1 Department of Physics Education and Lifelong Sports, Faculty of Sport Sciences, Poznan University of Physical Education, 61-871 Poznan, Poland; [EMAIL_ADDRESS] (Department of Physical Therapy and Sports Recovery, Faculty of Health Sciences, Poznan University of Physical Education, 61-871 Poznan, Poland; [EMAIL_ADDRESS] (Department of Anthropology and Biometrics, Faculty of Sport Sciences, Poznan University of Physical Education, 61-871 Poznan, Poland; [EMAIL_ADDRESS] (K.)  
phosphorylation when adenosine-5'-triphosphate (ATP) is produced [8]. NAD occurs in two forms: oxidized NAD\({}^{+}\) and reduced NADH. NAD takes part in many biological reactions including electron transport. The reduction of NAD\({}^{+}\) to NADH occurs almost exclusively in the mitochondria at the final stage of cellular respiration [9; 10]. In the human body, there is a pool of NAD that takes reduced (NAD\({}^{+}\)) and oxidized (NADH) forms, transforming into each other [8]. Importantly, the NAD pool is only constant for relatively short periods [8; 11]. In the long term, the NAD amount changes depending on several factors such as age, diet, physical activity, medicaments, boosters, time of the day, etc. [11]. NAD\({}^{+}\) metabolism is complex and includes many NAD\({}^{+}\)-consuming pathways as well as de novo and salvage pathways [8]. [PERSON] and [PERSON] [1] have claimed that the monitoring of the NADH level in tissue provides important information about the mitochondrial metabolic state (energy production, amount of intracellular oxygen). In addition, changes in the NAD\({}^{+}\)/NADH ratio reflect cellular respiration processes in mitochondria, thus indirectly represent their function [1; 5]. Studies on changes in NADH in response to physical exercise were performed on animal and human skeletal muscle samples, but not in the skin [8; 9; 12]. Early reports including animals did not provide a clear answer as to how NADH levels were modified by exercise [13; 14]. Subsequent human research had shown that intensive exercise, unlike light exercise, shifted the NAD\({}^{+}\)/NADH balance toward NADH [8; 15]. Only [PERSON] et al. [16] have examined the influence of endurance training on changes in NAD\({}^{+}\) level in rat muscles and showed that training resulted in an increase in NAD\({}^{+}\) biosynthesis. Studies on skeletal muscle mitochondria are valuable, but usually invasive due to the use of the biopsy technique [17; 18] and expensive if transmission electron microscopy is used [19]. However, it has been suggested that physical exercise brings beneficial changes not only in skeletal muscle mitochondria, but also in skin mitochondria [20]. It has been demonstrated that physical exercise results in several beneficial mitochondrial adaptations [19; 21; 22; 23; 24; 25]. Various changes were extensively studied in skeletal muscle mitochondria [19; 21; 25; 26; 27], while only one study dealt with the changes in the skin [20]. However, we do not know whether training only affects muscle mitochondria, or the adaptations also take place in skin mitochondria that are easily accessible to study because they lie superficially. To the best of our knowledge, there is a lack of studies describing the effect of physical training on changes in NADH fluorescence in the skin. The novel, noninvasive, and cheap flow mediated skin fluorescence method can be a source of valuable information about the mitochondrial activity. Therefore, the study aimed to evaluate the changes in NADH fluorescence in the superficial skin layer resulting from a 7-week training period in highly trained competitive athletes. We hypothesize that physical training results in an increase in the NADH fluorescence levels in athletes.