## 1 Introduction  
Sterols belong to a group of steroids. They have three six-carbon rings, one five-carbon ring, a side chain located at carbon 17, a double bond between the fifth and sixth carbon, and a hydroxyl group located at the third carbon atom (Figure 1) [1]. Sterols can also be divided according to their origin. There are three groups of sterols: those of animal origin (for instance, cholesterol, cholesterol, and coprostanol), plant sterols--photosterols (for example, \(\upbeta\)-stosterol, campesterol, stigmasterol, and brassicasterol), and mycosterols found in mushrooms (i.e., ergosterol). A large part of cholesterol in the human body, about 60-70%, is produced by the liver, smaller amounts by the adrenal glands, intestines, and skin, and about 20-25% may come from a diet containing animal products [2, 3]. The sources of cholesterol in a diet are mainly products of animal origin, such as cream, lard, and eggs, and also fatty cheese and meat [4]. One of the most important dietary recommendations is to limit cholesterol intake to a maximum of 300-350 mg per day [5]. Too high a level of cholesterol in the blood may lead to an increased risk of diseases such as atherosclerosis and coronary heart disease and may also promote the appearance of gallstones [6, 7]. However, a low intake of cholesterol, especially in the presence of iron in the diet, may increase the risk of Parkinson's disease [8]. The increasing interest in phytosterols is related to their effect in reducing cholesterol levels in human blood serum, which is increased in over 60% of Poles [9]. Along with high cholesterol levels (hypercholesterol), phenomena such as hypertension and atherosclerosis often occur. By consuming phytosterols, which reduce the absorption of cholesterol in the intestines and intensity its excretion together with feces, the level of cholesterol in the blood is reduced (total cholesterol and LDL cholesterol fraction), and the risk of the above-mentioned diseases are decreasing [10]. Plant sterols have several other positive properties--for instance, they reduce the risk of stomach, colon, and prostate cancer--and they also have anti-inflammatory properties and antioxidants [11]. For people with dyslipidemia, it is recommended to consume about 2 g of phytosterols per day [12]. The best sources of plant sterols in the human diet are vegetables and oils. Oxysteros are oxidized derivatives of animal or plant sterols. If cholesterol is oxidized, the resulting products are called cholesterol oxidation products, abbreviated as COPs. If the substrate were phytosterols, we are talking about phytosterol oxidation products (POPs). Oxysteros can be formed in the human body in oxidation reactions with or without enzymes, e.g., by autoxidation. Oxysteros may also occur in food due to the oxidation of lipids that have been heat-treated or due to exposure to light and oxygen in the air [13,14]. Products resulting from the oxidation of sterols have many negative properties, including the effect they have on DNA synthesis, interfering with the proper functioning of cell membranes, and also have mutagenic, angiotoxic, and cytotoxic properties, which can lead to the development of atherosclerosis and promote the occurrence of some cancers (breast and prostate). However, it should be emphasized that the products of the oxidation of animal sterols (cholesterol) are more toxic than the products of the oxidation of plant sterols [15,16]. The most toxic cells are characterized by cholestanetriol, 7-ketocholesterol, 7\(\alpha\) and 78-hydrocholesterol and 25 and 26-hydroxycholesterol, while 5\(\alpha\),6\(\alpha\) and 5\(\beta\),6\(\beta\)-epoxysterols are important factors causing cellular steatosis [17]. Cholesterol oxidation products may enhance the occurrence of inflammatory reactions in the body; this action involves the initiation of the expression and synthesis of molecules such as inflammatory cytokines, chemokines, and adhesion molecules [14].