L-Cystine: A Comprehensive Reference Article
1. Identity and Chemical Profile
1.1 Nomenclature and Chemical Structure
Cystine is the oxidized dimeric derivative of the amino acid cysteine and has the formula (SCH₂CH(NH₂)CO₂H)₂. 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 conversion from cysteine to cystine can be viewed as an oxidation: 2 HO₂CCH(NH₂)CH₂SH + 0.5 O₂ → (HO₂CCH(NH₂)CH₂S)₂ + H₂O. Cystine contains a disulfide bond, two amine groups, and two carboxylic acid groups.
When cystine is formed, bonds are created between the sulfur atoms. These covalent bonds are called disulfide bridges. The great majority of the literature concerns l,l-cystine, derived from l-cysteine. Other isomers include d,d-cystine and the meso isomer d,l-cystine, neither of which is biologically significant. It is a white solid that is poorly soluble in water.
In cell biology, cystine residues found in proteins only exist in non-reductive (oxidative) organelles, such as the secretory pathway (endoplasmic reticulum, Golgi apparatus, lysosomes, and vesicles) and extracellular spaces. Under reductive conditions (in the cytoplasm, nucleus, etc.), cysteine is predominant.
1.2 Relationship to Cysteine
Through oxidation-reduction reactions, cystine can be reduced to cysteine. When the pH is low, the form that occurs is cysteine, whereas when the pH is higher, the sulfur atom and its hydrogen atom are oxidized to L-cystine. 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.
1.3 Natural Sources and Occurrence in Foods
L-cystine is a sulfur-containing amino acid found in dietary sources such as eggs, meats, dairy products, and whole grains. The highest concentrations of cystine are found in dried egg powder and sesame seeds at between 1000 and 2000 mg/100 g of product, and in sunflower seeds, offal, beans and white fish at between 500 and 1000 mg/100 g of product. Meats such as pork, beef, and poultry; fish; eggs; and dairy products like cheese and low-fat yogurt rank among the highest contributors per serving. Beef liver offers about 388 mg of cystine per slice, while pork, chicken, and eggs commonly deliver 200–400 mg per 100 g depending on preparation.
Cystine is particularly abundant in skeletal and connective tissues and in hair, horn, and wool. Animal feathers and human hair are natural sources of L-cysteine, which is the main component of keratin. The body gets 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 there.
1.4 Supplement Forms and Preparations
The supplement market packages L-cysteine in several forms, most commonly as N-acetylcysteine (NAC), as the oxidized dimer L-cystine, or as free L-cysteine itself. L-cystine is commercially available as standalone oral capsules and tablets, and as a component in multi-ingredient formulations targeting hair, nail, and skin health. Cystine is extracted with activated charcoal after the acidic hydrolysis of feathers and hair. After desorption from the activated charcoal matrix, the isolated and purified L-cystine is used in various applications. Due to safety and environmental issues in extracting L-cysteine from animal hair and feathers, the fermentative production of L-cysteine offers an attractive alternative using renewable feedstocks.
L-cystine, together with L-cysteine, is also 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.
2. Historical Discovery and Traditional Use
2.1 Discovery and Early Scientific History
Cystine was discovered in 1810 by the English chemist William Hyde Wollaston, who called it "cystic oxide." In 1833, the Swedish chemist Jöns Jacob Berzelius named the amino acid "cystine." The name was subsequently shortened to cystine, derived from the Greek word kystis (κύστις), meaning "bladder," in reference to its origin in bladder stones. This was the second amino acid to be discovered.
Discovered in 1810, cystine was not recognized as a component of proteins until 1899, when it was isolated from animal horn. In 1884, German chemist Eugen Baumann achieved the first isolation of cysteine by reducing cystine with zinc dust in acidic conditions, yielding a monomeric compound he named "cysteïne" to denote its derivation from cystine.
The sulfur within the structure of cysteine and cystine has been subject of historical interest. In 1902, Osborne partially succeeded in analysing cystine content via lead compounds. An improved colorimetric method was developed in 1922 by Folin and Looney. An iodometric analysis method was developed by Okuda in 1925.
