**1. Introduction**

The establishment and reevaluation of nutrient requirements in poultry is a dynamic process due to constant increases in feed consumption, growth rate, and the amount of dietary protein, and amino acid (AA) transformed into body protein in chicken meat over time, as a result of genetic selection programs [1–3]. The advancement of each of these factors necessitates the constant labor of AA to maintain high levels of productivity, especially when we consider that in modern animal production, new concepts such as sustainability in meat production are emerging, as a result of the constantly increasing societal demand for food production through environmentally friendly practices.

Decades ago, one of the responses to achieving high levels of efficiency in the transformation of feed into high-quality protein was the development of the concept of the ideal AA ratio, which seeks to maximize efficiency.

However, when comparing the AA profiles recommended by various sources, there is a significant difference between them. For example, among the main essential AA in poultry, arginine (Arg), methionine (Met), threonine (Thr), and tryptophan ratios of 94–120, 36–46, 58–73, and 14–20 have been proposed over time. One reason for these disparities is that these available AA ratios were proposed over 50 years, from 1965 to 2014 [3, 4]. During this time, significant advances in broiler growth have been made, as well as the development of other concepts such as the use of digestible AA ratios, in conjunction with a steady improvement in the nutritional quality of vegetable feedstuffs through selection, and an ever-increasing amount of new information on the composition and availability of nutrients in feed ingredients.

Another strategy, in addition to the use of ideal AA ratios, to improve broiler production efficiency is the use of low-CP diets supplemented with available feed-grade amino acids [3, 5, 6] to fulfill the AA requirements according to different recommendations [7, 8], ensuring the birds productivity at least at the same level shown by birds. Furthermore, using low-CP rations improves nitrogen efficiency by avoiding excess nitrogen excreted in the form of uric acid, reducing environmental pollution caused by nitrogen and ammonia emissions, and lowering the carbon footprint of feed manufacturing through changes in the type and amount of raw material included in the feeds.

While these feeding strategies are still being refined in terms of research and practical application, they must keep up with changes in broiler genetic potential. Furthermore, they must adjust to new nutritional concepts, such as the use of functional AA. Functional AAs are defined as AAs that participate and regulate key metabolic pathways that improve organism health, survival, growth, development, lactation, and reproduction [4, 9]. Functional AA's descriptive roles in nutrition and health, as well as the metabolic pathways involved, have been documented [4]. In recent years, there has been a large number of publications on this topic, as well as several outstanding reviews on the use of functional AA to improve the immune response and digestive capabilities in chickens, including the embryonic developmental stage and the growth stage on the farm [10–12]. According to a recent review [12], AA supplementation strategies can positively contribute to immune and gut health. In the present chapter, an attempt was made to analyze the available information on the use of Arg, Thr, and Met as functional AA aimed at establishing a pattern between the improvement in immune response and digestive physiology with improvements in broiler growth, estimate an AA requirement, and discuss the implications regarding the readjustment of feeds based on functional AA formulation and their application in broilers kept in commercial settings.
