1. Introduction

The contemporary severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), responsible for coronavirus disease 2019 (COVID-19), was first observed in December 2019 in the city of Wuhan, China ^[1]. Then, the fast diffusion of the disease led the World Health Organization (WHO) to declare a status of international health emergency, considering the effects the virus could have all over the world and in particular in underdeveloped countries with lower-quality health infrastructure ^[2]. The COVID-19 outbreak has deeply changed human life and brought new challenges to worldwide healthcare systems, which are currently allocating significant efforts in the development of vaccines, in the identification of therapeutic solutions and in the containment of the infection through restrictive measures such as social quarantine. Clinical manifestations of SARS-CoV-2 range from asymptomatic infection to the onset of serious pneumonia, acute respiratory syndrome, acidosis, coagulation dysfunction, organ failure, and death ^[3][4]. In such a multifaceted spectrum of clinical manifestations, it is crucial to identify predictive factors associated with mild, severe, or critical outcomes.

2. Malnutrition in the COVID-19 Pandemic

The retrospective assessment of data of the Spanish flu in 1918, which killed an estimated 50 million victims worldwide, suggested that disease severity resulted from an articulated interaction between viral, societal and individual factors ^[5]. According to recent clinical evidence in the COVID-19 pandemic scenario, several aspects have been correlated with more critical patients’ admissions to hospital, higher rate of complications, longer recovery time, and even higher mortality rate. In particular, recent studies have reported how malnutrition is one of the crucial elements that may be predictive of slower recovery, or no recovery at all, for the affected subjects ^[6]. Malnutrition refers to the incorrect intake of both energy and macronutrients (carbohydrates, proteins, fats), as well as to micronutrient (minerals and vitamins) deficiency. In the case of shortage of energy intake, the food energy fails to meet the individual’s needs, whereas micronutrient deficiency refers to a lack of vitamins and minerals which are needed, in small amounts, for healthy growth and development. Although it might seem contradictory, individuals might be overfed in terms of energy but be deficient in one or more micronutrients in their routine diet. Inadequate intake of these nutrients is currently largely diffuse, leading to an impaired resistance to infections and, consequently, to an increase in disease seriousness ^[7].

In January 2016, the major clinical nutrition societies organized the Global Leadership Initiative on Malnutrition (GLIM) to agree on shared key criteria for the classification of malnutrition in adult people in clinical contexts ^[8]. These allow defining malnutrition by first screening for malnutrition risk, and then assessing for diagnosis and severity classification. The main criteria involve assessing three parameters concerning clinical manifestation (reduced body mass index, involuntary weight loss, and low muscle mass) and two etiologic parameters (low food intake or assimilation, inflammation or any burden related to the disease). Malnutrition is diagnosed when at least one clinical manifestation and one etiology criterion are present. Malnutrition may be staged as moderate (Stage 1) or severe (Stage 2) according to phenotypic parameters, and fall within one of four categories concerning its etiology, by considering if it is caused by a chronic disease, distinguishing if inflammation is either present or absent, by an acute inflammatory disease, or by starvation (even when related to socio/economic or environmental causes implying food shortage or hunger).