Cystine: A Comprehensive Encyclopedic Reference
1. Identity, Chemical Properties, and Common Forms
Chemical Identity
Cystine is the oxidized dimeric derivative of the amino acid cysteine and has the formula (SCH2CH(NH2)CO2H)2. L-Cystine is a covalently linked dimeric nonessential amino acid formed by the oxidation of cysteine. Two molecules of cysteine are joined together by a disulfide bridge to form cystine. Cystine contains a disulfide bond, two amine groups, and two carboxylic acid groups. As with other amino acids, the amine and carboxylic acid groups exist in rapid equilibrium with the ammonium-carboxylate tautomer.
The systematic IUPAC name for the naturally occurring form is (2R)-2-amino-3-{[(2R)-2-amino-2-carboxyethyl]disulfanyl}propanoic acid, specifying the configuration at the chiral centers. The great majority of the literature concerns L,L-cystine, derived from L-cysteine. The molecular formula of cystine is C6H12N2O4S2. It is a white solid that is poorly soluble in water.
As a residue in proteins, cystine serves two functions: a site of redox reactions and a mechanical linkage that allows proteins to retain their three-dimensional structure. Because of the facility of the thiol-disulfide exchange, the nutritional benefits and sources of cystine are identical to those for the more-common cysteine.
Relationship to Cysteine
Because L-cysteine is quickly oxidized to L-cystine in normoxic circumstances, L-cystine is the predominant form of L-cysteine in blood, tissues, and foods. Because of the extremely reducing circumstances inside cells, L-cysteine is the most common type. In 1884, German chemist Eugen Baumann found that reduction of cystine with zinc gave the monomer, which he named "cysteïne." The easy redox interconversion of cysteine and cystine has "provided more puzzles to protein chemists than any of the other amino acids."
Natural Sources
L-cystine is a sulfur-containing amino acid found in dietary sources such as eggs, meats, dairy products, and whole grains. Cystine is particularly abundant in skeletal and connective tissues and in hair, horn, and wool. Human hair and skin contain approximately 10–14% cystine by mass.
L-cystine, together with L-cysteine, is produced through the enzymatic conversion from 2-amino-Δ2-thiazoline-4-carboxylic acid by microorganisms, and cystine can be formed via oxidation of cysteine in normoxic conditions. The body obtains L-cysteine from three different places: food absorption, the transsulfuration pathway (which starts with L-methionine degradation), and the breakdown of proteins that are already present.
Common Preparation and Commercial Forms
Cystine is commercially available as the free amino acid L-cystine in crystalline powder form and is incorporated into oral dietary supplement tablets, capsules, and stick-form sachets. Cystine-enriched food supplements are increasingly popular due to their beneficial health effects; however, the lack of industry standards and market regulations has resulted in quality issues with cystine food products, including cases of food adulteration and fraud. Studies have established reliable methods for determining cystine in food supplements and additives using quantitative NMR (qNMR). Research has found that four of eight food supplement samples were inaccurately labeled or even falsely labeled, with the relative actual amount of cystine ranging from 0.3% to 107% of the stated content.
Cysteine per se is too unstable to be employed as a concentrated supplement, and the cystine which it readily gives rise to is poorly absorbed. As a result, the more bioavailable derivative N-acetylcysteine (NAC) is frequently used in supplements and pharmaceutical contexts when intracellular cysteine elevation is the primary goal, though L-cystine itself is used in specific supplement formulations targeting hair, skin, and immune function.
2. Historical Discovery and Traditional Use
Discovery and Naming
Cystine was first discovered by the English chemist William Hyde Wollaston in 1810, as the principal component of a new type of urinary calculus (stone in the urinary tract). Wollaston had samples of two calculi which did not match the properties of those he had previously studied. Wollaston described the compound as a novel crystalline substance with unique solubility properties, initially naming it "cystic oxide" because it dissolved equally well in acids and alkalis. The name was subsequently shortened to cystine, derived from the Greek word kystis (κύστις), meaning "bladder," in reference to its origin in bladder stones.
