## 1 Introduction  
Oxidative stress is a condition that arises from an imbalance between the production of reactive oxygen species (ROS) and antioxidants. It is related to many diseases, physiological processes, and lifestyles including cardiovascular diseases, cancer, rheumatoid arthritis, aging, pregnancy, smoking, stress, and physical activity [1, 2]. Exercise-induced oxidative stress is a well-documented phenomenon, with both aerobic and anaerobic exercise leading to an increase in oxidized molecules. Moderate physical activity is generally considered beneficial for promoting the balance between ROS production and antioxidant defense mechanisms. However, excessive or intense exercise without adequate recovery may lead to an imbalance, resulting in increased oxidative stress and potential negative clinical effects for the athlete such as impaired and delayed recovery, muscle strength loss, delayed-onset muscle soreness, and chronic fatigue [3; 4; 5]. On the other hand, exercise-induced oxidative stress may serve as an attractive model in humans where the redox-modulating properties of dietary patterns and supplements can be tested in real dynamic conditions [6]. Exercise-induced oxidative stress can be mitigated through the use of food supplements, particularly antioxidants such as vitamins E and C, grape polyphenols, glutathione, and N-acetyl-cysteine (NAC) [7; 8; 9; 10; 11]. These supplements have been shown to reduce lipid peroxidation and minimize the detrimental effects of oxidative stress, potentially improving health and performance. However, the use of antioxidant supplementation during exercise training is a topic of debate, with some studies suggesting that it may interfere with exercise-induced adaptations [12]. Therefore, while food supplements can play a role in reducing exercise-induced oxidative stress, further research is needed to determine their optimal use. Spirulina has gained considerable recognition as an algae supplement, primarily because of its abundant bioactive compounds believed to offer potential health benefits. It contains a variety of possible antioxidants, including phycocyanin, beta-carotene, and tobacco, which could protect against oxidative stress and inflammation [13; 14]. Studies on both rats [15] and untrained human subjects [16; 17] found that spirulina supplementation increased the time to exhaustion during exercise, reduced levels of oxidative stress markers, and enhanced antioxidant status. These findings were further supported by a systematic review [18] that highlighted the potential benefits of spirulina in reducing muscle damage and improving exercise tolerance but concluded on the need to conduct further research on the topic. Traditional biomarkers of oxidative stress extensively used in research studies include thiobarbituric acid reactive substances (TBARSs), malondialdehydedehyde (MDA), and F2-isoprostanes [19]. All of them are lipid peroxidation markers that can be measured in blood samples. While blood analysis remains a valuable tool for studying oxidative stress, it involves invasive sampling that may not be feasible for frequent or repeated measurements, especially in certain populations such as pediatric or elderly individuals or those with underlying health conditions. An alternative method of oxidative stress monitoring is breath analysis. In the last decades, it has attracted great attention mostly because of its non-invasive character, allowing for repeated measurements from the same individual over time without causing discomfort or harm, and the comprehensive assessment of overall health status [20; 21]. It has been widely used for studying several health-related issues including early diagnosis of diseases, pharmacokinetic studies, and metabolism monitoring [22]. A variety of gases and volatile organic compounds (VOCs) in exhaled air have been associated with many physiological and pathological processes [23]. For instance, breath acetone is considered a potential biomarker of diabetes [24].