Overview
Glycine is the simplest and smallest amino acid and is classified as a non-essential amino acid. It is non-polar, highly flexible, and allows tight packing of protein structures, making it a major component of structural proteins such as collagen.
It plays a dual role in the body as both a metabolic amino acid and a key inhibitory neurotransmitter in the central nervous system. It contributes to protein synthesis, heme synthesis, creatine formation, glutathione production, and bile acid conjugation. Because of its small size and flexibility, it is crucial for maintaining normal protein folding, connective tissue strength, and neurological balance.
Symptoms
Abnormal levels mainly affect the nervous system and metabolism. Excess is associated with symptoms such as seizures, hypotonia, apnea, feeding difficulties, nausea, vomiting, visual disturbances, developmental delay, intellectual disability, and, in severe neonatal cases, coma or early death. These features are characteristic of nonketotic hyperglycinemia. Its deficiency may present with weakness, fatigue, hypotonia, seizures, poor growth, delayed development, brain fog, indigestion, bloating, and reduced exercise tolerance. Neurological symptoms reflect it’s role in inhibitory neurotransmission and NMDA receptor modulation.
Causes
Elevated levels are caused by defects in glycine metabolism, most commonly due to glycine cleavage system deficiency seen in nonketotic hyperglycinemia. Excess may also result from impaired conversion to serine or defective mitochondrial metabolism.
Deficiency can occur due to inadequate synthesis, malnutrition, increased metabolic demand, or defects in enzymes involved in glycine synthesis, such as AGAT deficiency, which affects creatine production. Secondary alterations may be seen in liver disease, metabolic stress, or critical illness.
Risk Factors
Risk factors include inherited metabolic disorders, especially nonketotic hyperglycinemia, the neonatal and infant age group, and a family history of metabolic disease. Patients with liver dysfunction, mitochondrial disorders, or severe metabolic stress are also at risk.
Improper fasting, delayed sample processing, or lack of concurrent plasma and CSF analysis may affect interpretation. Neonates with unexplained seizures or hypotonia represent a high-risk group for glycine metabolism disorders.
Prevention
Prevention focuses on early detection and management of metabolic disorders affecting glycine metabolism. Newborn screening and early metabolic evaluation help identify severe forms of hyperglycinemia and reduce neurological damage.
Adequate nutritional support and management of underlying metabolic or hepatic conditions help maintain normal balance. Proper laboratory practices, including fasting sample collection, rapid separation, freezing, and paired plasma-CSF analysis when indicated, improve diagnostic accuracy. Early diagnosis and timely intervention help prevent long-term neurological and developmental complications associated with imbalance.
