The building blocks of all the proteins in the body. Essential amino acids (EAAs) are ones that the horse can't manufacture "in house" and which therefore have to be provided in the diet. The essential amino acid that is most commonly inadequate in the equine diet is known to be lysine, which is thought to followed by methionine and threonine. These three are called limiting amino acids. Providing a supplementary source of these limiting EAAs in the diet enables the horse to make best use of the protein it eats without having to oversupply total crude protein to make up for any shortfall. Conditionally essential amino acids (eg glutamine) are those which the horse can manufacture but can be in short supply when demand is high (eg illness, stress, hard exercise).