Lysine is an essential amino acid and is a basic building block of all protein. This nutrient was first isolated in 1889 from casein. Lysine has a net positive charge at physiological pH values, making it one of the three basic (with respect to charge) amino acids. This polar amino acid is commonly found on the surfaces of proteins and enzymes, and sometimes appears in the active site. Lysine is incorporated into proteins at the rate of 7 percent on a molar basis compared to the other amino acids.

Lysine is heat sensitive and is not present in processed foods. It helps regulate the pineal gland, mammary glands, and ovaries. Lysine is important for growth and bone development, promotes calcium absorption, maintains nitrogen balances, ability to fight viral infections such as the herpes simplex virus, aids in the production of antibodies, hormones and collagen, and helps to build muscle tissue.

It is required for growth and bone development in children, assists in calcium absorption and maintaining the correct nitrogen balance in the body and maintaining lean body mass. Furthermore it is needed to produce antibodies, hormones, enzymes, collagen formation as well as repair of tissue.

Since it helps with the building of muscle protein, it is useful for patients recovering from injuries and recovery after operations, and there might be use in lysine to help maintain healthy blood vessels. It also seems to assist in fighting herpes and cold sores.

Although a deficiency of lysine is rare, since it is found in so many protein foods, the symptoms may include anemia, enzyme disorders, lack of energy, hair loss, bloodshot eyes, weight loss and retarded growth as well as reproductive problems, poor appetite and poor concentration.

People on a vegan or vegetarian diet, using grains as their only source of protein are often deficient in this nutrient.

Toxicity has not been determined but animals fed high amounts of lysine have shown a tendency to gallstones as well as elevated cholesterol. These tendencies have not been proven in humans. Diarrhea and stomach cramps may be indicative in high dosage, but these are not consistent symptoms.

Good sources of lysine are found in cheese, eggs, lime beans, potatoes, milk, meat and brewer's yeast.