L-lysine, an essential amino acid, is required for healthy growth, tissue repair and enzyme production. It is a protein-building block that strengthens and nourishes the structural, circulatory and immune systems. It is not produced by the body like other essential amino acids and must be obtained from the diet. Some natural food sources for L-lysine include Lima beans, kidney beans, potatoes, corn, red meat, fish and milk. The mean requirement of lysine in healthy human adults is about 30 mg/kg body weight/day or 50 mg/g protein intake. Studies have shown that lysine therapy improves immune status and is used as a therapeutic agent in herpes simplex viral infection. A double-blind randomized trial of L-lysine treatment found it to be an effective agent for significant reduction in occurrence, severity and healing time for recurrent herpes simplex viral infection. Lysine fortification increased the blood levels of complement 3 (C3), CD-4 and CD-8 T-cells as compared with controls. These two studies suggest that supplementation of lysine through food fortification enhances immunological status. A few recent studies have also shown an effect of lysine on diarrheal incidence and severity. An experimental model suggested inhibitory effect of L-lysine on incidence of diarrhea induced by stress and 5-hydroxytryptamine. A community trial among adults in Syria showed that the period prevalence and mean duration of diarrheal illness were significantly lower in the female participants who consumed lysine-fortified wheat bread compared to the control group as well as better immunological parameters such as C3c delta.