# Erythrocyte Phospholipid Fatty Acid Profile in High-Level Endurance Runners
## 1 Introduction  
Endurance runners engage in regular training sessions characterized by predominance of moderate and low-intensity sessions alternated with high-intensity ones [1]. Fatty acids (FAs) are an important source of energy during exercise, and their oxidation increases during low and moderate intensity [2]. Nutrition plays a crucial role in the recovery and adaptation processes of these athletes [3]. For years, runners have restricted lipid intake due to concerns about its accumulation as ectopic fat and potential negative impact on performance [4]. Lipids are macronutrients with essential functions in the body. They act as an energy source, hormone precursors, and participate in immune function [5]. FAs are essential components of erythrocyte membranes, and it is known that the composition of FAs ingested in the diet is related to those stored in the membrane [6]. Changes in the composition of erythrocyte membrane FAs can modify their properties and function [7]. It has been reported that dietary FA intake is reflected in plasma within days but takes months to be incorporated into erythrocytes [8]. Erythrocytes have a lifespan of 120 days and are considered an ideal matrix for evaluating long-term FA intake [9]. Polyunsaturated fatty acids (PUFAs) are essential FAs that must be ingested through the diet and have significant benefits for human health [10]. The fluidity and flexibility of the cell membrane are primarily determined by the presence of PUFAs in the lipid bilayers [11]. Omega-3 (\(\omega\)-3) and Omega-6 (\(\omega\)-6) families have different functions and compete for the same enzymatic pathways and incorporation into the cell membrane [12]. Proinflammatory eicosanoids are derived from arachidonic acid (AA), an \(\omega\)-6 FA that in high concentrations contributes to thrombus formation, atheroma, and increased inflammation [13]. However, the intake of \(\omega\)-3 FAs such as docosahexaenoicenoic acid (DHA) and eicosanetanoic acid (EPA) increases the concentrations of eicosanoids with vasodilator, platelet aggregation inhibitory, and anti-inflammatory properties [14]. In athletes, various studies have reported that \(\omega\)-3 FAs optimize recovery processes and contribute to improving the immune system [15]. Physical exercise is another factor that can modify the FAs of erythrocyte membranes. It is known that physical exercise increases the amount of reactive oxygen species (ROS) and free radicals [16]. Since erythrocyte membranes contain a large amount of PUFAs and are exposed to high levels of oxygen, they are susceptible to lipid peroxidation and potential muscle damage [17]. It has been demonstrated that moderate and low-intensity aerobic exercise favors vasodilation and an improvement in immune and anti-inflammatory response [18]. However, high-intensity training increases the production of superoxide radicals in lipid bilayers, leading to increased inflammation and the possibility of oxidative stress and damage to erythrocytes [19]. Additionally, the high intake of \(\omega\)-6 from current diets is another factor contributing to an increased \(\omega\)-6/\(\omega\)-3 inflammatory ratio, which relates to the proportion of \(\omega\)-6 and \(\omega\)-3 FAs [12]. Different studies have reported the erythrocyte profile of different sports specialities such as football and basketball [20], water polo [21], rugby [22], and hockey [23]. These sports are characterized by highly competitive activity and the performance of high-intensity actions and training [24]. These athletes showed altered profiles compared to the control population, with insufficient \(\omega\)-3 values as a consequence of chronic training and inadequate FA intake. However, endurance runners have lower competitive density during the season and different physiological demands. Runners undergo high training volumes, with 70-80% of the completed kilometers below the second ventilatory threshold (VT\({}_{2}\)) and a reduced percentage above VT\({}_{2}\)[25].