Discovered in 1810, cystine was not recognized as a component of proteins until 1899, when it was isolated from animal horn. Karl Axel Hampus Mörner was the first to isolate cystine from a protein hydrolysate (animal horn). Richard August Carl Emil Erlenmeyer, Jr. was the first to synthesize cystine. This was the second amino acid to be discovered.
The name "cystine" comes from its original description as "bladder calculi" in 1833. Early chemical analyses in the mid-19th century confirmed its sulfur content, distinguishing it from other organic compounds found in calculi.
Pre-Modern and Traditional Context
Cystine does not have a distinct documented traditional ethnobotanical or herbal medicine history in the sense of ancient cultures deliberately isolating and using it as a purified compound, as it is a naturally occurring constituent of foods and bodily tissues rather than a plant-derived herbal remedy. Its discovery was entirely a product of 19th-century analytical chemistry. However, foods naturally rich in sulfur amino acids — including cystine — such as meats, eggs, dairy, and legumes, have been staples of human nutrition across all cultures and historical periods. The therapeutic significance of sulfur-rich compounds in keratin-containing structures (hair, nails, skin) has been recognized in cosmetic and nutritional traditions across many cultures, though these were associated with dietary protein intake rather than specifically identified cystine supplementation.
The first medical recognition of cystine's pathological role came through its characterization as the primary component of a subset of urinary stones, establishing the condition now known as cystinuria. Cystine urolithiasis is the only clinical expression of cystinuria, an autosomal recessive genetic defect of the transepithelial transport of cystine and other dibasic amino acids in the kidney. Management of this condition has evolved from dietary and hydration measures to modern pharmaceutical interventions.
3. Key Constituents and Established Mechanisms of Action
Molecular and Biochemical Identity
Cystine is a sulfur-containing amino acid consisting of 2 cysteine molecules connected by an S-S bond. This sulfur-containing amino acid is one of the precursors of glutathione (GSH), which is vital for antioxidant reactions in the body, and its supply is considered to be a rate-limiting factor in GSH synthesis. In addition to its function in protein and enzyme synthesis, cysteine is a precursor of taurine and coenzyme A.
Disulfide Bond Formation and Protein Structure
The oxidation of the thiol groups on cysteinyl residues in proteins allows them to form disulfide bonds. This helps ensure that proteins, particularly those found outside of cells, fold correctly and remain stable. Cysteine residues play a valuable role by crosslinking proteins, which increases the rigidity of proteins and also functions to confer proteolytic resistance. Inside the cell, disulfide bridges between cysteine residues within a polypeptide support the protein's tertiary structure. The bonded sulfur atoms form a disulfide bridge, a principal factor in the shape and function of skeletal and connective tissue proteins and in the great stability of structural proteins such as keratin.
Glutathione Synthesis (Primary Antioxidant Mechanism)
The Km for GCL's rate-limiting substrate cysteine is close to usual intracellular levels of this amino acid; it follows that an increase in dietary cysteine intake can be expected to boost glutathione synthesis, particularly when cellular glutathione levels are relatively low. L-cysteine is now widely recognized as a conditionally essential or indispensable sulphur amino acid. It plays a key role in the metabolic pathways involving methionine, taurine and glutathione (GSH), and may help fight chronic inflammation by boosting antioxidant status. In stressed and inflammatory states, sulphur amino acid metabolism adapts to meet the increased requirements for cysteine as a rate-limiting substrate for GSH.
Cystine is a sulfur-containing amino acid made of two disulfide-bonded cysteines and is known as a precursor to glutathione (GSH). Theanine (gamma-glutamylethylamide) is an amino acid that is absorbed in the small intestine and hydrolyzed in the intestine and liver to glutamic acid and ethylamine. GSH is a tripeptide consisting of glutamic acid, cysteine, and glycine. When cystine is consumed alongside theanine, the theanine contributes the glutamate precursor (via glutamic acid), while cystine provides the cysteine component, jointly promoting GSH synthesis.
