The Role of Nutrition in Covid-19 Pandemic: A Review

On December 12, 2019, Covid-19 an infectious disease pandemic caused by the corona virus has become a major global threat to the city of Wuhan, China. Coronaviruses are a large family of viruses that can cause disease in humans and other animals. The new coronavirus seriously attacks lung epithelial cells and causes respiratory disorders that require ventilatory support in severe cases. Elderly people, especially those with underlying diseases such as diabetes, cardiovascular diseases, respiratory diseases, and high blood pressure, such as people who have a suppressed immune system, are exposed to severe symptoms and mortality. Also, in addition to the lung tissue, the new coronavirus also targets other organs such as the heart, kidney, digestive system and brain. In the pathogenesis of this disease, there is a cytokine storm and changes in immune responses. Two factors, the uncontrolled inflammatory response and the reduction of the immune system in people with corona can cause the symptoms to worsen during and after infection. The immune system and the digestive system both play key roles in terms of access to nutrients and the defense of the host body as the intestinal microbiota against external foreign factors. Therefore, one of the biggest concerns is related to reducing inflammation and not reducing the patient's immune response, and to solve this concern, in addition to drug therapy, diet therapy should also be at the top of these approaches. The importance of proper eating patterns and habits in the Covid-19 epidemic showed that this issue is not only to prevent the presence of non-communicable diseases that can subsequently lead to more severe infections but is also considered a solution for modulating the inflammatory condition of patients. It is a fact that ignoring the importance of nutrition in patients with Covid-19 leads to the worsening of symptoms and complications of the disease in these patients. Therefore, the purpose of this study is to investigate the relationship between nutrition, the body's immune system, and covid-19 to formulate appropriate food plans and patterns for prevention and improvement during treatment and after.

In this review study, we have examined the importance of proper nutrition on the immune system and its effects on the prevention and treatment of covid-19 disease. For this purpose, articles from international databases such as PubMed, Google Scholar, Science Direct, and Scopus were analyzed by entering the key words of the Covid-19 pandemic, immune system, nutritional factors, and food supplements.

The immune system needs sufficient amounts of vitamin A for its various defense functions, such as keratin and mucus expression, cell death, growth, development, immune cell activity, antibody production, and cytokine expression against pathogens. Several studies have shown the effect of vitamin A deficiency on the disease severity of patients with coronavirus. Vitamin A and retinoid cause the secretion of compounds that inhibit virus replication in immune cells. Therefore, using this vitamin as a supplement in patients suffering from viral diseases reduces complications and mortality. This group of vitamins is an essential micronutrient that has different roles such as improving the function of the immune system and promoting the growth of cells. Also, this vitamin has an anti-inflammatory and inhibitory role in the entry of neutrophils into the damaged lung. Vitamin B6 plays a coenzyme role in T-cells production and interleukin 2. Therefore, when faced with a deficiency of group B vitamins, the immune system response in dealing with foreign agents becomes weaker. With these interpretations, group B vitamins can be considered as an auxiliary treatment in Covid-19 treatment. Vitamin C or ascorbic acid is a water-soluble vitamin that has a very strong antioxidant role, which becomes very important in the conditions of oxidative stress caused by coronaviruses. Vitamin C can strengthen the immune system and resistance against the infection caused by coronaviruses through various ways such as the migration of leukocytes to the sites of infection, phagocytosis and killing of bacteria, the activity of natural killer cells, the function of T lymphocytes (especially cytotoxic T lymphocytes) and the production of antibodies. Vitamin C supplementation has also been shown to reduce the duration and severity of upper respiratory tract infections such as the common cold, especially in people who are under severe physical stress. The role of vitamin D in bone health is known, but another essential role of this vitamin is in strengthening the response of the immune system and the growth and maturation of immune cells. Vitamin D can reduce the risk of viral infections such as coronaviruses by several mechanisms. Vitamin D moderates this process by reducing the secretion of inflammatory cytokines and increasing the expression of anti-inflammatory factors. New studies also reported that by detecting the presence of the virus, vitamin D can prevent the rapid multiplication of the virus through programmed death and autophagy in tissues infected with infectious agents. Therefore, this vitamin can be effective in treating and preventing the covid-19 coronavirus in different ways. Studies related to the roles of vitamin E in host immunity and susceptibility to infection have been reported. When infected with coronaviruses, cell membranes, especially lungs, undergo oxidation of unsaturated fatty acids and subsequently increase their permeability. By maintaining the of the T-Cell membrane, vitamin E plays a significant role in improving the performance of immune system responses and clearing bacterial and viral infectious agents. Vitamin E supplementation increases antibody production, lymphocyte proliferation, T helper 1 cytokine production, natural killer cell activity, and macrophage phagocytosis. Therefore, vitamin E can be used as an effective antioxidant in the prevention and improvement of symptoms caused by Covid-19.Selenium deficiency occurs in two forms: mild deficiency and impairment of human health, and severe deficiency and defects in the response of the humoral and cellular immune system.Zinc deficiency may exacerbate the effects of Covid-19 in several ways; including by destroying the balance between innate and acquired immune cells, intensifying inflammatory responses that can lead to failure of various organs, excessive thrombus formation that can lead to an increased risk of thromboembolism, and finally, by reducing the function of the mucous barrier in many organs and tissues. Therefore, the nutrient zinc can be helpful for the optimal functioning of the immune system when the symptoms of the disease of Covid-19 appear in affected people. The favorable effect of iron on the functioning of the immune system and protection against foreign factors is well known. Probiotics, especially some lactobacilli and bifidobacteria, reduce the incidence of respiratory infections in humans and improve their consequences. However, a total of the evidence suggests that lactobacilli and bifidobacteria may improve immune function. The role of probiotics in the prevention of viral respiratory infections has also been recently noticed. Even though oral probiotic supplements are not currently part of any specific protocol for the treatment and prevention of respiratory viral infections, many studies have shown that their use can be beneficial in modulating the systemic immune system.

Considering that a proper diet is effective in all diseases, including cardiovascular diseases, cancer, and metabolic diseases, there is no doubt that nutrition is also important in infectious diseases. A suitable diet that contains helpful macronutrients and micronutrients, by affecting homeostasis in immune cells, maintains their function throughout life and strengthens immune mechanisms, especially in people at risk like the elderly, pregnant women, and children, and is a key element in improving the functioning of the immune system and strengthening resistance against viral and microbial infections. Therefore, insufficient lack of nutrients inception such as zinc, selenium, iron, vitamins A, B, C, D, and E, proteins, and omega-3 fatty acids can cause weak immune responses and make people susceptible to infectious diseases such as covid-19.