## 1 Introduction
Similarly, breath isoprene has been associated with many metabolic effects and diseases such as chronic heart disease, chronic liver disease, hypoglycemia, and others. However, because of its lack of specificity to a disease, isoprene is not considered a useful biomarker [25]. Additionally, aldehydes, particularly pentanal, hexanal, and heptanal, presented significant elevations in expired air of patients with lung cancer [26]. The effect of exercise on VOC levels is much less studied, but the studies so far clearly show a change in the profile of VOCs after different exercise modalities. Recently, [PERSON] et al. [27] compared the volatilome of ultraendurance runners before and after an ultramarathon. Out of 793 VOCs, 63 changed significantly, and the alteration of several of them, such as acetate, acetone, isoprene, 2,3-butanedione, 2,3-butanedid 4-heptanone and 2-butanone, implied changes in lipid oxidation, inflammation, and gut microbiome activity. In addition, several researchers have already used exercise as a stimulus to assess VOCs, which may serve as markersof the inflammatory profile of patients with inflammatory bowel disease [28] or asthma features in asthmatic children [29]. Among the various types of VOCs in exhaled air, alkanes are particularly important in monitoring human health status due to their potential as biomarkers for various physiological and pathological states. Exhaled alkanes are generated by oxidative processes in the body associated with the lipid peroxidation of polyunsaturated fatty acids (PUFAs) by ROS [30]. Lipid peroxidation is initiated by the abstraction of a H radical from an allylic position followed by the reaction of the allylic radical, giving lipid hydroperoxides. Lipid hydroperoxides are subsequently transformed into unsaturated oxocarboxylic acids that react with carbon radicals leading to shorter-chain alkanes. Depending on the number and position of double bonds and the side of the allylic position (alkyl side or -COOH side), different volatile and semi-volatile end products are expected, including alkanes and alkenes [30]. Therefore, their concentration is believed to reflect the level of oxidative stress in the body, and they have been used as oxidative stress biomarkers in many studies [31; 32; 33; 34; 35; 36; 37; 38; 39]. Among those studies, there are only a few focusing on exercise-induced oxidative stress [40; 41]. In the current work, the non-invasive approach of breath analysis is implemented for the first time in an intervention study that investigates the effect of spirulina supplementation on exercise-induced oxidative stress. All previous, similar, research works were based on invasive blood analysis using traditional biomarkers of oxidative stress such as TBARSs, reduced glutathione (GSH), protein carbonyls, Total Antioxidant Capacity, and Superoxide Dismutase (SOB) [16; 17]. However, in this work, a novel breath analysis method is proposed that uses 15 target alkanes, ranging from C5 to C14, as potential oxidative stress biomarkers in exhaled air, to investigate the effect of spirulina on exercise-induced oxidative stress. The main purpose of this study was to examine if the proposed breath analysis method could provide insights regarding the potential effect of spirulina supplementation on exercise-induced oxidative stress.