Olive oil production is increasing year by year, resulting in increased quantities of by-products, such as olive leaves and cake. Olive leaves and cake are not a good source of nutrients and energy, but are an abundant source of polyphenols. Polyphenols act as antioxidants and thus help to protect the animals from oxidative stress. The goal of this study was to evaluate the effect of adding olive leaves and cake to feed mixtures on parameters of oxidative stress in growing pigs. As a part of the experiment, 24 growing pigs, weighing 10–12 kg and aged 40–55 days, were individually housed in metabolic cages. They were divided in three groups of eight animals. Control group (Kontrola) was fed with basic feeding mixture, leaves group (Listi) was fed with basic feeding mixture with added olive leaves in concentration of 7.5 %, and the cake group (Tropine) was fed with basic feeding mixture with added olive cake in concentration of 7.5 %. During the experiment, faeces and urine were collected, and at the end of the experiment blood samples were collected. The results showed a beneficial effect of adding olive leaves on fatty acid composition in plasma and on increase in the activity of the desaturase enzymes in plasma but also showed an impairment of the absorption of fatty acids. Supplementation of olive leaves and cake had no effect on the concentrations of vitamin E, lipid-soluble antioxidant capacity (ACL), and water-soluble antioxidant capacity (ACW) in blood plasma. The supplements also had no effect on the markers of oxidative stress: malondialdehyde (MDA), F2-isoprostanes, oxidised low-density lipoprotein (oxLDL), and 8-hydroxy-2’ –deoxyguanosine (8-OHdG).
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