2.2 Traditional and Pre-Scientific Use
L-cystine as a defined molecular entity did not feature in traditional herbal or dietary medicine in the way that botanical preparations have; it was not isolatable until the 19th century. However, the foods richest in cystine — eggs, meat, dairy, legumes, and seeds — have been dietary staples across virtually all human cultures for millennia. The deliberate therapeutic use of sulfur-rich animal materials, including horn hydrolysates and meat broths, has appeared in Ayurvedic, traditional Chinese, and European folk medicine traditions, but these preparations were understood in terms of their gross nutritional or energetic properties rather than their cystine content specifically. The identification of cystine as a distinct chemical constituent from protein hydrolysis belongs entirely to the modern scientific era (post-1810). No traditional medical system has a documented specific use for isolated L-cystine as a supplement prior to its chemical characterization.
3. Key Biochemical Constituents and Mechanisms of Action
3.1 Role as a Sulfur Amino Acid
Sulfur amino acids are a kind of amino acids which contain sulfhydryl, and they play a crucial role in protein structure, metabolism, immunity, and oxidation. L-cysteine plays a key role in sulfur metabolism in all organisms and is used in the synthesis of proteins, glutathione, biotin, lipoic acid, methionine and other sulfur-containing metabolites. L-cysteine also serves as a precursor for the biosynthesis of coenzyme A.
L-cysteine is now widely recognized as a conditionally essential or indispensable sulfur amino acid. It plays a key role in the metabolic pathways involving methionine, taurine and glutathione (GSH). There is a requirement for adequate sulfur amino acid intake that extends beyond the need for adequate amounts to maintain normal protein synthesis and turnover.
3.2 Glutathione Synthesis: The Rate-Limiting Role of Cysteine/Cystine
Glutathione (GSH) is a tripeptide, γ-L-glutamyl-L-cysteinylglycine, present in all mammalian tissues at 1–10 mM concentrations (highest concentration in liver) as the most abundant non-protein thiol that defends against oxidative stress. Key determinants of GSH synthesis are the availability of the sulfur amino acid precursor, cysteine, and the activity of the rate-limiting enzyme, glutamate cysteine ligase (GCL), which is composed of a catalytic (GCLC) and a modifier (GCLM) subunit.
GSH serves several vital functions including antioxidant defense, detoxification of xenobiotics and/or their metabolites, regulation of cell cycle progression and apoptosis, storage of cysteine, maintenance of redox potential, modulation of immune function and fibrogenesis. As an important intracellular antioxidant, it acts as a regulator of cellular redox state protecting cells from damage caused by lipid peroxides, reactive oxygen and nitrogen species, and xenobiotics.
3.3 Structural Role in Keratin
The great stability of keratin results from the numerous disulfide bonds of cystine. The amino acid composition of keratin differs from that of collagen. Cystine may account for 24 percent of the total amino acids. Being a constituent of the amorphous part of keratin (the so-called "matrix") and forming numerous intra- and intermolecular disulfide links, they stabilize the quaternary structure and promote resistance of keratins in solution and action of proteolytic enzymes.
Cysteine is an important part of keratin, the major protein in hair and nails and the exoskeleton of several species. Skin and hair contain almost 10–14% cysteine. The cysteine-rich nature of keratin allows the formation of multiple disulfide bonds, which impact the integrity and stability of keratin.
3.4 Downstream Metabolic Pathways
Once generated, cysteine is consumed by various metabolic pathways such as protein synthesis, generation of sulfur-containing molecules such as glutathione, taurine, lanthionine, coenzyme A and the gasotransmitter hydrogen sulfide (H₂S). L-cysteine can also have a direct effect on post-translational modification or S-glutathionylation of proteins, which can cause modification of structure and function and thereby provide protection against oxidative signaling events.
Cysteine is used for biosynthesis of glutathione (GSH), coenzyme A, taurine and sulfate. GSH functions in redox regulation and detoxification of oxidants and reactive electrophiles. Coenzyme A is central to fatty acid metabolism and the citric acid cycle; taurine is utilized for bile acid synthesis and osmotic regulation; sulfate is used as a structural component of oligosaccharides, transport of steroid hormones and detoxification of foreign compounds.
