Research Progress on Amino Acid-Chelated Iron and Zinc and Their Applications in Pig Raising

         Iron and zinc are trace elements indispensable for the growth and development of livestock and poultry. Iron performs multiple functions in animals, primarily by serving as an essential component of hemoglobin, myoglobin, cytochromes, and various oxidases. As an oxygen carrier, iron ensures the normal delivery of oxygen to body tissues. Moreover, iron is an active constituent of numerous metabolic enzymes within the body, such as iron-sulfur proteins, cytochromes, cytochrome oxidase, and peroxidases. The activity of certain enzymes, including acetyl-CoA synthetase, succinate dehydrogenase, xanthine oxidase, and cytochrome reductase, is closely associated with iron and plays a crucial role in cellular bio-oxidation processes. Additionally, iron influences protein synthesis and immune function in animals. Zinc is an essential trace element required for animal metabolism; due to its wide range of physiological and biochemical functions in the body, it is often referred to as the "element of life." In higher animals, there are over 200 metalloenzymes containing zinc. Many enzymes exhibit their maximum activity only in the presence of zinc, and zinc also helps maintain the structural conformation of certain enzyme’s organic molecular cofactors and acts as a coenzyme during enzymatic reactions. Zinc participates in the synthesis of DNA and RNA polymerases and is closely linked to protein metabolism.
In production, ferrous sulfate and zinc sulfate are commonly added to diets to supplement deficiencies in iron and zinc. Although trace elements added in the form of inorganic salts have the advantages of low cost and chemical stability, they also have significant drawbacks, such as low bioavailability, potential antagonism and degradation of other nutrients, and widespread overuse. This not only results in a waste of resources but also impairs the absorption of nutrients and increases the likelihood of severe environmental pollution. Therefore, in recent years, both domestically and internationally, there has been growing interest in researching and promoting the use of organically bound, bio-complexed trace mineral elements to replace inorganic trace elements and incorporate them into animal feed. Organic trace elements primarily refer to amino acid chelates and protein chelates of trace minerals. Many researchers believe that organic trace elements exhibit higher biological activity and bioavailability, can enhance the body’s immune function, have good palatability, require lower dosages, and are more environmentally friendly.
Currently, the unscientific supplementation of essential trace element additives is a fairly common problem—specifically, the practice of over-supplementing these elements. Moreover, when formulating compound feeds, little consideration is given to the natural content and bioavailability of essential trace elements in the feed ingredients themselves, particularly when high-Zn and high-Fe formulations are used in feed. Since foreign reports indicated that high levels of Zn and Fe can promote growth and reduce post-weaning diarrhea in piglets, the amounts of Zn and Fe added to feed have steadily increased, with most of these additions being made in the form of inorganic salts. Because inorganic Fe and Zn have low absorption rates, the Zn and Fe added to feed are largely excreted undigested by the animal’s body. This not only results in a waste of resources and causes residues in livestock and poultry but also leads to pollution of soil and water bodies.   

1. Nutritional properties of amino acid-chelated iron and zinc
    The most prominent feature of trace elements in organic form is their high bioavailability. Bioavailability refers to the proportion of ingested trace elements that are digested, absorbed, transported to their sites of action, and converted into biologically active forms—relative to the total amount consumed. Amino acid chelates not only represent the primary form through which the body absorbs metal ions but also serve as intermediate substances in the protein synthesis process within animals. They are not only rapidly absorbed but also reduce the number of biochemical reactions required, thereby conserving energy expenditure and exhibiting higher bioavailability. After being chelated by amino acids, iron and zinc can form remarkably stable chelates that effectively inhibit interactions among different minerals. Moreover, under the protection of the coordinating amino acid ligands, iron and zinc ions can efficiently prevent the formation of insoluble inorganic salts with other impurity ions. Consequently, we can add significantly less amino acid-chelated iron and zinc to feed compared to their inorganic counterparts.
The biological availability of iron and zinc varies significantly depending on the source of these minerals. Wedekind et al. (1992) reported that, in piglets fed a corn-soybean meal-based diet, if ZnSO4 is taken as the reference substance with a biological availability assumed to be 100%, then the biological availability of ZnO is 61%, whereas the relative biological availability of methionine-chelated zinc can reach as high as 206%. According to Uenry and Millerl (1995) and Cunha (1997), if FeSO4 is used as the reference standard with a biological availability of 100%, the relative biological availability of amino acid-chelated iron ranges from 125% to 185%. Zhou Guilian et al. (2000), through their study on the biological availability of iron from different sources, found that the biological availability of lysine-chelated iron and glycine-chelated iron was 1.73% to 48.31% and 2.75% to 47.19% higher, respectively, than that of ferrous sulfate.
2. Mode of Action of Amino Acid-Chelated Iron and Zinc
It has been demonstrated that the absorption of amino acid-chelated iron and zinc occurs via the same mechanisms used for the absorption of amino acids or small peptides, which differ from the absorption mechanisms of ordinary metals in the small intestine (Ashmead et al., 1985). Found (1974) proposed that iron and zinc ions located at the centers of five- and six-membered ring chelates can be absorbed through the brush-border membrane of small intestinal villi in the form of amino acids or peptides. This theory is based on the fact that iron and zinc ions are coordinated to amino acid ligands via both covalent and ionic bonds, thereby being protected within the core of the complex from certain physicochemical factors. Moreover, metal-amino acid chelates are primarily absorbed in the intestinal mucosa; they cross the mucosal cell membranes, as well as the membranes of the mucosal and basal cells, in their entirety, entering the bloodstream and ensuring more efficient absorption of the iron and zinc they carry. When amino acid-chelated iron, copper, zinc, and manganese are used, the antagonistic effects among these elements are significantly reduced (Hemken et al., 1996). For all animals, there is evidence indicating that amino acids are absorbed either as peptides or in free amino acid form—indeed, absorption as peptides may even be more important (Webb et al., 1992). Small peptides can be absorbed intact, passing through the intestinal mucosal cells into the systemic circulation (Bronk et al., 1993). The absorption mechanisms of iron and zinc—whether via amino acids or peptides—make their absorption more efficient.
