## 1 Introduction  
In sports, the monitoring of body composition is a key factor and its variation over time can affect the athletes' performance [1]. Indeed, to obtain success in sport competitions, it is mandatory to achieve an optimal body composition in athletes through the careful adjustments of their training and nutritional habits according to specific sports demands. In the context of healthy eating habits, it has been documented that a higher adherence to the Mediterranean Diet (MD) boosts sports performance [2, 3], suggesting that MD represents a valid pattern for preventing a good health status in the human population that practice physical activity [4, 5, 6, 7, 8]. Hence, it has been reported that adjustments to the MD plan should be made in athletes according to sport nutrition practices to optimize sport performance [9, 10]. To measure and monitor the body composition of athletes during pre- and post-competition, different methods are currently available, including the bioelectrical impedance analysis (BIA), which represents the most used non-invasive tool. BIA, by detecting resistance (Rz) and reactance (Xc) along with impedance (Z), as the resultant from Rz and Xc, allows to evaluate body composition parameters and cell hydration status [11]. From these raw measurements, using prediction equations, several body composition parameters, such as free fat mass (FFM), fat mass (FM), total body water (TBW), and intracellular and extracellular water (ICW and ECW), are calculated. Among the bioimpedance parameters measured with BIA, the phase angle (PhA) is one of the most clinically important parameters, which is used as an indicator of cellular health, quality of cells, cell function, and membrane integrity [12; 13; 14]. PhA values between 5\({}^{\circ}\) and 7\({}^{\circ}\) indicate good cell structure and a good membrane integrity, lower values of PhA are related to fluid accumulation and loss of membrane integrity, while higher values are related to dehydration. Thus, the evaluation of the PhA is a qualitative approach included in the analysis of the body composition through BIA. In previous decades, technological improvements have made the BIA a more reliable and, therefore, more acceptable way of measuring body composition through the qualitative analysis bioelectrical impedance vector analysis (BIVA), which consists of the interpretation of the raw parameters recorded in BIA and plotting them as a vector within a graph. Of note, avoiding the use of regression equations, BIVA shows the vector position within tolerance ellipses drawn for each specific population. Indeed, tolerance ellipses reflect the percentile in body composition parameters and can be useful to identify the specific athlete's profile for each sport [15] to create a reference target zone for each sport and each competitive level [16]. Specifically, the analysis through a rightward or leftward displacement of the BIVA vector to the major axis may represent a decrease or increase in the ICW/ECW ratio and the amount of TBW [17]. In addition, the vector migration laterally to the left with respect to the major axis indicates a greater cell mass, while displacements to the right with respect to the major axis indicate a smaller amount of cell mass [18]. The vectorial approach appears to be more efficient, since it considers both variables, PhA and vector length. Indeed, in groups of individuals characterized by identical PhA, but different vector lengths, and showing different body fluids or % FM [19], qualitative analysis may be used to monitor changes in bioelectrical parameters and in body composition. BIVA is useful to compare data with population-specific references in physiological conditions, such as in athletic subjects or in the elderly, and in pathological conditions such as in obesity [20]. Moreover, vector displacement by BIVA can indicate changes in soft tissues and body fluids in response to changing eating habits, use of supplements, and training programs or competitions [15; 16].