3.5 Immune Function
Adequate concentrations of GSH are required for mixed lymphocyte reactions, T-cell proliferation, T- and B-cell differentiation, cytotoxic T-cell activity, and natural killer cell activity. Adequate GSH levels have been shown to be necessary for microtubule polymerization in neutrophils. Because L-cystine is the predominant extracellular form of cysteine and the direct substrate for intracellular GSH synthesis, its availability significantly influences these immunological processes.
4. Scientific Evidence by Area of Use
4.1 Hair Loss (Telogen Effluvium and Androgenetic Alopecia)
This is the area with the most direct human clinical evidence for L-cystine supplementation. Clinical studies have confirmed that the hair-growth-promoting effect of approved oral drug combinations is beneficial for the treatment of diffuse telogen effluvium, which is characterized by the excessive loss of telogen club hairs.
Pantovigár® (multi-ingredient formulation): An oral formulation containing L-cystine, thiamin, calcium d-pantothenate, medicinal yeast, keratin and p-aminobenzoic acid (Pantovigár®) has demonstrated clinical efficacy for the treatment of diffuse telogen effluvium. This product has been used in European clinical practice and has been the subject of multiple clinical evaluations, though it is a multi-ingredient formula and attributing specific effects to L-cystine alone from these data is methodologically limited.
L-Cystine alone and with keratin hydrolysate (randomized controlled trial, 2019): A study investigated the efficacy of L-cystine alone or combined with a commercially available hydrolysate of natural keratin obtained from a non-human source (feathers) in improving both acute telogen effluvium and brittle nail syndrome symptoms. A randomised, parallel group study was carried out for 3 months on 60 female subjects. A statistically significant increase of hair density, hair and nail brightness, and nail plate growth rate were observed both in the L-cystine and the combination treatment group. The study demonstrated that L-cystine alone or a mix Kera-Diet®+L-cystine, associated with trace elements and specific vitamins at the right dosage, can enhance hair and nail conditions. The L-cystine dose used in this study was 250 mg L-cystine per capsule in the CYS arm.
L-Cystine with Serenoa repens, Cucurbita pepo, and Pygeum africanum (double-blind RCT, 2024/2025): The objective of this study was to evaluate the safety and efficacy of an oral supplement containing l-Cystine, Serenoa repens, Cucurbita pepo, Pygeum africanum, vitamins, and micronutrients in chronic telogen effluvium and androgenetic alopecia. Eighty patients of both sexes aged 18–60 years with CTE or AGA were randomized to receive one capsule daily of the oral supplement or placebo for 6 months. Dermatological evaluations, clinical pictures and phototrichograms were done at baseline, 3 months and 6 months. The oral supplement significantly increased hair density and anagen hairs after 3 and 6 months, as well as overall hair volume, hair and scalp appearance, and hair shedding compared to a placebo. Moreover, it could have a beneficial effect on quality of life, and it was well tolerated.
Cystiphane® (arginine, L-cystine, zinc and B6, open-label study, 2023): Telogen effluvium is a common cause of non-cicatricial hair loss with no treatment-standardized protocol. An oral supplement based on arginine, L-cystine, zinc and B6 vitamin (Cystiphane®, Laboratoires Bailleul) was investigated for its hair-growth properties. Twenty patients, aged between 18 and 70 years old, affected by TE were recruited. Patients were asked to take the oral supplement as a monotherapy, four tablets daily, in one or two administrations during meals. The study lasted 3 months. Results were very promising, with improvement of hair density and thickening of the hair shaft diameter in most patients seen with both global photography and trichoscopy. However, this was an open-label, uncontrolled study of only 20 subjects, and the evidence should be considered preliminary.
In vitro mechanistic evidence: The four core compounds of an oral hair-growth formulation enhanced proliferation and metabolic activity of human hair follicular keratinocytes compared to those cultivated in a minimal growth medium only. Functional grouping of differentially expressed genes confirmed the regulation of cell cycle-/proliferation-associated genes (cdk1, HJURP) and revealed regulation of cell death- and oxidative stress-associated gene groups. A supportive effect of the compounds on cell viability was demonstrated by lower sensitivity to solar-simulated UV-radiation and increased protection against oxidative stress. This study suggested that L-cystine primarily contributes to the observed protection against endogenous oxidative stress.
