ARTICLE 72 FEED & ADDITIVE MAGAZINE October 2022 • Leaching of mineral from the feeds prior to consumption • Availability of suitable test diet • Limited data on mineral bioavailability & nutritional requirement of many species • Investigations in fish are comparatively complicated as both dietary intake and waterborne mineral uptake have to be considered in determining the mineral budgets. • The exchange of ions from the aquatic environment across gills and skin of fish complicates the determination of the quantitative dietary requirements. • Many trace elements are required in such small amounts that it is difficult to formulate purified diets low in mineral and maintain water sufficiently free of the test element. • A critical factor in the determination of ultra-trace elements, such as manganese, vanadium, and chromium, is the need for meticulous sample preparation. • Often normal values of trace elements in fish tissue vary widely in reports from laboratory to laboratory. • Information available on fish mineral requirement is fragmentary and incomplete. • Relatively little is known about the uptake, function, and biological availability of many trace elements. BIOAVAILABILITY OF MINERALS • Bioavailability of an element can differ markedly when supplied from different feedstuffs and within the same element from feed in different diets. • Many factors can influence the bioavailability of minerals. These include the intake level of the nutrient, its chemical form, the digestibility of the diet that supplies the element, the particle size, interactions with other nutrients, chelators, inhibitors, physiological and pathological states of the animal, the water chemistry, the type of feed processing, and the species of animal being tested. • The biological availability of an element in a diet can differ depending on the molecular form in which the element is present, its valence state, the ligands present when the element is ingested from different diets. • Mechanisms that involve the formation of insoluble and nonabsorbable substances in the gut may either hinder or facilitate the mucosal uptake transport, and metabolism of an element in the body. Certain inorganic elements may compete with the test element for important binding sites during these processes. MINERAL INTERACTIONS A wide range of potential mineral–mineral and mineral–vitamin interactions has also been reported (Hilton, 1989) in fish. Antagonistic relationships occur when elements with a similar electronic configuration and ionic radius compete for binding sites, e.g. - Zinc and cadmium in metallothionein, - Magnesium/manganese substitutions at enzyme active sites, - Synergistic relationships in which one element enhances the role of another, (Mertz, 1986; Davis, 1980) e.g., iron and copper, - The complex interrelationship among copper, zinc, iron, and calcium, as well as that of copper, molybdenum, and sulfur. Another type of interrelationship involves the interactions between the elements themselves. Selenium has a high affinity for certain toxic elements such as mercury and silver; hence selenium exerts a protective effect against the toxicity of these metals by forming complexes in vitro, resulting in a decrease in the biological availability of both selenium and the heavy metal. Minerals also interact with other nutrients. A synergism between dietary selenium and vitamin E is Figure: Biological dose–response curve. Dependence of animal function on intake of an essential nutrient according to Mertz (1986).
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