Like all the other essential amino acids, lysine is needed to make new body proteins, but it is also a critical agent in the intestinal absorption of calcium. Lysine is a standout nutrient for the immune system because it helps produce antibodies and has important antiviral properties. As a nutritional supplement, lysine seems to be active against herpes simplex viruses (HSV). While lysine is abundant in many animal proteins (red meats, fish, and dairy products), it is typically the limiting amino acid in plant proteins. Vegetarians and especially vegans must be diligent in choosing proteins or opt for supplements to ensure adequate lysine intake.