## 1 Introduction  
Cardiovascular disease (CVD) is the most significant global non-communicable disease (NCD), which is responsible for a third of total mortality [1]. The search for an effective antidote and prevention of this disease's mechanism is still in progress [2, 3]. Many risk factors including lifestyle modifications, drug use, tobacco smoking, and dietary considerations have been defined [4]. Accumulating scientific evidence suggests that oxidative stress (OS) is also a common pathway for developing CVD. Concurrently in the last decade, the diet antioxidant capacity as a factor in preventing oxidative stress mechanisms has been raised in scientific discussions. It has been shown that patients at risk of CVD have enhanced OS and could benefit from dietary antioxidant treatment [5]. Although the benefits associated with antioxidant-rich foods intake are known, the precise doses capable of lowering oxidative stress or improving the body's antioxidant potential have not yet been determined. Some research revealed relationships between OS and dietary antioxidant intake [6; 7; 8; 9; 10; 11; 12]. It has been shown that OS markers improved after serving fresh fruits, vegetables, chocolate, and red wine [7; 8; 10; 13]. Other results revealed that consuming 160 g of potato chips daily increased oxidative stress and inflammatory markers [11]. Although these studies showed the possibility of regulating the body's antioxidant capacity, their results do not allow for a straightforward and practical interpretation of dietary recommendations for CVD patients. It is worth noting that studies on well-monitored organism antioxidant capacity changes related to a dietary supply of natural antioxidants are missing. To understand how dietary intervention (DI) increases the body's antioxidant capacity, it is essential to note that firstly, the antioxidant capacity of the dietary intake should be monitored; and secondly, biomarkers of OS should be fitted precisely and analysed. The dietary antioxidant capacity is analysed in the literature with many markers. Since antioxidants are a chemically diverse group of compounds, one popular compound is the oxygen radical absorbance capacity (ORAC) [14; 15; 16]. The biomarkers of OS (C-reactive protein, interleukin-6, tumour necrosis factor-\(\alpha\), and F2 isoprostane) and its reactions to DI have also been studied in the literature [6; 7; 8; 9; 10; 11; 12]. Nevertheless, their reproducibility and sensitivity regarding dietary intervention varied, especially when a practical approach to dietary recommendation was needed. It is worth noting here that changing the dietary patterns leading to permanent modifications in dietary intake is challenging for many patients and thus should be related to simple and realistic recommendations [17]. Therefore, the focus should be on minor nutritional modifications based on existing eating habits, such as snacking [18; 19]. Therefore, when undertaking DI we decided to use red beetroot and chokeberry snacks, which are popular plants with known health-promoting properties recognised by respondents in Poland. Another issue is monitoring diet-induced changes in the body's antioxidant capacity. Scientific debates on OS markers and the organism's antioxidant capacity were presented in the literature [17; 20; 21; 22]. It was concluded that antioxidant capacity biomarkers are better antioxidant defence forecast in pathophysiological conditions than OS markers [23; 24; 25]. Under physiological conditions, an antioxidant system defends the cells from ROS-induced damage [26]. The most important antioxidant enzymes are SOD, catalase (CAT), and GPx [27]. These enzymes represent the primary line of ROS defence, and glutathione, among them, is perceived as the most vital antioxidant for the body. Therefore, it is pertinent to observe the glutathione defence-related markers.