Age-related decreases in levels of several amino acids may influence cognitive function. Amino acids are important intermediates in the production of neurotransmitters in the brain and are involved in neuromodulating activities. l-Tyrosine is an amino acid naturally synthesized in the body. It is the precursor of the neurotransmitters l-dopa, dopamine, norepinephrine and epinephrine, supporting a positive outlook and calmness. DL-Phenylalanine provides a mixture of two essential amino acids, d-phenylalanine and l-phenylalanine. In the body, l-phenylalanine converts to tyrosine, enhancing neurotransmitter support. A separate pathway allows both d- and l-phenylalanine to support levels of phenylethylamine, a function not provided by l-tyrosine. The serotonin precursor l-tryptophan is included to support relaxation and restful sleep. In a double-blind, placebo-controlled study, l-tryptophan supported healthy processing and promoted positive outlook in women. Taurine is considered an inhibitory amino acid, helping to maintain healthy glutamate and other excitatory amino acid activity in the brain. This provides neuroprotective action important for maintaining healthy brain cell function. Acetyl-l-carnitine plays an important role in multiple metabolic pathways in the brain. It also contributes to the production of the neurotransmitter acetylcholine and may support dopamine receptor function. As a result, acetyl-l-carnitine plays a role in memory and attention.