The Cystine–Glutamate Antiporter (System xc−)
Cystine serves as a substrate for the cystine-glutamate antiporter. This transport system, which is highly specific for cystine and glutamate, increases the concentration of cystine inside the cell. In this system, the anionic form of cystine is transported in exchange for glutamate. Cystine is quickly reduced to cysteine once it enters the cell, where it participates in GSH biosynthesis and other redox reactions. Cysteine is then converted into cystine, and high levels of cystine stimulate the exchange of glutamate for cystine through the cystine-glutamate antiporter. This results in an increase in nonsynaptic glutamate, which then acts on the glutamate receptors (mGluR2/3) of presynaptic neurons. Binding of this receptor leads to the inhibition of glutamate release in the nucleus accumbens.
Keratin and Structural Protein Role
Keratin is a protein found in the cortex and is composed of 18 amino acids, the most abundant of these amino acids being cysteine, cystine, serine, glutamic acid, threonine, arginine, leucine, isoleucine, and valine. Cystine's high abundance in keratin underpins its structural role in hair and nails, where dense disulfide cross-linking determines mechanical strength and resilience.
Melanin Synthesis Modulation
Various mechanisms of action have been considered to explain the skin-lightening property of thiol compounds, one of them being the skewing of the melanin synthesis pathway toward the production of lighter pheomelanin instead of darker eumelanin, consequently producing a lightening effect. Thiol compounds can inhibit enzymatic activity by binding to copper ions at the active site of tyrosinase (TYR), and act as an antioxidant scavenging reactive oxygen species and free radicals or as a modulator of redox balance, thereby inhibiting overall melanin synthesis.
Redox Status and Epidermal Defense
Deficiency in glutathione biosynthesis is efficiently compensated in keratinocytes by the cysteine/cystine and thioredoxin systems. This highlights a remarkable antioxidant capacity of the epidermis that ensures skin integrity and efficient wound healing.
4. Scientific Evidence by Area of Use
4.1 Hair Loss (Telogen Effluvium and Androgenetic Alopecia)
Hair follicle keratinocytes are among the most rapidly proliferating cells in the body, and cystine — as the dominant amino acid in keratin — has been studied as a supportive nutrient in hair loss conditions.
In vitro research: Clinical studies have confirmed that the hair-growth-promoting effect of approved oral drug combinations is beneficial for the treatment of diffuse telogen effluvium, characterized by the excessive loss of telogen club hairs. Since data elucidating the mode of action of such combinations are limited, researchers focused on the identification of cellular processes potentially supporting the treatment of hair loss, using a minimal growth culture system (MGM) to mimic the reduced activity of human hair follicular keratinocytes (HHFKs). The effect of four core compounds (L-cystine, thiamine, calcium D-pantothenate, and folic acid) of the marketed oral combination Pantovigar®, approved for the treatment of diffuse hair loss, was examined. The tested compound combination had positive effects on metabolic activity, cell viability, and proliferation of keratinocytes. Furthermore, this study suggested that L-cystine primarily contributes to the observed protection against endogenous oxidative stress. This was an in vitro study; its findings cannot be directly extrapolated to clinical outcomes.
Clinical evidence (combination supplement, uncontrolled study): Telogen effluvium (TE) is a common cause of non-cicatricial hair loss with no treatment-standardized protocol. A study evaluated the efficacy, tolerability, and patient compliance of a treatment with an oral supplement based on arginine, L-cystine, zinc and B6 vitamin (Cystiphane®) administered 4 times daily in 20 patients aged 18 to 70 years old affected by TE, over 3 months. After 3 months of taking the supplement, the researcher rated an average improvement of 2.89 at the clinical evaluation. The mean trichoscopic value had risen by +2.055 for hair quantity and the mean trichoscopic diameter value had increased by +1.83. After 3 months of treatment, patients gave an average efficacy opinion of 3.61. The oral supplement proved effective as an adjuvant in the treatment of TE in this cohort. This study was limited by its small sample size (n=18 evaluable), lack of a placebo control, and the multi-ingredient formulation making it impossible to attribute effects to L-cystine alone.
