## 1 Introduction  
Relative energy deficiency in sport (RED-S) has gained a lot of attention during the last few years. This is a syndrome resulting from low energy availability that impairs health and performance [1]. Its effects are often subtle and unrecognized but can critically alter an athlete's physical and mental status. Due to excellent work from some research groups, we now better understand RED-S effects of health in male and female athletes [2]. Unfortunately, we can only speculate to which extent and when RED-S affects performance. We know even less about performance effects among competitive athletes since LEA measurements are quite a great burden for participants. This under-researched phenomenon has many negative performance outcomes that are mostly supported by theory and some indirect evidence from research in athletes with eating disorders. In one of the first studies assessing performance in elite female swimmers, [PERSON] et al. [3] concluded that ovarian suppression impairs performance. Energy availability (EA), calculated with exercise energy expenditure (EEE) from diary logs, was associated with poorer performance. Unfortunately, evidence observing performance while actually measuring EA is lacking and is currently limited to only a few studies [4; 5; 6]. The decline in performance due to LEA could appear earlier than detrimental health effects, and more evidence is needed to support this. In females, the Low Energy Availability in Females (LEAF) questionnaire is available as a short and simple screening tool to recognize risk for RED-S [7; 8]. Screening with LEAF can be performed before any evident clinical signs are present. This validated tool has already indirectly shown poorer performance in 1000 athletes [9]. In this study, EA was assessed by two surrogate markers, eating disorder and disordered eating, which were measured by three questionnaires (the Brief Eating Disorder in Athletes Questionnaire, Eating Disorder Screen for Primary Care, and self-reported current or past history of eating disorder or disordered eating). Performance was subjectively reported with single yes/no questions for decreased training response, impaired judgment, decreased coordination, decreased concentration, irritability, and depression. In addition to these performance effects, increased injury risk was also estimated (>3 separate injuries in the last year, resulting in missed training/race days in the past year) [9]. Novel findings indicate that not only is daily EA important for an athlete's well-being and results, but it is also important for the distribution of EA throughout the day [10; 11]. The so-called within-day energy balance (WDEB) has already been associated with suppressed resting energy expenditure (REE) and blood parameters of catabolism in male athletes [10]. In elite athletes with amenorhea, potentially indicating LEA, the catabolic state was also associated with severe WDEB [11]. This severe state, also called within-day energy deficiency, was set at energy balance (EB) greater than \(-\)300 kcal/h (WDEB\({}_{-300}\)). A value of 300 kcal comes from the predicted liver glycogen amount in small subjects [12], [PERSON] et al. [12] suggested that WDEB\({}_{-300}\) in elite athletes is associated with higher body fat, which could be due to a reduction of REE. The REE can be measured or predicted, and the ratio between these two parameters (measuredREE/predictedREE or mREE/pREE) has been suggested to be a possible screening marker for RED-S [2; 13]. The importance of this ratio has yet to be confirmed as it needs more evidence [8]. This study aims to find direct evidence of LEA on performance. This paper was set as original research, with direct measurements of EA and WDEB to support the performance effects of LEA on athletes. We aim to find any associations between LEA, as detected by objective and subjective methodology and performance-related parameters obtained under strict laboratory conditions.