L-Lysine

Lysine is an essential amino acid that’s often in short supply in cereal-based foods. It’s also the amino acid most easily damaged by heat: during cooking it reacts with sugars and becomes unusable. A study of 67 commercial pet foods found that most of the lysine was still usable in most of them, but as little as two-thirds in one kibble for growing cats, and that some dry foods for growing dogs were close to the minimum.

Makers add L-lysine to make up the difference. Claims that extra lysine prevents or treats illness in cats are a separate question. Ask your veterinarian about your own pet.