Clinical evidence (RCT, multi-ingredient): Chronic telogen effluvium and androgenetic alopecia can significantly impact quality of life, and deficiencies in essential micronutrients may play a role. A double-blind, placebo-controlled, randomized clinical study assessed the effectiveness and safety of an oral supplement containing L-cystine, Serenoa repens, Cucurbita pepo, Pygeum africanum, as well as vitamins and micronutrients. The study involved 80 participants aged 18 to 60, both male and female, randomly assigned to the oral supplement or placebo daily for 6 months. Results showed that participants who took the oral supplement experienced a significant increase in hair density compared to those who took the placebo, with improvements observed as early as 3 months into the study. A significant limitation is that the formulation contained multiple active components, making it impossible to isolate L-cystine's individual contribution to the observed outcomes.
Evidence strength: Preliminary to moderate. Positive signals exist in clinical studies, but most trials use multi-ingredient formulations making it impossible to attribute effects specifically to L-cystine. Well-controlled, L-cystine-only RCTs with adequate power are lacking.
4.2 Skin Pigmentation and UV Protection
Clinical evidence (RCT, combination with glutathione): To evaluate the skin lightening and anti-dark spot effects of oral supplementation with L-Cystine associated with L-Glutathione as compared to placebo and benchmark, effects were investigated in a 12-week randomized, double-blind, parallel-group, benchmark- and placebo-controlled trial involving 124 Asian female subjects. Women were randomly allocated into 4 equal groups (500 mg L-Cystine and 250 mg L-Glutathione, 250 mg reduced L-Glutathione, 500 mg L-Cystine, or a placebo, daily). Results published in the Journal of Cosmetic Dermatology (2022) showed skin-lightening outcomes in the active groups versus placebo, though the study design again makes it difficult to distinguish the contribution of L-cystine from that of glutathione.
Clinical evidence (RCT, mixture with L-cystine and vitamin C): A clinical trial used both vitamin C for anti-oxidation and L-cystine for decreasing melanin biosynthesis. All subjects administered 1 test capsule (containing 60 mg of L-cystine, 125 mg of vitamin C, and citrus peel extract) or placebo for two months. Researchers objectively measured changes in the melanin index (MI), skin lightness (L*), and skin color (ITAO). The MI values were significantly lower in the test group compared with the placebo group from 4 weeks onward. In addition, the active group had significant improvement in lightness and color at the end of the examination period. Again, the multi-ingredient design limits conclusions about L-cystine specifically.
Clinical evidence (RCT, cysteine peptides): A randomized, placebo-controlled, double-blind, parallel-group study was conducted on 90 healthy Japanese males and females aged 30–59 years to assess the effect of oral supplementation with cysteine peptides for 5 weeks on UV-B-induced erythema and pigmentation. This study focused on cysteine-containing peptides from yeast extract rather than free L-cystine directly.
Evidence strength: Preliminary. Human RCT data support anti-pigmentation effects of thiol-containing preparations, including L-cystine in combination, but the evidence base is small, involves primarily Asian female populations, and is confounded by co-administered ingredients. Independent replication in diverse populations is needed.
4.3 Immune Function and Exercise Recovery
The combination of cystine and theanine (C/T) has been the subject of a series of clinical studies examining immune modulation during exercise and surgical stress.
Clinical evidence in athletes (small RCTs): Intense exercise induces increased blood neutrophil counts and decreased lymphocyte counts, and leads to inflammation and immunosuppression. It was previously reported that cystine and theanine (CT) supplementation by long-distance runners before a training camp suppressed the changes of these blood parameters observed in unsupplemented control subjects after the camp. A subsequent study examined 16 long-distance runners allocated to one of two groups given CT supplements (700 mg cystine + 280 mg theanine daily) or placebo (8 in each group) for 7 days prior to and during a 9-day training camp, during which daily run training averaged 19.9 km/day prior to the camp and 28.6 km/day during the camp. The results indicated a significant increase in the high-sensitivity C-reactive protein (hs-CRP) and neutrophil count in the blood, as well as a decreasing tendency for lymphocytes in the placebo group, but not the CT group. It was presumed that cystine/theanine inhibited the excessive inflammatory reaction and suppressed the decline of immunological function. Study limitations include very small sample sizes (n=8 per group), making findings preliminary.
