Effect of soya bean and fish oil inclusion in diets on milk and plasma enzymes from sheep and goat related to oxidation

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Supplementation of ruminant diets with vegetable oils, rich in linoleic acid, induces an increase in conjugated linoleic acid (CLA) content in the milk (Martínez Marín et al., 2012), while the inclusion of marine lipids, such as fish oil, enhances the n‐3 polyunsaturated fatty acid (PUFA) content in milk fat (Capper et al., 2007), which is very important because the contribution of n‐3 PUFAs to milk and dairy products in order to meet the human nutritional requirements is low in a balanced diet (Shingfield et al., 2008).
Nevertheless, this approach either could increase the risk of plasma lipid peroxidation with deleterious consequences on animal health or milk spoilage (Grandelli et al., 1998), or could cause unpleasant oxidation (Palmquist et al., 1993), because fatty acids, especially PUFAs, are easily oxidized. As a result, the inclusion of PUFA in ruminant diets, in most cases, is accompanied by the addition of antioxidant compounds (Gobert et al., 2009; Santos et al., 2014).
Recently, it has been proven that some feedstuffs, such as flax hulls, are not only rich in PUFA, but also rich in antioxidant compounds that improve the oxidative status of cows (Côrtes et al., 2012; Schogor et al., 2013) without an extra administration of an antioxidant source. The soya bean oil could be another option because it has the highest antioxidant capacity compared to other vegetable oils, such as extra virgin olive, corn and sunflower oil (Pellegrini et al., 2003), and it also contains high tocopherol content (Castelo‐Branco and Torres, 2012). In addition, the use of soya bean oil as a possible dietary source to improve the oxidative status of ruminants has been proposed by Scislowski et al. (2005a). Positive impact on the antioxidant defence system may also be observed with the fish oil too, because its inclusion in the diets of rabbits (Hsu et al., 2001) and rats/hamsters (Erdogan et al., 2004; Muga and Chao, 2014) inhibited the oxidative stress and prevented the formation of free radicals. Although fish oil is not rich in antioxidants per se, generally it is supplied with natural antioxidants, such as tocopherols, by the food industry to prevent oxidative deterioration due to its high content of unsaturated fatty acids (Rossell, 2009; Yi et al., 2011).
Oxidative stress in ruminants occurs not only during periparturient period or heat stress but also when they receive diets rich in PUFA (Scislowski et al., 2005a,b). Although dietary PUFAs are subjected to ruminal biohydrogenation, some of them escape from this process. Thus, before their absorption by various tissues, including in the milk, dietary PUFAs are absorbed by small intestine and recycled in blood mainly as TG‐rich lipoprotein particles. At this stage, PUFA becomes preferential targets for the action of free radicals that induce an oxidative stress. The oxidative stress is faced normally by the body with a wide range of antioxidant mechanisms that can be divided into enzymatic and non‐enzymatic (e.g. metabolites) (Ye et al., 2015). This type of stress is facilitated if an imbalance occurs between the respective amounts of PUFA (which can be attacked by free radicals) and antioxidant systems (involved in PUFA protection against free radicals). Several endogenous enzymes, such as superoxide dismutase (SOD), glutathione reductase (GR), catalase (CAT) and glutathione transferase (GST) (Miller et al., 1993; Board and Menon, 2013), found both in the blood and in the milk represent the main intracellular antioxidant defence system that regulates reactive oxygen species accumulation within the tissues (Celi, 2010; Sordillo, 2013) (Fig. 1). Furthermore, the enzyme lactoperoxidase (LPO) is related to the oxidation of milk lipids (O'Connor and O'Brien, 2006).
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