Aspartic acid

Aspartic acid, particularly the L-form, is thought to act as a neurotransmitter, exciting neurons in the brain.
While less common, D-aspartic acid is used as a dietary supplement and may influence hormone levels, such as increasing testosterone. This is more of a supplement claim and not a standard pharmacological use, with mixed research results.
Is non-essential, as the body can synthesize it from oxaloacetate and glutamate, particularly in the liver and muscles (Aspartic Acid in Health and Disease).
Research suggests that aspartic acid, particularly L-aspartic acid, acts as a potent neuronal excitant, activating the same receptors as glutamic acid.
The excitatory nature of aspartic acid can contribute to excitotoxicity under pathological conditions, where excessive activation of excitatory amino acid (EAA) receptors may lead to brain injury. This is managed by Na+-dependent transport mechanisms to clear EAAs, highlighting the importance of its transport in limiting potential neurotoxicity (Aspartic Acid - an overview | ScienceDirect Topics).
It is particularly important in maintaining intracellular NAD(H) redox balance via the malate-aspartate shuttle and facilitating amino acid metabolism (Aspartic Acid in Health and Disease).
D-aspartic acid, distinct from L-aspartic acid, has been studied for its potential pharmacology actions in hormone regulation, particularly testosterone. It is naturally found in the pituitary gland, hypothalamus, and testes, and is often used as a dietary supplement to increase testosterone and growth hormone levels, potentially improving sexual potency and athletic performance (D-Aspartic Acid benefits, dosage, and side effects).
Research, such as studies on resistance-trained men, shows mixed results, with some indicating increased testosterone in sedentary individuals but less effect in trained athletes, suggesting dosage and baseline hormone levels may influence outcomes (Three and six grams supplementation of d-aspartic acid in resistance trained men | Journal of the International Society of Sports Nutrition).

Action	Description	Context	Evidence Level
Neurotransmitter Role	Acts as a neuronal excitant, potentially excitatory in CNS, similar to glutamic acid	Neurological signaling, cerebellum	Sparse, mixed studies
Nutritional Support	Provides amino acids for protein synthesis, gluconeogenesis, and metabolic processes	Total parenteral nutrition	Well-established
Hormone Regulation (D-form)	May increase testosterone and growth hormone levels, used as supplement	Reproductive health, athletics	Mixed, debated efficacy
Liver Function Support (Indirect)	Part of LOLA, reduces ammonia levels in hepatic encephalopathy	Liver disease treatment	Combined with ornithine