EXPERT TOPIC
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SHRIMP SALMON
TRACE MINERALS
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A study of Trace minerals in farmed Atlantic salmon by Senior researcher Dr Katerina Kousoulaki MSc, PhD
ll seafood and fish, be they wild or farmed, are rich sources of essential and highly bioavailable trace minerals, as are other foods of animal origin (Yildiz 2008; Sarti et al., 2015). Some of these trace minerals support protein function and accelerate enzymatic activities in some of the most important metabolic processes in the animals. For instance, iron (Fe) is bound to heme proteins in vertebrates facilitating the storage and transport of oxygen and electrons (Greenwood and Earnshaw, 1998). Zinc (Zn) is involved in over three hundred enzymes (in Scarpa & Gatlin 1992; Soudek et al., 2016), among the most important functions of which are CO2 regulation, tissue damage healing, protein digestion and DNA transcription.
"It is estimated that nearly 2 billion
people in developing countries suffer from Zinc deficiency resulting in around 800,000 deaths among children. Can farmed Atlantic Salmon be the solution?" Manganese (Mn) is an important trace mineral and antioxidant, also involved in protein metabolism (Law et al., 1998). Copper (Cu), found in cytochrome c oxidase, is involved in energy metabolism and the production of ATP, and in hemocyanin, the oxygen carrying protein in molluscs and arthropods (Coates and Nairn, 2014). Selenium (Se) is another antioxidant trace mineral essential in low concentrations, involved in stress responses of the cells (Watanabe et el., 1997).
Trace mineral distribution in farmed Atlantic salmon
In salmon, the trace mineral distribution varies depending on the tissue. Zn, important in wound healing and the most abundant trace mineral in salmon body and tissues, is found in higher levels in the gills and the skin of the fish, as is also Mn (Figures 1 A and B). Those tissues are directly exposed to the aqueous
environment and thus incur higher damage risks. Cu is abundant in the liver of salmon, as this organ is the most important one for energy metabolism. Fe was analysed mainly in the gills and liver of salmon where there is higher blood content and haemopoetic function, respectively. Se is present in lower amounts in all tissues analysed, at higher levels in salmon liver. The highest amounts of minerals are present in the fillet as this is the largest organ of salmon. It is however worth noting that other tissues, small organs such as liver and the gills, or larger ones such as the skin, can provide nearly the same amounts of trace minerals as the whole of the fish fillet. Although, those parts of the fish are not common in our diet. Nevertheless, novel ingredients made from filleting plus-products, such as hydrolysates also developed in Nofima (Aspevik et al., 2016), can be used as nutritional supplements or added in different processed fish products or soups. Similar to the gills of salmon in our study, high Zn levels are present also in fish eyes (Do Carmo E SĂĄ et al., 2005).
Feed trace minerals and Atlantic salmon production performance, lipid metabolism and health
Trace minerals such as Fe and Zn together with Mg, act as co-factors and coenzyme precursors for long chain n-3 polyunsaturated fatty acids (LC n-3 PUFA) biosynthesis by bioconversion of alpha linolenic acid (ALA) to EPA and DHA, facilitating the catalytic activity of desaturases, elongases as well as peroxisomal β-oxidation (Mahfouz and Kummerow, 1989; Eder and Kirchgessner, 1994; Zhou et al. 2011). LC n-3 PUFA, such as EPA and DHA, are susceptible to lipid peroxidation, and salmon with higher levels of n-3 PUFAs may be even more exposed to oxidative stress if the red-ox system is ineffective due to low mineral-status, especially when the requirement of vitamin B2;3;6;7 (in Lewis et al., 2013) and the antioxidant vitamins are not fully covered (Tacon, 1996; Mourente et al., 2002). Mn, Cu and Se are the functional red-ox centres of the antioxidant enzymes superoxide dismutase (SOD) and glutathione peroxidase (GPX), where SOD handles superoxide and GPX handles both lipid and water soluble hydroperoxides (Mariotti et al., 2012). A GPX situated in the intestinal mucosa, specifically handles fatty acid hydroperoxides from the diet and converts them to non-toxic hydroxyl fatty acid (Esworthy et al. 1998; Lygren et al. 1999). Sufficient intake of these trace minerals is therefore important to protect fish against lipid oxidation and the resulting oxidation products, which can be
32 | February 2017 - International Aquafeed