Lysine is an essential amino acid that is not produced by the body and must be taken from the diet.It is found in large amount in legumes, cereals, red meat, sea fish, cheese and eggs. It is crucial for the synthesis of proteins in the body and multiple metabolic functions.Often used as a dietary supplement without scientific evidence that would support any benefit of it. Breastmilk contains it naturally with a highest concentration in the colostrum followed by a decrease within the first month after birth (Harper 1984, 1986 Janas, Spevacek 2015).The concentration of lysine together with other amino acids and proteins is higher in the milk of mothers of preterm infants than in mothers of term infants (Atkinson 1980).Pasteurization slightly decreases the concentration of lysine and other amino acids (Silvestre 2006, Valentine 2010) but increases the amount of bioavailable lysine (Baro 2011). It is excreted into breast milk in clinically non-significant amount (0.5% of the administered dose) and as a part of proteins (Irving 1988, Thomas 1991).When used as a supplementation is considered to be safe during breastfeeding (Amir 2011, The Royal Women's Hospital Fact Sheet 2013)