Lysine is an essential or indispensable amino acid. In other words, the dog and cat are unable to produce an adequate quantity to support their normal physiological functions and must therefore obtain it from the diet. If the diet is inadequate it can result in depressed intake, retarded growth and development, and graying of feathers and fur among various species.
Diets based on high levels of starch are often limited by the amount of lysine and have to be supplemented. At the other extreme, excessive lysine relative to arginine can create an antagonism interfering with normal urea cycle function resulting in amino aciduria-a potentially lethal condition. Interestingly, this antagonism has been exploited as a therapy for feline herpes virus (FHV-1) sufferers by providing high dose oral L-Lysine to intentionally antagonize the high arginine appetite for the disease. Some nutritionists consider indexing the requirement for all other amino acids off of the lysine requirement. That's how important this amino acid is to getting fundamental animal nutrition correct.