Clinical evidence (resistance training and NK cell activity): Training experiments have shown that L-theanine and cystine supplementation can increase glutathione (GSH) levels, restore NK cell activity reduced by high-intensity and high-frequency resistance exercise, and enhance immune responses. Oral supplementation of L-theanine and cystine can alleviate the fluctuation of blood immunocompetent cells caused by high-intensity endurance exercise, inhibit the excessive inflammatory response, prevent various infectious diseases, reduce related muscle damage, and alleviate immunosuppression.
Evidence strength: Moderate but limited by small sample sizes. Multiple small RCTs consistently show blunting of exercise-induced immunosuppression markers with the C/T combination, but larger, independently replicated trials are needed.
4.4 Perioperative Recovery and Surgical Stress
In clinical studies of body-builders and long-distance runners, the intake of CT suppressed excessive inflammatory reactions and a decline in immune functions after intense training. Surgery, as well as intense exercise, induces excessive inflammatory reactions. In mice, the preoperative administration of CT suppressed excessive inflammatory reactions associated with surgery and promoted postoperative recovery. Moreover, in clinical studies of gastrectomized patients, CT intake suppressed excessive postoperative inflammatory reactions and induced early recovery.
RCT evidence (esophagectomy): In a randomized controlled trial, the dosages of cystine (700 mg) and theanine (280 mg) were determined in conformity with those used in previous studies. The test food consisted of 700 mg cystine, 280 mg theanine, and inert excipients. Basic and animal model studies have shown that the oral administration of both cystine and theanine (CT) increases tissue GSH levels, reduces inflammatory responses, regulates immune responses, and reduces the biological response to surgical invasion. A reduction in biological reactions from surgical invasion is necessary to promote postoperative recovery. Previous studies have proven that GSH concentration in blood and organs decreases during biological invasion. GSH is an antioxidant derived from living tissue and is meant to protect the body from oxidative stress.
Evidence strength: Emerging. Multiple small RCTs in surgical patient populations suggest C/T supplementation may reduce postoperative inflammatory markers and accelerate functional recovery, but these findings require larger, multi-center trials to confirm clinical significance.
4.5 Chemotherapy Support
Previous research demonstrated that the incidence rate of diarrhea induced by S-1 adjuvant chemotherapy in colorectal cancer patients after surgery was significantly less in the cystine/theanine group (4.5%) than in the control group (41.7%). The duration and completion rate of S-1 adjuvant chemotherapy were significantly longer and higher, respectively, in the cystine/theanine group (complete ratio: 75.0%, duration: 24.8 ± 5.8 days) than in controls (complete ratio: 35.5%, duration: 20.0 ± 7.7 days). The oral administration of cystine and theanine attenuated the adverse events of S-1 adjuvant chemotherapy and increased the S-1 completion rate, suggesting that cystine and theanine may be useful supportive care for chemotherapy.
GSH is a substrate in conjugation reactions for detoxification, and works as a vital substance for antioxidation reactions that reduce reactive oxygen species, such as peroxides and free radicals. Glutamine, which is also a precursor of GSH and is similar to cystine/theanine, is reported to have a positive effect in preventing and treating oral mucositis in patients receiving mucotoxic cancer chemotherapy. In addition, GSH was reported to show neuroprotective effects for patients undergoing oxaliplatin-based chemotherapy.
Evidence strength: Preliminary but intriguing. Results from small trials are promising, but the combination design, small sample sizes, and focus on Japanese patient populations limit broad applicability.
4.6 Cystinuria and Kidney Stone Disease
In this context, cystine is not a supplement but rather a metabolic substrate whose accumulation causes disease. Cystine urolithiasis is the only clinical expression of cystinuria, an autosomal recessive genetic defect of the transepithelial transport of cystine and other dibasic amino acids in the kidney. Stones form due to the increased excretion of cystine, which is poorly soluble at normal urine pH. Cystine stones account for only about 1% to 2% of all kidney stones but represent roughly 6% to 8% of all pediatric calculi. Eighty percent of cystinuria patients will have their first stone during their first two decades.
