Más allá del umbral: respuestas individuales a la baja disponibilidad energética en deportistas

Abstract

Energy availability (EA) is a key factor for maintaining athletes’ health, performance, and adaptive capacity. EA is defined as the amount of energy remaining to sustain essential physiological functions after subtracting the energy expended during exercise. When EA is insufficient to support these basic functions, a state of low energy availability (LEA) arises, which may compromise endocrine function, metabolism, and recovery capacity. Traditionally, LEA has been defined using a fixed threshold (<30 kcal/kg FFM/day); however, recent evidence suggests that individual responses to this condition are highly variable and influenced by multiple physiological, metabolic, and behavioural factors.

The main aim of this doctoral thesis was to examine the physiological, metabolic, and behavioural determinants of LEA in elite and pre-elite athletes, considering their potential interactions and individual variability. To this end, an integrative approach was adopted, based on the assessment of body composition, resting metabolism, free-living physical activity and sedentary patterns, and validated symptom-based screening tools.

Regarding body composition, its association with symptoms of testosterone deficiency in men and with the risk of the Female Athlete Triad in women was examined. In male athletes, body fat was not associated with symptoms of testosterone deficiency; however, those who exhibited higher values of fat-free mass had higher risk of suffering symptoms. In female athletes, the total LEAF-Q score did not correlate with any body composition parameters; nevertheless, body fat percentage and visceral adipose tissue were positively associated with the LEAF-Q injury subscale.

The relationship between EA, resting metabolic rate (RMR), and the measured-to-predicted (RMRratio) was also examined, together with their association with subjective indicators of well-being and recovery. Although no significant associations were observed between EA and either RMR or RMRratio, athletes with lower RMRratio reported poorer subjective perceptions of well-being.

Finally, the relationship between physical activity and sedentary patterns with EA and perceived health was evaluated. The results showed that none of the physical activity variables were significantly associated with EA, and this finding remained consistent when reallocating time across different activity intensities. However, higher levels of sedentary time were related to greater menstrual symptoms and poorer well-being indicators, whereas low-intensity ambulatory activity was associated with better menstrual symptomatology.

Taken together, the findings of this thesis advocate for a reformulation of how LEA is conceptualised, detected, and managed. The multi-systemic nature of LEA, demands an assessment paradigm that integrates objective physiological measures with subjective indicators, behavioural profiling, and sport-specific contextualisation. This thesis advocates that no single variable, when examined in isolation, is sufficient to characterise the energetic status of athletes or to predict the presence of symptoms. Instead, these markers offer complementary yet incomplete insights.