Sulphur amino acids are essential to the synthesis of the main hair protein keratin. A sulphur amino acid deficiency results in hair loss, a slow down in growth and a generally dry, brittle appearance of the hair.
The synthesis required for the maintenance of the skin and the hair may represent up to 30% of an adult dog’s daily protein requirement.
Only methionine is considered to be an essential amino acid. If cystine is provided in sufficient quantities however, it helps free up methionine for other functions. The metabolism of sulphur amino acids produces sulphuric acid, which is eliminated through the urine. A carnivore’s natural diet, which is rich in sulphur amino acids therefore tends to produce acidic urine.
Generally speaking, amino acids are present in animal proteins in large quantities. These amino acids are rarely missing from the diet of cats or dogs, unless the animal is fed an unsupplemented vegetarian diet. Dogs are less sensitive to these deficiencies than cats.
Methionine and cystine are particularly abundant in egg and fish proteins, as well as milk casein. Wheat and maize glutens are also very rich in methionine and cystine.