Hydroxylysine

Other Names

(2S,5R)-2,6-diamino-5-hydroxyhexanoic acid(2S,5R)-5-Hydroxylysine(5R)-5-Hydroxy-L-lysine2,6-diamino-3-hydroxyhexanoic acid2,6-Diamino-5-hydroxy-hexanoic acid2,6-Diamino-5-hydroxycaproic acid2,6-diamino-5-hydroxyhexanoic acid3-Hydroxylysine5-hydroxy-2,6-diaminohexanoic acid5-Hydroxy-DL-lysine5-Hydroxy-Lys5-Hydroxylysine5HylallohydroxylysineD-Hydroxylysinedelta-hydroxylysineDL-5-HydroxylysineDL-α-Hydroxylysineerythro-5-hydroxy-L-lysineHydroxy-L-lysineHydroxy-LysineHylL-5-HydroxylysineL-HydroxylysineL-Lysine, 5-hydroxy-, (5R)-Lysine, 3-hydroxy-Lysine, 5-hydroxy-α,ε-diamino-δ-hydroxycaproic acidδ-DL-hydroxylysineδ-hydroxylysine

Synopsis

Hydroxylysine: A Comprehensive Encyclopedic Reference

1. Identity, Chemical Classification, and Natural Sources

1.1 Chemical Identity and Nomenclature

Hydroxylysine (abbreviated Hyl) is an amino acid with the molecular formula C₆H₁₄N₂O₃. It is a hydroxylated derivative of the essential amino acid lysine, carrying an additional hydroxyl (–OH) group on its side chain. It was first discovered in 1921 by Donald Van Slyke as the 5-hydroxylysine form. Its full systematic name is 2-amino-6-hydroxy-6-aminohexanoic acid; it is also catalogued in the Human Metabolome Database as HMDB0000450 and in PubChem under multiple CIDs depending on stereoisomer (e.g., L-Hydroxylysine, CID 3032849; 3-Hydroxylysine, CID 419865; 4-Hydroxylysine, CID 550919).

The most common form is the (5R) stereoisomer found in collagen. However, the enzyme JMJD6 has recently been shown to be a lysyl hydroxylase which modifies an RNA splicing factor, producing the (5S) stereoisomer. Hydroxylysine arises from a post-translational hydroxy modification of lysine and is therefore not encoded directly by any codon in the standard genetic code.

1.2 Classification: A Post-Translational Modification Product, Not a Standard Dietary Amino Acid

Unlike the 20 standard amino acids that directly form most proteins, hydroxylysine is a modified version of lysine. It is not incorporated into proteins during their initial construction but is formed afterward through a specific chemical change. This modified amino acid is primarily recognized for its significance in collagen, the body's most abundant protein.

Hydroxylysine is a glycogenic amino acid uniquely found in collagen, the chief structural protein of mammalian skin and connective tissue, and in some similar structural plant proteins. Hydroxylysine also exists in collagen-like domains of several other proteins that are not defined as collagens.

1.3 Natural Sources and Distribution

Hydroxylysine is found only in animal proteins and mostly in collagens. The hydroxyl groups of hydroxylysine residues have two important functions: they serve as attachment sites for carbohydrates and they play a crucial role in stabilizing intra- and intermolecular crosslinks.

The content of hydroxylysine in the triple-helical domains of collagen peptide chains varies from 6 residues per 1,000 amino acids. Only 17% of the lysyl residues are hydroxylated in collagen type III, whereas almost 90% are hydroxylated in collagen type IV and about 80% in collagen type VI. Hydroxylysine is therefore present whenever collagen-rich animal tissues are consumed: meat (particularly connective tissue-rich cuts), bone broth, gelatin, cartilage, and hydrolyzed collagen preparations all contain hydroxylysine as a component amino acid.