Evidence strength: Moderate — there are multiple small-to-medium randomized controlled trials supporting a beneficial effect on hair density and anagen rate in telogen effluvium, but most used L-cystine in combination with other active ingredients (vitamins, minerals, botanical extracts), making it impossible to isolate the contribution of L-cystine specifically. Studies are generally short (3–6 months), conducted in predominantly female populations, and many involve industry-supported formulations. Some studies lack the dose-relationship effect, employed methods are not reliable or standardized, and the study design sometimes does not take the placebo group into account.
4.2 Nail Health (Brittle Nail Syndrome)
Acute telogen effluvium and brittle nail syndrome are two medical conditions affecting both males and females. A dietary approach based on amino acids and/or on protein hydrolysate could be a safe and effective approach in reducing hair loss during acute telogen effluvium and in improving brittle nail conditions. Studies demonstrate the efficacy of oral minerals (e.g., zinc and iron), B-vitamins, and L-cystine on hair and nails. Some of these studies demonstrate that oral supplementation can have a positive effect on hair or nails while some others demonstrate that the lack of nutrient intakes with the diet has a detrimental role on hair and nail conditions.
The same 2019 RCT described under hair loss (Nobile et al.) also reported a statistically significant increase of hair density, hair and nail brightness, and nail plate growth rate were observed both in the CYS and KDC treatment groups. Evidence for nail-specific outcomes from L-cystine supplementation largely derives from these same combination-product trials and should be considered of similar moderate-to-preliminary strength.
4.3 Skin Pigmentation and Lightening
Glutathione has become a potential skin-lightening ingredient after the discovery of its anti-melanogenic properties, 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.
Key clinical trial (Duperray et al., 2022 — Journal of Cosmetic Dermatology): A 12-week randomized, double-blind, parallel-group, benchmark- and placebo-controlled trial involved 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). Skin color was measured at baseline, after 6 and 12 weeks by spectrophotometry. Size and color of facial dark spots were determined from digital photographs. A significant skin lightening was observed after 12 weeks of oral supplementation with L-Cystine associated with L-Glutathione. This combination also induced a significant reduction in the size of facial dark spots after 6 and 12 weeks.
Notably, the group treated with glutathione and cystine together exhibited the most excellent skin lightening effect, and there were significant differences in skin lightness and ITA° values compared to the group treated with glutathione or cystine alone as well as the placebo group. In the clinical trial, a skin lightening effect was observed after a combination of glutathione 250 mg plus cystine 500 mg was orally administered, and a weaker but significant skin lightening effect was observed when only 500 mg of cystine was administered.
Evidence strength: Overall, clinical validation of the skin lightening effects of cysteine, N-acetyl cysteine, and cystine is still insufficient to draw any conclusion, and additional research is needed. Clinical trials on cysteine, N-acetyl cysteine, and cystine are still insufficient. The Duperray et al. 2022 trial is currently the most methodologically robust study, but the evidence base is thin, consisting essentially of a single primary RCT. The study was conducted in an Asian female population; generalizability to other ethnicities and to males has not been established.
4.4 Antioxidant Status and Oxidative Stress
L-cysteine is widely recognized as a conditionally essential or indispensable sulfur amino acid. It plays a key role in the metabolic pathways involving methionine, taurine and glutathione, and may help fight chronic inflammation by boosting antioxidant status. In stressed and inflammatory states, sulfur amino acid metabolism adapts to meet the increased requirements for cysteine as a rate-limiting substrate for GSH.
Critically ill patients receiving enteral or parenteral nutrition, enriched with cysteine, exhibit decreased cysteine catabolism and improved GSH synthesis. Studies indicate that L-cysteine supplementation and an improvement in GSH status is potentially useful for prevention of oxidative stress and insulin resistance. However, most of this evidence derives from research on cysteine and its NAC derivative rather than from trials specifically testing free L-cystine as an oral supplement in healthy or ambulatory populations. Evidence for L-cystine's isolated antioxidant effects in clinical (non-critical care) settings remains preliminary.
