Introduction: Regular physical activity is associated with substantial health benefits, provided the body has sufficient energy sources. However, a long-term low-calorie intake can cause the syndrome of Relative Energy Deficiency in Sport (RED-S) with a significant health threat to athletes. Therefore, the objective of the current study was to evaluate markers of RED-S in a cohort of female athletes. Methods: The study was carried out in a cohort of female endurance athletes (n = 23) and healthy female control subjects (n = 21) recruited from a total of 42 athletes and 45 controls who underwent the Low Energy Availability in Females Questionnaire (LEAF-Q) survey. Anthropometric, nutritional, and laboratory analyses were performed on study subjects. Results: A higher LEAF-Q score signifying low energy availability (LEA) was observed in athletes compared to controls (8 vs. 5, p < 0.005). Menstrual problems were significantly more common in athletes, with 33.3% of athletes having amenorrhea (p < 0.001). Compared to control women with LEAF-Q < 8, energy expenditure was higher (p < 0.001) in athletes with LEAF-Q = 8, and athletes had a negative energy balance (90%) with a very low value of energy availability (24.3 kcal/kg FFM/day). Athletes with a LEAF-Q score =8 had lower serum concentrations of estradiol (p < 0.001), progesterone (p < 0.001), leptin (p < 0.001), white blood cells (p < 0.005) and phosphorus (p < 0.005). Furthermore, they had significantly higher concentrations of hepcidin (p < 0.05) and free T3 (p < 0.05). Discussion: LEA is prevalent in female endurance athletes, and its diagnosis deserves a multifactorial approach with anthropometric and nutritional analyses, and the use of a wider range of laboratory markers.
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