Mood is regulated by a complex set of molecules in the brain, including the hormone known as serotonin. L-lysine is a very different molecule found in the diet that nevertheless has a direct effect upon serotonin levels. It regulates serotonin and thus has a relationship with behavior. Deficiencies with L-lysine have even been linked to behavioral problems in humans.
L-lysine is an amino acid, a building block of proteins. It is an essential amino acid, meaning that it cannot be manufactured by the body and must be obtained through the diet. According to the University of Maryland Medical Center, lysine plays an important role in proper growth. It helps absorb calcium, form collagen in the bones and connective tissue and produce carnitine, a nutrient responsible for converting fatty acids into energy and lowering cholesterol. The L in L-lysine stands for the fact that the side chain on the amino acid is "left-handed."
Serotonin is a hormone and neurotransmitter — a molecule that transmits signals through the body — found in the gastrointestinal tract, blood platelets and central nervous system. It regulates mood, appetite, sleep, muscle contraction and some cognitive functions such as memory and learning. Low levels have been associated with mood disorders, particularly depression.
Serotonin is also known as 5-hydroxytryptamine, or 5-HT for short. It is associated with a receptor molecule simply called a 5-HT receptor, which mediates the effects of the neurotransmitter. It is located on the cell membrane of nerve cells so that it can modify the activities of the cell, which then accepts the serotonin neurotransmitter and its effects.
The 5-HT receptor must first bind to a specific molecule to trigger the response by the cell. This binding molecule is known as an agonist. An antagonist molecule, on the other hand, blocks the action of the agonist. A study led by Miro Smriga and published in the December 23, 2003, edition of the "Proceedings of the National Academy of Sciences" found that L-lysine acts like a partial serotonin receptor antagonist, regulating some aspects of mood related to anxiety.
Because of L-lysine's role, its absence can have an effect upon behavior. Another Smriga-led study, published in "The Journal of Nutrition" in December 2002, found an L-lysine deficiency in rats interfered with the normal circadian release of serotonin, increasing stress-induced anxiety and fecal excretion. This is a small but nevertheless important effect.