Lysine

Studies say no. A systematic review found no evidence that lysine prevents or treats feline herpesvirus, and one shelter study linked it to worse signs.

Lysine is an essential amino acid that cats need for growth and muscle. FDA lists lysine (L- and DL-forms) as GRAS for animal feed under good manufacturing or feeding practice (21 CFR 582.5411). Meat and fish supply most of it, and the added form fills gaps in plant-heavy recipes. If your cat has a medical condition, ask your veterinarian before adding lysine products on top of its food.

Source: govinfo.gov

Lysine is added to raise protein quality. Grains and some plant proteins are low in lysine, so recipes that use them add L-lysine to bring the amino acid balance closer to what a cat needs. Meat supplies lysine on its own, so heavily meat-based foods rarely need much added.

AAFCO cat food nutrient profiles set a lysine minimum of 0.83% of dry matter for adult maintenance and 1.20% for growth and reproduction. Those numbers count lysine from all ingredients, not just the added form. A food labeled complete and balanced for a life stage is meant to meet the matching minimum.

Source: aafco.org

Lysine is often the first amino acid to run short in diets built on grain. When one essential amino acid runs low, the body can only build protein up to that level, so the low one limits the rest. Adding lysine lets the other amino acids in the food be used more fully. That is why plant-heavy recipes list L-lysine more often than meat-heavy ones.

Pettings gives lysine 4 out of 5 and lists it as an amino acid. It appears on 42 of the 149 dry cat foods we list. The count includes labels that say L-lysine or lysine hydrochloride. Each food's page shows how it scores overall.