Cystinuria is the result of an autosomal recessive disorder caused by mutations in one of two genes, either SLC3A1 on chromosome 2 (type A) or SLC7A9 on chromosome 19 (type B), which code for components of the major proximal renal tubule cystine and dibasic amino acid transporter. Complications of cystinuria include recurrent urinary tract infections, chronic kidney disease (CKD), and hypertension. Some individuals develop end-stage kidney disease.
Medical preventive treatment is based on high diuresis (≥1.5 l/m2 per day) well distributed throughout the day and night, and urine alkalinization up to pH 7.5 by means of sodium bicarbonate and/or potassium citrate. When these basal measures are ineffective at preventing stone recurrence or dissolving pre-existing stones, sulfhydryl agents such as D-penicillamine or tiopronin, which form highly soluble mixed disulfides with cystine moieties, are added to urine dilution and alkalinization.
Evidence strength: Well-established in terms of pathophysiology; established medical consensus on treatment strategies exists, with management guided by clinical practice guidelines from European and US bodies.
4.7 Glutathione Replenishment and Antioxidant Status
Critically ill patients receiving enteral or parenteral nutrition, enriched with cysteine, exhibit decreased cysteine catabolism and improved GSH synthesis. Cystine is a nonessential amino acid. In a clinical trial targeting surgery patients, cystine became essential during the perioperative period. In addition, the synthetic pathway from methionine to cysteine remains inhibited in rats under surgical stress. These reports support a role for cystine in suppressing the decrease in GSH during surgical stress.
Evidence strength: The role of cystine as a GSH precursor is biochemically well-established. Clinical evidence that oral L-cystine alone substantially raises tissue GSH in healthy humans is limited; most evidence derives from the C/T combination or from parenteral nutrition contexts.
5. Body Systems and Health Areas Associated with Cystine
- Integumentary system (hair, skin, nails): Novel interventions based on cystine- or cysteine-based oral supplementation have been garnering interest for their ability to improve skin, hair, and nail conditions. Cystine is the dominant sulfur amino acid in keratin, underpinning structural integrity of hair shafts and nails.
- Immune system: CT intake is considered to exhibit an immunomodulatory effect by suppressing the decrease in GSH due to invasive stress.
- Urinary/renal system: Cystine is a chemical substance which naturally occurs as a deposit in the urine, and can form a calculus (hard mineral formation) when deposited in the kidney.
- Antioxidant/redox system: As a rate-limiting precursor of glutathione, cystine directly influences the cellular capacity for oxidative stress management across tissues including the liver, lungs, and skin.
- Nervous system: Cystine levels influence the cystine-glutamate antiporter, which results in increased nonsynaptic glutamate acting on mGluR2/3 receptors of presynaptic neurons. Binding of this receptor leads to the inhibition of glutamate release in the nucleus accumbens.
- Musculoskeletal and connective tissue: Cystine is particularly abundant in skeletal and connective tissues.
- Gastrointestinal system (in the chemotherapy context): The incidence rate of diarrhea induced by S-1 adjuvant chemotherapy in colorectal cancer patients after surgery was significantly less in the cystine/theanine group than in the control group.
6. Dosage Forms and Dosages Reported in Studies
The following dosages are reported from peer-reviewed clinical studies and should be understood strictly in the context of those studies:
- Hair loss (Cystiphane® combination): A supplement based on arginine, L-cystine, zinc, and B6 vitamin was administered as four tablets daily, in one or two administrations during meals, over 3 months.
- Hair and nail condition (combination with keratin hydrolysate): A study of L-cystine combined with Kera-Diet® and specific vitamins and trace elements demonstrated hair and nail benefits. Specific dosage for L-cystine was not extractable from available source text.
- Skin pigmentation (L-Cystine + L-Glutathione RCT): Women were randomly allocated into four groups receiving either 500 mg L-Cystine and 250 mg L-Glutathione, 250 mg reduced L-Glutathione alone, 500 mg L-Cystine alone, or a placebo, administered daily for 12 weeks.