3. Application Effects of Amino Acid-Chelated Iron and Zinc in Pig Raising
3.1 Piglets
Oral administration of amino acid-chelated iron to piglets can increase hemoglobin levels and plasma iron concentrations, prevent iron-deficiency anemia in piglets, and promote the improvement of their immune function as well as their growth and development.
Feng ZhanYi (1992) reported that adding 60 mg/kg of methionine iron to the diet of pregnant sows one month before farrowing and for 20 days after farrowing, as well as supplementing piglet starter feeds with 100 mg/kg of methionine iron, could increase weaning litter weight by 11.24% to 18.76%, boost daily weight gain by 17.64% to 29.40%, and significantly elevate hemoglobin and plasma iron levels in piglets, thereby enhancing their immune function and disease resistance. Spears (1992) reported that feeding methionine iron to piglets from 3 days after birth reduced mortality by 30.4% compared to feeding ferrous sulfate. Furthermore, adding methionine iron to the diet of weaned piglets could reduce the incidence of diarrhea by 4.3%. Numerous other studies have also shown that administering amino acid-chelated iron to sows during the peripartum period can achieve similar effects on weight gain and anemia prevention as intramuscular injections of dextran iron in piglets. Additionally, it has been reported that adding methionine zinc (containing 40 mg/kg of zinc) to weaned piglet diets can reduce the incidence of diarrhea by 3.8%.
3.2 Growing and Finishing Pigs
Lou Xuebiao (1992) reported that adding 60 mg/kg of methionine iron to the diet of growing-finishing pigs aged 45–90 days could increase weight gain by 13%, reduce feed-to-gain ratio by 11.7%, and improve economic benefits by 13%. Kuznetsov (1996) reported that, for growing pigs aged 27–82 days, the bioavailability of iron from methionine chelate iron was 36% higher than that from ferrous sulfate. According to a study conducted by the University of Tennessee in the United States (1990), feeding methionine zinc to growing-finishing pigs resulted in an average daily weight gain increase of 8.8% (P<0.5) compared to inorganic zinc, and also improved feed efficiency by 5.9% (P<0.01). Laveme Schugel specifically pointed out that feeding methionine zinc has a favorable anti-stress effect on pigs during periods such as vaccination, castration, stress induction, disease outbreaks, sudden climate changes, and when dietary ingredients are altered. Lü Defu reported that feeding growing-finishing pigs with protein microspheres could increase daily weight gain by 33% and improve feed utilization by 24%.
3.3 Pregnant and Lactating Sows:
Adding methionine-chelated iron to the diets of pregnant and lactating sows can reduce stillbirths, increase birth weights and iron content in newborn piglets, prevent anemia in piglets, and lower mortality rates. According to available data, when sows are fed amino acid-chelated iron (500 ppm) from 21 to 28 days before farrowing, piglets no longer require supplemental iron after birth. As a result, piglet mortality is significantly reduced, weaning weights are higher, and the rearing survival rate of weaned piglets can reach as high as 94%. Xu Jianxiong (1993) reported that when sows’ diets were supplemented with 60 mg/kg of iron plus methionine-chelated iron, the weaning weights of piglets at 35 days of age increased by 19.4%.
Microelement amino acid chelates not only help maintain normal reproductive performance in animals but also reduce chronic uterine infections, lower embryo mortality rates, accelerate uterine tissue recovery, and improve the reproductive performance of female livestock. Gu Xiaohua (1994) reported that feeding amino acid chelated iron to sows during the last 21–24 days of gestation can reduce piglet mortality, improve the body condition of sows, decrease the culling rate of multiparous sows, reduce the amount of feed required for raising piglets, and prevent diseases such as anemia and diarrhea in piglets.
4. Issues with the Use of Amino Acid-Chelated Iron and Zinc
4.1 The positive effects of amino acid-chelated iron and zinc in livestock and poultry production are well established; however, their mechanisms of action, optimal addition levels, and factors influencing their efficacy still require further investigation—for instance, the metabolic differences between organic and inorganic forms in terms of their biological functions.
4.2 Whether imported or domestically produced, amino acid-chelated iron and zinc—whether in the form of single amino acid chelates or as part of complex additives—are currently generally priced too high, typically 5 to 10 times higher than the prices of inorganic salts. Although the absorption rate of inorganic iron and zinc is only one-fifth that of chelates, people still tend to prefer inorganic salt products. The price factor significantly hinders the wider adoption and use of amino acid-chelated iron and zinc.
5. Solutions for applying amino acid-chelated iron and zinc
5.1 We should further develop protein resources, fully utilize inexpensive protein raw materials to obtain low-cost amino acids, and continuously improve production processes by selecting appropriate process routes, thereby significantly reducing production costs and developing amino acid chelated iron and zinc products that meet the specific needs of the animal production market.
5.2 While exploring the mechanisms of action, it is important to rationally develop and utilize certain feed resources that are high in protein but low in biological potency, while maintaining strict quality control to ensure they meet the basic requirements for both additives and animal digestive physiology.