4.5 Skin Ageing and Collagen Matrix (Combined Formulations)
Collagen is an essential ingredient in dietary supplements for its anti-ageing benefits, and l-cystine-based supplementation has garnered interest for its ability to improve skin condition. Studies aimed to evaluate the effects of oral supplementation combining l-cystine and fish collagen peptides at two different dosages on mature (55–65 years) and young (18–30 years) skin types. Two randomised, three-arm, double-blind, placebo-controlled trials were conducted. A total of 198 Asian women were allocated into equal groups (5.5 or 11 g of active supplement or placebo daily for 12 weeks). These data come from combination collagen/cystine products, and isolating the contribution of L-cystine alone from the collagen effect is not possible from the study design.
4.6 Nitrogen/Amino Acid Nutrition and Body Weight
The body weight gain, food efficiency ratio, and the amino acid nutrition biomarker improved by adding the amino acids glycine, L-threonine and L-cystine to a 9% casein diet. The conversion percentage of L-tryptophan to nicotinamide decreased with the addition of the amino acids glycine, L-threonine and L-cystine. L-cystine improved the body weight gain, the food efficiency ratio and the urine ratio, and decreased the conversion percentage. This work was conducted in an animal model (rat), and the human translational relevance requires further investigation.
5. Body Systems and Health Areas Associated with L-Cystine
- Integumentary system (hair, nails, skin): Cysteine is an important part of keratin, the major protein in hair and nails and the exoskeleton of several species. The disulfide bonds formed from cystine residues give keratin fibres their tensile strength and resistance to proteolytic digestion.
- Antioxidant/redox system: Recent studies have highlighted the importance of GSH in key signal transduction reactions as a controller of cell differentiation, proliferation, apoptosis, ferroptosis and immune function. Since cystine/cysteine availability governs GSH synthesis rates, L-cystine is a fundamental upstream substrate for the cellular redox system.
- Immune system: GSH is a key determinant of redox signaling, vital in detoxification of xenobiotics, and modulates cell proliferation, apoptosis, immune function, and fibrogenesis.
- Connective tissue and structural proteins: Cystine is particularly abundant in skeletal and connective tissues and in hair, horn, and wool.
- Metabolic/sulfur pathways: Sulfur amino acids exert important functions through their metabolites, such as S-adenosylmethionine (SAM), polyamines, taurine, and glutathione.
- Skin pigmentation: Evidence from the Duperray et al. 2022 trial points to involvement in the melanogenesis pathway, likely via glutathione-mediated inhibition of tyrosinase and shifting of melanin synthesis toward pheomelanin.
- Renal system (pathological context): Cystine — a metabolic product of cysteine — has been linked to cystinuria. Individuals who suffer from cystinuria accumulate amino acids in the urine and the formation of kidney stones composed of cystine.
6. Dosage Forms and Reported Dosages
L-cystine is available for oral administration as standalone capsules or tablets and as a component of combination nutraceutical products. The following dosages are those reported in published clinical research:
- 250 mg L-cystine per capsule, used as the active intervention in the Nobile et al. (2019) randomized, placebo-controlled trial on hair and nails in 60 healthy females over 3 months.
- Women in the Duperray et al. (2022) skin pigmentation trial were randomly allocated into groups receiving 500 mg L-cystine and 250 mg L-glutathione, 250 mg reduced L-glutathione, 500 mg L-cystine, or placebo, daily for 12 weeks.
- Eighty patients in the Piquero-Casals et al. (2025) hair loss RCT were randomized to receive one capsule daily of an oral supplement containing l-cystine and botanical extracts or placebo for 6 months.
- Patients in the Cystiphane® study on telogen effluvium were asked to take the oral supplement as a monotherapy, four tablets daily, in one or two administrations during meals.
- Two RCTs evaluating l-cystine combined with fish collagen peptides administered either 5.5 or 11 g of active supplement daily for 12 weeks to 198 Asian women.
No established Recommended Daily Intake (RDI) has been set by major regulatory authorities specifically for L-cystine as a dietary supplement. Sulfur amino acid requirements in humans are established in terms of total methionine + cysteine combined. Sulfur amino acid intake in humans is variable and optimal intake has not been adequately defined. The average American diet contains about 100 g of protein daily; the mean sulfur amino acid intake is about 2.4 g.
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. A 2023 qNMR study found four of eight food supplement samples were found to be inaccurately labeled or even with fake labeling, with the relative actual amount of cystine ranging from 0.3% to 107% of the claimed amount.