- Melanin reduction (combination with citrus extract and vitamin C): Subjects administered one test capsule containing 60 mg of L-cystine, 125 mg of vitamin C, and citrus peel extract, or placebo, for two months.
- Exercise immune modulation and surgical recovery (cystine/theanine): Sixteen long-distance runners were given CT supplements at 700 mg cystine + 280 mg theanine daily for 7 days prior to and during a 9-day training camp. In the esophagectomy perioperative trial, the dosages of cystine (700 mg) and theanine (280 mg) were determined in conformity with those used in previous studies.
- Chemotherapy support (S-1 adjuvant): Cystine was used in combination with theanine; the specific cystine dose used was 700 mg per day based on the same C/T combination formula used across the series of Japanese trials.
Cystine is available commercially as free amino acid powder, capsules, and tablets. Cystine-enriched food supplements are increasingly popular, and it is also used as a food additive in some countries.
7. Safety Considerations and Interactions
General Tolerability
In clinical trials with the C/T combination (700 mg cystine + 280 mg theanine daily), adverse events were not prominently reported as a significant concern, and supplement administration in athletes, surgical patients, and chemotherapy patients was described as well tolerated. The oral administration of cystine and theanine attenuated the adverse events of S-1 adjuvant chemotherapy and increased the S-1 completion rate.
Poor Absorption at High Doses
Cysteine per se is too unstable to be employed as a concentrated supplement, and the cystine which it readily gives rise to is poorly absorbed. This poor bioavailability of free L-cystine is a relevant limitation for supplementation strategies aiming to elevate intracellular cysteine levels; N-acetylcysteine and other pro-drugs are generally preferred for this purpose in clinical settings.
Cystinuria and Kidney Stone Risk
The presence of cystine in urine is often indicative of amino acid reabsorption defects. As the result of a genetic defect in proximal tubular reabsorption of filtered cystine, increased urine levels of the poorly soluble amino acid result in recurrent cystine nephrolithiasis. Recurrent cystine stones not only adversely affect the quality of life of patients suffering from cystinuria but may also result in chronic kidney disease (CKD) from recurrent renal injury. Individuals with known cystinuria or a personal/family history of cystine stones have a specific pathophysiological reason to avoid supplemental cystine.
Cystinosis
In humans the excretion of high levels of cystine crystals can be indicative of cystinosis, a rare genetic disease. Various derivatives of cysteamine are used to address cystinosis. These derivatives convert poorly soluble cystine into more soluble derivatives. In individuals with cystinosis — a lysosomal storage disorder — cystine accumulates pathologically in cells and tissues; supplemental cystine would be contraindicated in this condition.
Supplement Quality and Labeling Accuracy
When current analytical methods were applied to investigate different food supplements and additives regarding cystine quantity, four of eight food supplement samples were found to be inaccurately labeled or even falsely labeled, with the relative actual amount of cystine ranging from 0.3% to 107% of the declared content. This represents a notable practical safety concern for consumers relying on labeled doses.
Interactions
Direct pharmacokinetic drug–drug interactions with free L-cystine as a dietary supplement are not well characterized in the peer-reviewed literature. Others have evaluated the literature linking oxidative stress and other clinical conditions to an imbalance of extracellular L-cysteine/L-cystine, suggesting that manipulating this balance could have downstream effects on redox-sensitive signaling. For the related compound NAC — which is metabolically connected to cystine — nitroglycerin interacts moderately with NAC, as their co-administration may result in hypotension and nitroglycerin-induced headache. Carbamazepine, when co-administered with NAC, can result in a lower-than-desired blood concentration of carbamazepine. Whether these interactions extend to dietary L-cystine supplementation is not established in the available literature.
Evidence Quality and Label Fraud
The lack of industry standards and market regulations has resulted in quality issues with cystine food products, including cases of food adulteration and fraud. This quality inconsistency is relevant to effective dosing as well as safety, particularly where consumers depend on accurate labeling to manage intake.
References
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