Vitamin A and D deficiencies in the prognosis of respiratory tract infections: A systematic review with perspectives for COVID-19 and a critical analysis on supplementation

Considering the rapid spread of COVID-19, the scientific community has been looking for ways to recognize factors that may interfere with the outcome of viral infection. Despite the lack of studies with the new coronavirus, it is known that adequate serum levels of micronutrients are essential for the organic response to infectious diseases. Thus, we aim to review the effects of vitamin A, D, iron, zinc, or folate deficiency on the prognosis of patients with respiratory infections with manifestations similar to COVID-19 and discuss about supplementation of the nutrients analyzed in this review. The search was conducted in the databases PubMed, Lilacs, and SciELO, including observational studies published between 2010-2020, with results for individuals with respiratory tract infections with manifestations similar to COVID-19. Six articles met the inclusion criteria, all of which were related to deficiencies of vitamins A and D. In general, vitamin A deficiency was associated with cough, fever, and greater total respiratory resistance. Regarding vitamin D, the lack of this nutrient led to higher rates of ICU admission, the need for mechanical ventilation, and mortality. Evidence linking specific relationships between nutritional deficiencies and COVID-19 remain lacking due to the small number of studies and heterogeneities in population subgroups. In conclusion, deficiencies of vitamins A and D seem to negatively affect the prognosis of respiratory tract infections. Supplementation of these nutrients for prevention or treatment of patients diagnosed with COVID-19 should respect serum levels, nutritional status and housing conditions (e.g.,endemic location) of individuals.