1.4 Common Forms and Preparations

Hydroxylysine is not commercialized as a stand-alone dietary supplement in the manner of lysine or other free amino acids. Instead, it reaches consumers as an integral component of:

Hydrolyzed collagen peptides — bovine, porcine, marine, and chicken-derived collagen hydrolysates, sold in powder or capsule form, all contain hydroxylysine residues as part of their peptide composition.
Gelatin — heat-denatured collagen that retains hydroxylysine in its primary structure. Collagen is insoluble, but heat denaturation (boiling in water, dilute acids, or alkalies) yields digestible, soluble gelatin.
Bone broth — a traditional food preparation in which connective tissue is simmered to release collagen-derived peptides including those containing hydroxylysine.
Free-form L-hydroxylysine — available as a research-grade or specialty chemical (CAS 1190-94-9 for L-hydroxylysine); its use as a standalone oral supplement lacks a well-established commercial market and clinical evidence base.

2. Historical and Traditional Use

2.1 Gelatin and Collagen-Rich Foods in Traditional Medicine

Hydroxylysine does not have a history of use as an isolated compound in any traditional medical system — its chemical identity was not established until 1921, and its specific role in collagen was not elucidated until decades later. However, the collagen-containing preparations that deliver hydroxylysine to the body have a deep and well-documented history of traditional use across multiple cultures.

In Chinese medicine, preparations made from animal hides, tendons, and bone — collectively rich sources of collagen and its constituent hydroxylysine — have been employed for centuries to support the sinews, bones, and skin. Classical texts from the Han dynasty (~200 BCE–220 CE) describe gelatin (阿膠, ē jiāo), prepared by prolonged boiling of donkey hide, as a tonic for blood nourishment, wound healing, and the strengthening of tendons and ligaments.

In European folk medicine and culinary tradition, bone broths and gelatinous stocks have long been regarded as restorative foods for convalescent individuals, those with joint ailments, and those recovering from fractures. Medieval European apothecaries prescribed calf's foot jelly as a strengthening preparation.

In Ayurvedic medicine, preparations based on bone ash and marrow (known collectively as Asthi varga) and gelatin-containing animal preparations were used to support bone integrity and joint health.

Across all these traditions, the rationale for use was empirical and corresponded to the biological concept — later confirmed by biochemistry — that collagen-rich foods support connective tissue health. The role of hydroxylysine as the specific active molecular component in those preparations was, of course, entirely unknown at the time of traditional use and was not articulated until modern structural biochemistry.

2.2 Discovery and Early Scientific History

Hydroxylysine was first discovered in 1921 by Donald Van Slyke as the 5-hydroxylysine form. Subsequent structural work in the mid-twentieth century identified hydroxylysine as a defining constituent of collagen and elucidated its role in crosslinking and glycosylation. The biochemical pathway linking vitamin C deficiency to impaired hydroxylysine synthesis was recognized in connection with the disease of scurvy, establishing one of the earliest molecular explanations for a nutritional deficiency disease.

3. Biosynthesis, Key Constituents, and Mechanisms of Action

3.1 Biosynthesis: The Lysyl Hydroxylase Pathway

The body does not directly obtain hydroxylysine from the diet; instead, it synthesizes it from the amino acid lysine. This conversion occurs through post-translational modification, meaning the change happens after the initial protein chain containing lysine has been assembled. Within the endoplasmic reticulum, an enzyme called lysyl hydroxylase catalyzes this transformation. Lysyl hydroxylase adds a hydroxyl group to specific lysine residues, converting them into hydroxylysine.

This enzymatic reaction requires vitamin C as an important cofactor. Vitamin C ensures that lysyl hydroxylase functions correctly, allowing the hydroxylation of lysine to proceed efficiently.

At the molecular level, proline and lysyl hydroxylases require ferrous iron (Fe²⁺) to function. In the course of hydroxylating proline or lysine, ferrous iron (Fe²⁺) is oxidized to ferric iron (Fe³⁺). Ascorbic acid is required to reduce Fe³⁺ to Fe²⁺, forming semidehydroascorbic acid in the process. This regeneration of the reduced iron cofactor is the biochemical basis for the vitamin C–collagen axis.

