# Natural Clays as Potential Amino Acids Carriers for Animal Nutrition Application
## 1 Introduction  
Over the past few decades, the precision nutrition of animal farming has developed more detailed tools and technologies to pursue the distribution and balance of essential nutrients in an effective animal diet. The final target is to fulfill more comprehensive and dynamic nutritional recommendations in order to improve both the animals' health and the quality of products of animal origin [1]. In this context, the efficient use of dietary proteins and protein digestibility play a pivotal role [2]; moreover, this approach satisfies sustainability and nutritional ecology principles. Indeed, the tendency to reduce crude protein (CP) levels in an animal diet is in the direction of nitrogen minimization in animal excretion, an increasing environmental issue [3, 4]. The need for an increased supplementation of synthetic essential amino acids in animal feeding is a crucial point. Among essential amino acids, methionine plays a key role, being related to methyl donation in protein synthesis, and involved in antioxidant and immune defense processes. For this reason, methionine is heavily used in animal feed, to improve their health status and growth performance. In high-yielding dairy cows, methionine is one of the most limiting amino acids, while it is the second most limiting amino acid in a typical swine diet. Therefore, an efficient methionine supplementation can improve the dietary protein utilization, when dietary methionine is limiting [5]. Protected dietary amino acids have been proposed to prevent their degradation by the rumen microbiota and along the gastrointestinal tract, and to guarantee a higher bioaccessibility and bioavailability. A variety of approaches have been experimented to reduce the degradation of essential amino acids during the gastrointestinal transit; in particular, one of the most applied approaches implies the delivery of the protected molecules or the use of a bioequivalent chemical analogue [6]. Until now, such a protection is guaranteed by means of expensive biodegradable polymeric coating, able to resist at the physiological pH. For example, a larger concentration of blood plasma methionine was reported in the case of a rumen-protected methionine form, where a methionine core was coated with a pH-sensitive copolymer (styrene and 2-methyl-5-vinylpyridine) [7]; or, an alternative, where methionine was microencapsulated in a lipid matrix of carnauba wax by a melt-emulsification process [8]. Despite the interesting results, these methionine protection methods were confined to ruminants, and/or required quite a complex protection technology. Moreover, in this protection process, the use of moderately high temperatures and organic chemicals are required, which implies cost and possible risk of the amino acid degradation. In this respect, the use of natural carriers and milder, environmentally friendly process conditions should be preferred. Regarding carriers, it is undeniable that clay minerals could represent good candidates, in view of their sorption capacity [9; 10]. Among others, both sodium and calcium pelleted bentonite have been used as a feed supplement for many years. As a matter of fact, bentonite is used in feed applications for its good absorption capability of pathogenic microorganisms, enzymes, toxins, mycotoxins, and heavy metal ions from feeds. Bentonite's high adsorption capacity has been related to particle size, shape, and to the high specific surface area [5; 6; 7; 8; 9]. Other clay minerals, such as kaolinite, palygorskite, etc., have been reported as effective for feed application. Indeed, due to their adsorbent properties, clay minerals are most suitable for the treatment of gastrointestinal disturbances, particularly diarrhea. Despite some concerns about clay minerals' nonselective binding interaction, which could result in the depletion of other important components of the diets, clay-based feed additives have been increasingly incorporated into animal diets to prevent adverse animal health conditions [11].