L-Lysine

L-lysine is a necessary building block for all protein in the body. L-Lysine plays a major role in calcium absorption; building muscle protein; and the body's production of hormones, enzymes, and antibodies.
Brewer's yeast, legumes, dairy, fish, and meat all contain significant amounts of L-lysine. Most people, including vegans (vegetarians who also avoid dairy and eggs), consume adequate amounts of L-lysine.
However, some very restricted diets can lead to L-lysine deficiency. This is because most grains, and particularly wheat (which is the most commonly eaten grain in the West) are very low in L-lysine; too low for them to give enough for humans. Legumes (peas and beans) fill in that gap, and the grains in turn provide the L-methionine that the legumes don't supply. Hence, because of its good protein balance (at least with respect to L-lysine and L-methionine) beans on toast is highly nutritious - at least as far as the protein is concerned.
Athletes involved in frequent vigorous exercise have an increased need for essential amino acids, including L-lysine.
Taking normal amounts of L-lysine, no consistent problems have been reported in humans, though abdominal cramps and transient diarrhoea have occasionally been reported at very high intakes of 15 - 40 grams.
L-lysine supplementation has been shown to increase the absorption of calcium and it may reduce its excretion. L-lysine works with other essential amino acids to maintain growth, lean body mass, and the body's store of nitrogen.
L-lysine has one very specific use; it is often used, in fairly high doses, to speed up recovery from flare-ups of herpes-class viruses. These include the viruses responsible for cold sores, genital herpes, chickenpox and shingles (chickenpox and shingles are caused by the same virus).
In this case, it is probably a good idea to take L-lysine in combination with vitamin C and/or immune boosting herbs such as the Ayurvedic herb andrographis.