During collagen biosynthesis, lysine residues undergo extensive post-translational modifications through the alternate action of two distinct metal ion-dependent enzyme families (LH/PLODs and GLT25D/COLGALT), ultimately producing the highly conserved α-(1,2)-glucosyl-β-(1,O)-galactosyl-5-hydroxylysine pattern.

3.2 Three Lysyl Hydroxylase Isoforms (PLOD Genes)

Three genes (PLOD1, PLOD2, and PLOD3) encode unique isoforms of procollagen-lysine, 2-oxoglutarate 5-dioxygenase. PLOD1 and PLOD2 hydroxylate lysine residues in the triple helical and telopeptide domains, respectively, whereas the substrate specificity of PLOD3 is less fully characterized.

During collagen maturation, the hydroxylysine residues in the helix region are often modified by O-linked glycosylation. These reactions are catalyzed by hydroxylysine galactosyltransferase (GT) and galactosylhydroxylysine–glucosyltransferase (GGT). The enzymatic activities of GT and GGT are found in multifunctional PLOD3, but not in PLOD1 and PLOD2.

3.3 Dual Functional Roles of Hydroxylysine in Collagen

The hydroxylysine residues formed in the lysyl hydroxylase reaction in collagen have two important functions: first, their hydroxy groups serve as sites of attachment for carbohydrate units, either the monosaccharide galactose or the disaccharide glucosylgalactose; and second, they stabilize intermolecular collagen crosslinks.

Glycosylation function: Hydroxylysine residues are glycosylated by Mn²⁺-dependent galactosyltransferase enzymes, through an inverting reaction that adds the galactose moiety of UDP-α-galactose to the hydroxylysine hydroxyl group, generating galactosyl-hydroxylysine. Galactosyl-hydroxylysine is further glycosylated by the multifunctional LH/PLOD enzymes through a retaining reaction requiring Mn²⁺ and UDP-α-glucose, ultimately generating glucosyl-galactosyl-hydroxylysine.

Crosslinking function: Tropocollagen molecules gather to form collagen fibrils via covalent cross-linking by lysyl oxidase, which links hydroxylysine and lysine residues. The chemical nature of mature collagen crosslinks (lysyl pyrrole, hydroxylysyl pyrrole, hydroxylysyl pyridinoline, or lysylpyridinoline) is determined by the extent of hydroxylation of the lysines involved in crosslinking.

Lysyl oxidases initiate collagen crosslinking through the oxidation of the ε-amino group of lysine or hydroxylysine on collagen side-chains, which subsequently dimerize to form immature, or trimerize to form mature, collagen crosslinks.

3.4 Glycosylated Forms

Hydroxylysine is present in part as galactosyl hydroxylysine and in part as glucosyl-galactosyl hydroxylysine. The relative proportion and total content of galactosyl hydroxylysine and glucosyl-galactosyl hydroxylysine vary in bone and soft tissues, with a higher content of galactosyl hydroxylysine in bone, suggesting that its urinary excretion might be a more sensitive marker of bone resorption than urinary hydroxyproline.

3.5 Role in Collagen Triple-Helix Stability

Hydroxyproline and hydroxylysine are important for stabilizing collagen by cross-linking the propeptides in collagen. Collagen molecules are composed of three polypeptide chains that intertwine to form a triple helix. The hydroxylysine-derived crosslinks prevent collagen fibers from sliding past each other, reinforcing the overall structure of connective tissues. The presence and proper modification of hydroxylysine are important for the mechanical properties of tissues like bone and tendon.

4. Scientific Evidence by Area of Application

4.1 Bone Metabolism and Resorption Biomarker

One of the most extensively studied scientific applications of hydroxylysine measurement is as a biomarker of bone and collagen turnover, not as a therapeutic agent per se but as a diagnostic indicator.

