Of the 700 amino acids that exist in nature, only 20 alpha-amino acids are used by living organisms for protein synthesis. The branched-chain amino acids (BCAA: valine, leucine and isoleucine) are among those that have no known derivatives of metabolic importance, and thus their main role is protein metabolism. Because BCAA cannot be synthesized de novo by higher organisms such as mammals and birds, they are characterized as essential or indispensable nutrients, and therefore they must be obtained from their diet. The three BCAA make up about 35 percent of the 10 essential amino acids in muscle proteins and about 40 percent of those same essential amino acids required by monogastric animals. In a typical corn-soybean meal-based diet, however, BCAA comprises about 50 percent of the essential amino acids, and excesses rather than deficiencies are therefore more likely the norm.

In the field of monogastric animal nutrition, recent scientific evidence suggested that older requirement estimates of BCAA for swine and poultry were in need of serious re-evaluation.

Apart from protein synthesis, BCAA and their products of catabolism contribute carbon skeletons and nitrogen groups to the synthesis of intermediates of the tricarboxylic cycle, ketone bodies, lipids, other amino acids (alanine, glutamate and glutamine), or they can be catabolized to CO2 for production of ATP (energy). Today, there is increasing interest in the physiology and pathophysiology of BCAA and their keto-analogs in the area of human nutrition because they have been found to play a crucial role in clinical conditions such as starvation, exercise, sclerosis, chronic liver disease, renal failure, uremia, cachexia, sepsis and post-surgery recovery.

In the field of monogastric animal nutrition, recent scientific evidence suggested that older requirement estimates of BCAA for swine and poultry were in need of serious re-evaluation. Indeed, valine has been already shown to be equally limiting, after lysine, with methionine, threonine and tryptophan in a low protein corn-soybean meal diet, a finding that places valine as fifth limiting, to be followed by isoleucine. Considerable uncertainty existed until a few years ago over the valine requirement of lactating sows, but several recent reports suggested a much more reasonable, albeit higher, requirement. In poultry, confusion still surrounds the ideal ratios to lysine for most of the BCAA. Recent evidence suggests that ratios of BCAA may need to be somewhat higher than originally thought. Thus, a highly active scientific field that is far from giving conclusive answers.