What is cystine?

Cystine is a nonessential amino acid, which can be composed in your body from 2 molecules of the amino acid cysteine, so you do not need to get it from food in order to be healthy [1].

Cystine is a nonproteinogenic amino acid, which is not regularly incorporated into proteins [1], which means it is found only in some proteins as opposed to proteinogenic amino acids that are found in most proteins in the human.

Cystine Functions in the Human Body

Cystine is [1]:

  • A part of the protein alpha-keratin in the skin, hair and nails
  • Involved in the production of glutathione, which is an antioxidant
  • A source of sulfur

Cystine Supplements

Available nonprescription oral forms of cystine:

  • L-cystine
  • N-acetyl cystine

Cystine Benefits

Cystine supplements are used in treatment of certain hereditary metabolic disorders.

There is INSUFFICIENT EVIDENCE about the effectiveness of cystine supplements in the prevention or treatment of bronchitis, cystic fibrosis, chronic obstructive pulmonary disease (COPD), end-stage renal disease (ESRD), epilepsy, fibrosing alveolitis, heart related chest pain (angina) or influenza, in promoting hair growth, improving brittle nails, boosting immunity, removing toxins from the body, wound healing or relieving pain in rheumatoid arthritis [9].

Cystine Safety: Side Effects, Toxicity

The use of a single amino acid supplement may lead to negative nitrogen balance, decreasing the metabolic efficiency and increasing the workload of the kidneys. In children, taking single amino acid supplements may also harmfully affect growth.

Not enough is known about the safety of cystine supplements during pregnancy and breastfeeding, so women in these periods should avoid them.

Who else should avoid cystine supplements?

Individuals with:

  • Maple syrup urine disease (MSUD)
  • Cystinuria
  • Cystinosis

Cystinuria and Cystine Kidney Stones

Cystinuria is a hereditary metabolic disorder in which cystine is excreted in the urine, which may result in the formation of cystine kidney stones [2].

Symptoms can appear for the first time in infants, children or adolescents and may include sulfur or rotten egg-smelling urine, blood in urine, pain in the flank (usually only on one side and often radiating toward the groin) [3].

Diagnosis of cystine kidney stones is made by a CT; urine test may reveal cystine crystals [4].

Cystinuria can be decreased by a vegetarian diet, which is low in the amino acid methionine (which is metabolized to cystine in your body) [5].


Cystinosis is a hereditary metabolic disorder with accumulation of cystine within the cells, mainly in the kidneys and eyes [6]. The affected individuals usually have blond hair and blue eyes, but the disorder can be also found in some with dark hair and brown eyes [7]. Symptoms, which can appear in infants, children or adults, may include failure to thrive, vomiting, craving for salty and spicy foods and excessive urination. Main complications include kidney failure or blindness. Treatment includes a low-protein diet, cysteamine and other drugs [6]. Patients may live up to their 5th decade.

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