and feel general Maximum SARS-CoV-2 malaise. For most people, with adequate self-care, the infection is health Maintenance limited to this stage3. Stage 2 describes the pulmonary phase of the New baseline 3 Stage 1 infection . People who enter this stage develop pulmonary inflam- Resilience health established mation and pneumonia, either without hypoxia (stage 2a) or with hypoxia (stage 2b)3. These people require hospitalization. Patients 2a Stage 2 with prolonged hypoxia tend to require mechanical ventilation3. Health Patients can then progress to stage 3. These patients are in criti- 2b Stage 4 cal condition and can develop ARDS and extrapulmonary systemic hyperinflammation syndrome. In addition, they can develop shock, Minimum Stage 3 vasoplegia, respiratory failure, cardiopulmonary collapse, myo- health Termination carditis, acute kidney injury and other extrapulmonary complica- 3 Time after infection tions . The prognosis for these patients is poor, and some continue to decline toward mortality, whereas the others enter stage 4, the Fig. 1 | The disease phases of patients with COVID-19. After infection, recovery stage, and survive and exhibit a resilience-of-health phe- 4 patients can remain healthy and show no signs of sickness (maintenance of notype (Fig. 1). health). For patients who become symptomatic, the disease course can be The metabolic health of an individual is represented by the proper described by four stages. Stage 1 is mild, and patients exhibit fever, malaise functioning of organismal metabolic processes coordinated by and a dry cough. Stage 2 is characterized by a pneumonia phase without or multiple physiological systems. Disruption of these systems causes with hypoxia (2a and 2b). Patients who progress further along the disease dysfunctional organismal metabolic processes and a decline in met- course develop acute respiratory distress syndrome, shock or multiorgan abolic health (Fig. 2). The major risk factor for severe COVID-19 is failure (stage 3 III). Patients who recover (stage 4) from the infection show poor metabolic health (Fig. 2). In past coronavirus outbreaks, T2D a resilience phenotype. Some patients may never return to their original was one of the most common comorbidities in infected individu- 13,14 health state, thus establishing a new baseline for health. Patients who peak als . In agreement with this finding, T2D, obesity and hyperten- in stages 1 or 2 will bypass stage 2 or 3, respectively, and enter into their sion appear to be major comorbidities in people with COVID-19 recovery phase. and are associated with more severe and critical COVID-19 disease courses15–17. The reasons for this finding are likely to be multifacto- rial and are conventionally thought to involve immune dysfunction. For example, individuals with metabolic syndrome and T2D have more susceptible to developing COVID-19-associated pathologies impaired immune function in general18, and their antiviral response independently of viral burden. Finally, people who survive SARS against SARS-CoV-2 may consequently be impaired. Moreover, the infections and other critical illnesses are predisposed to develop- physiological complications of T2D and metabolic syndrome are ing metabolic complications during their recovery process, thus also