ISSUE FOCUS FEED & ADDITIVE MAGAZINE May 2024 59 not fully dissociated and thus have very poor bioavailability (Cardoso et al., 2021). In the stomach, most trace minerals dissociate from ligands to which they are bound, allowing the resulting free metallic ions to enter the small intestine. However, if not absorbed, the increase in pH conditions leads to a de novo chelation processes (Vacchina et al., 2010). Moreover, the bioaccessibility of zinc (Zn) and other metal ions is drastically dependent on chemical interactions in the small intestine. A higher degree of dissociation in the small intestinal lumen enhances phytic acid's capacity to chelate cations. Consequently, in conditions characterized by high phytate and low phytase activity, trace mineral sources that are less soluble in water may be less susceptible to interact with phytate. The acid-induced solubilization and dissociation set the stage for the subsequent absorption of metallic ions in the small intestine, ensuring the effective utilization of trace minerals. Then, in the small intestine, specific transporters aid the uptake of metallic ions into the intestinal cells, which are transported by their chemical ligands. These transporters recognize and enable the absorption of different trace minerals (Richards et al., 2010). Nowadays, some of the trace minerals are being chelated to improve their bioavailability and minimize antagonistic interactions with phytase, calcium, or other dietary components. This process helps them resist degradation in the stomach's acidic conditions, thereby sustaining mineral homeostasis. Some advocates of chelation assert that these chelated trace minerals utilize active transport pathways supported by amino acid and peptide transporters across enterocytes, thereby optimizing mineral absorption and tissue deposition. However, according to recent European Food Safety Authority (EFSA) opinions, studies have shown that Zn deposition in animal tissues from chelates of glycine, hydroxy analogue of methionine, or amino acids hydrate have demonstrated no significant differences when compared to that of Zn sulfate or inorganic Zn. Additionally, previous studies using radioisotope labeling have shown varying ratios of Zn to C and S isotopes from Zn-methionine at the gut barrier and within enterocytes (Beutler et al., 1998, Hill et al., 1987a, and Hill et al., 1987b). These studies have also revealed distinct time kinetics of absorption of these labelled Zn ions compared to C and S ions in enterocytes, suggesting differing absorption pathways for Zn and methionine. Similar findings have been observed with other chelated sources. In addition, studies applying X-ray absorption structure spectroscopy determined identical Zn-speciation within intestinal cells of sheep and broilers fed either inorganic or organic Zn, respectively, providing further evidence that entry routes for Zn into the organism do not differ between feed Zn sources (Sui et al., 2011 and Liu et al., 2014). Recent research in pigs and poultry have illustrated that chelating agent alone significantly reduces phytate antagonism with Zn from Zn sulfate. This suggests that the occasional superiority of chelates under high phytate conditions is mainly attributable to altered chemical interactions within the gastrointestinal lumen rather than by alternative, molecular transport mechanisms (Windisch et al., 2002 and Boerboom, 2021). Under semi-synthetic conditions, denoted by the absence of phytic acid in the diet, the superiority small intestine pH: 6.0-8.0 small intestine pH: 5.0-7.5 small intestine pH: 7.0-8.0 gizzard pH: 2.5 rumen pH: 6.5-7.0 esophagus pH: 6.5-7.0 esophagus pH: 6.5-7.0 reticulum pH: 6.0-7.0 omasum pH: 7.0-8.0 abomasum pH: 2.2-3.8 stomach pH: 1.5-4.0 crop pH: 5.5 proventriculus pH: 3.5 Figure 1. The digestive system of swine, poultry, and ruminant with certain pH conditions along the GI tract (authors).
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