Though sometimes overlooked, nutrition plays a critical role in the treatment and recovery of COVID-19 patients. In particular, adequate protein intake during both the acute and post-acute phases of the illness is linked to decreased morbidity and mortality.
So how does this single macronutrient exert such a profound influence on COVID-19 outcomes?
It’s well-established that protein deficiency has a negative effect on immune functions due to decreased viable immunoglobulins and gut-associated lymphoid tissue (Fernández-Quintela et al. 2020). In addition, research shows that protein malnutrition increases susceptibility to viral infections such as Zika and influenza, in part by decreasing the antibody response (Iddir et al. 2020).
One complication of COVID-19 involves the so-called cytokine storm, a hyper-inflammatory response caused by the over-release of cytokines that often results in multi-organ dysfunction and death. In addition to anti-inflammatory and antiviral drugs, amino acids can play a role in mitigating the release of cytokines, thus decreasing overall mortality (Ferrara et al. 2020). One small study found that patients receiving L-glutamine supplements (10 g/3 times per day) were hospitalized for fewer days and were less likely to require admission to the intensive care unit (ICU) than their counterparts not receiving the supplements (Cengiz et al. 2020). Glutamine may help to preserve lung and intestinal function and has been shown to significantly reduce the amount of pro-inflammatory cytokines. Arginine and glycine may reduce the number of certain inflammatory agents in the alveoli, thus limiting damage to the lungs (Ferrara et al. 2020). Complete proteins, such as those found in animal products, contain all essential amino acids and may also exert an anti-inflammatory effect (Fernández-Quintela et al. 2020).
People admitted to the hospital with COVID-19 may already be experiencing protein-calorie malnutrition. Nausea, vomiting, and diarrhea are just some of the symptoms that can affect oral intake, and these symptoms are directly correlated with the aforementioned cytokine storm (Fernández-Quintela et al. 2020). To this end, the European Society for Clinical Nutrition and Metabolism (ESPEN) recommends the immediate initiation of oral nutrition supplements for patients who are able to eat by mouth. These supplements should provide at least 400 cal and 30 g of protein per day, according to ESPEN guidelines (Barazzoni et al. 2020).
In addition to modulating the immune response, protein is required in order to prevent the loss of muscle mass and function that follows the catabolic responses to COVID-19 and other inflammatory diseases. One metabolic derangement associated with severe COVID-19 is impaired protein metabolism, manifested through increased protein and muscle breakdown, decreased muscle synthesis, and increased synthesis of acute phase proteins (Stachowska et al. 2020). ESPEN recommends providing 1.3 g protein per kg of body weight per day for critically ill COVID-19 patients (Barazzoni et al. 2020). Others recommend protein intake of up to 1.5–2.0 g/kg/day (Stachowska et al. 2020). To put this in perspective, the standard protein recommendation for healthy individuals is around 0.8 g/kg. Increased protein intake during critical illness has been shown to improve survival, likely due in part to its muscle-sparing effects (Barazzoni et al. 2020).
Prolonged stays in the ICU, especially those longer than two weeks, are likely to increase muscle catabolism. Physical and mental impairments may result from this time in the ICU and influence the long-term outcomes for patients (Barazzoni et al. 2020). Once again, delivering adequate protein, in tandem with other nutritional interventions and physical activity when possible, may help to limit such impairments and enhance muscle anabolism (Barazzoni et al. 2020).
COVID-19 and the recommendations surrounding it continue to evolve. While we know that protein plays a critical role in the treatment of and recovery from the disease, there is still not a consensus on the ideal amount of protein to provide to those infected. Moreover, the specific anti-inflammatory mechanisms of amino acids such as glutamine are not fully understood. Nevertheless, nutrition therapy, including protein supplementation, is clearly relevant and should be used along with pharmacological interventions to help more patients recover from COVID-19.