Lysine is an essential amino acid for cats and dogs, which must be provided in the diet for the synthesis of all the proteins the body needs. Lysine deficiency in kittens and puppies can slow down growth for example.
Lysine is very sensitive to heat, which means that overly aggressive thermal treatments provoke a reaction with the sugars (Maillard reaction), which makes lysine unavailable to the body. Milk that is too warm for example causes a reaction between lysine and lactose.
Besides its major role in protein synthesis, studies have shown the benefit of increasing dietary intake of lysine to combat the herpes virus in cats, which is one of the agents responsible for diseases of the upper respiratory tract, collectively termed feline rhinitis. The herpes virus generally provokes more serious clinical signs than other agents, especially in the eye. Kittens that have not been immunised may die as a result of infection. The supplementation of lysine limits the intensity of viral excretion and clinical signs in infected animals.
Lysine is abundant in animal proteins, especially meats and milk (casein) as well as soy proteins. There is however a risk that lysine will be missing from a cereal-based diet, which necessitates the supplementation of this amino acid.