7. Safety Considerations and Interactions
7.1 Cystinuria: A Critical Safety Concern for a Specific Population
Cystinuria is the most common genetic cause of recurrent kidney stones. L-cystine stones, which are larger and more likely to cause chronic kidney disease than calcium oxalate monohydrate stones, form as a consequence of excessive levels of L-cystine in the urine due to defective reabsorption of filtered cystine. This autosomal recessive disorder is caused by mutations in one of two genes coding for components of proximal renal tubule amino acid transporters. Affected genes are either SLC3A1 on chromosome 2 leading to type A cystinuria, or SLC7A9 on chromosome 19 leading to type B.
In patients with cystinuria, the excretion of l-cystine in the urine is estimated to be >400 mg/day, as compared to <30 mg/day in normal subjects. The clinical consequence of high levels of the poorly soluble l-cystine in these patients' urine is the precipitation of l-cystine to form urinary tract stones, primarily in the kidneys and less often in the bladder.
A published case report in PMC documented rapidly accelerated cystine stone formation in a cystinuric patient who began taking L-cystine-containing dietary supplements. Patients and physicians should be aware of the potential harm of these types of dietary supplements, whose intake has become very popular in the western world, in patients with cystinuria.
7.2 Management of Cystinuria: Current Approaches
Current methods of prevention of stone formation in cystinuria include increasing fluid intake, dietary modifications, alkali therapy, and cystine-binding thiol drugs (CBTDs), which help increase the solubility of cystine in the urine. At severe conditions, chelation therapy is necessary, utilizing the reaction of D-penicillamine or α-mercaptopropionylglycine with L-cystine to generate more soluble asymmetric disulfides. These drugs have side effects including loss of taste, fever, proteinuria, serum sickness-type reactions, and even frank nephritic syndrome.
7.3 General Safety Profile at Supplemental Doses
L-cystine at the dosages reported in hair/nail/skin clinical trials (250–500 mg/day as a standalone or in combination products) was generally described as well tolerated. The oral supplements studied could have a beneficial effect on quality of life, and were well tolerated. No serious adverse events attributable to L-cystine were reported in the reviewed RCTs at these dosages.
The concern about high-dose free amino acid intake is well-established in the scientific literature more broadly. While problems associated with cysteine are less common and have not been exhaustively studied, they are nonetheless documented. Free cysteine can cause problems in a segment of the population.
7.4 Quality and Labeling Issues
A concern specific to L-cystine supplements is product quality. As noted above, the lack of industry standards and market regulations resulted in quality issues with cystine food products, including cases of food adulteration and fraud. Independent testing using qNMR found substantial discrepancies between labeled and actual cystine content in commercially available supplements.
7.5 Relationship to Overall Sulfur Amino Acid Balance
Others have evaluated the literature linking oxidative stress and other clinical conditions to an imbalance of extracellular L-cysteine/L-cystine. While not all tissues have had their L-cysteine and L-cystine metabolism thoroughly investigated, prior research has shown that transportation plays a major role in maintaining a balance between the amounts of these compounds outside of cells and inside them. Elevated oxidative stress in various tissues from uncontrolled hyperglycemia may cause increased L-cysteine utilization and the lower levels seen in diabetes. This suggests that populations with metabolic disease may have altered requirements, though clinical data directly testing L-cystine supplementation in these groups are limited.
8. Summary of Evidence Quality by Application Area
- Hair loss (telogen effluvium, androgenetic alopecia): Multiple small-to-medium RCTs; evidence is moderate but mostly derived from multi-ingredient formulations.
- Nail brittleness: Supported by limited RCT data largely from the same combination trials used for hair outcomes; evidence is preliminary.
- Skin pigmentation/lightening: One adequately powered RCT (Duperray et al. 2022); evidence is early, restricted to Asian women, and confined primarily to the combination with glutathione.
- Antioxidant/GSH support: Mechanistically well-established for cysteine/cystine as the rate-limiting GSH precursor; most clinical evidence involves NAC or critically ill populations rather than oral L-cystine in healthy adults.
- Skin ageing (collagen combination products): Preliminary; L-cystine cannot be distinguished from collagen contributions in available studies.
- Metabolic/immune health: Largely indirect, mechanistic, or from animal models; insufficient direct human evidence to draw conclusions specific to supplemental L-cystine.
References
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