Assessment of bone resorption can be achieved with measurement of urinary hydroxylysine glycosides, urinary excretion of the collagen pyridinium cross-links, urinary excretion of type I collagen telopeptide breakdown products (cross-linked telopeptides), and urinary hydroxyproline.

Like hydroxyproline, hydroxylysine is not reused for collagen biosynthesis, and although it is much less abundant than hydroxyproline, it is a potential marker of collagen degradation. This non-reutilization is a critical property: because hydroxylysine released during collagen breakdown cannot be recycled into new collagen chains, its appearance in urine directly reflects the net rate of collagen catabolism.

Galactosyl hydroxylysine (GH) as a bone-specific marker: A study by Moro et al. (published in Calcified Tissue International) measured β-1-galactosyl-O-hydroxylysine in the urine of 107 individuals aged 30–79 years using HPLC. Vertebral mineral density measured by quantitative computed tomography (QCT) and urinary GH were inversely correlated (r = −0.74; P < 0.001). High rate of bone mineral loss is associated with high urinary GH excretion. Measurement of GH in urine provides a simple and noninvasive method for the evaluation of the extent of bone resorption in large groups of subjects and appears to be more specific than urinary hydroxyproline excretion.

Serum-based biochemical markers of bone resorption may provide better clinical information than urinary markers because direct comparison with serum markers of bone formation is possible and because the within-subject variability of serum markers may be lower. Researchers have described a method for the measurement of free β-1-galactosyl-O-hydroxylysine (Gal-Hyl) in serum. In a study published in Clinical Chemistry (1999), serum free Gal-Hyl correlated strongly with urine free Gal-Hyl in women and pubertal girls (r = 0.84; P < 0.001). Serum Gal-Hyl was higher during puberty and increased after menopause, consistent with the known acceleration of bone resorption at those life stages.

Galactosyl hydroxylysine (GHYL) is considered a better bone resorption biomarker than glucosylgalactosyl hydroxylysine (GGHYL) and hydroxyproline. The concentration of GHYL in urinary excretion has been applied to evaluate the occurrence of fracture in postmenopausal osteoporotic women without fragility fractures and postmenopausal osteoporotic women with fragility fractures. High levels of GHYL in fracture patients suggest a possible defect in bone collagen, and urinary GHYL may indeed identify such an abnormality.

GHYL can be easily measured without preanalytical hydrolysis and extraction by high performance liquid chromatography (HPLC). Although GHYL has revealed the potential as a biomarker of bone resorption, the use of GHYL has not been extensively studied for the assessment of osteoporosis and the evaluation of osteoporotic treatment.

Evidence strength: The evidence supporting hydroxylysine glycosides as biomarkers of bone resorption is moderate to good for research and investigational clinical use, based on multiple cross-sectional and longitudinal human studies. However, due to the complexity of measurement and lack of standardization, these markers have not achieved the routine clinical adoption of later-generation resorption markers such as serum CTX-1.

4.2 Connective Tissue Structural Integrity

Hydroxylysine is understood to be fundamentally required for normal connective tissue architecture. A loss of lysyl hydroxylase 1 activity greatly reduces the amount of hydroxylysine, which impairs cross-linking between collagen molecules. This disruption in the network of collagen fibers weakens connective tissues. This insight comes not from supplementation studies but from the clinical pathology of enzyme-deficiency syndromes (see Section 5.1). No controlled human trials have evaluated the administration of free hydroxylysine as a supplement to improve connective tissue integrity, and no human interventional evidence for this use exists in the peer-reviewed literature.

4.3 Skin and Wound Healing (Collagen Hydrolysates Context)

Hydroxylysine is a constituent of the collagen hydrolysate (CH) products that have been evaluated in human clinical trials for skin health. It is important to note that the clinical evidence accumulated to date is attributable to the entire matrix of collagen-derived peptides and amino acids, not to hydroxylysine in isolation.

Collagen hydrolysates in functional foods and supplements are dietary sources of amino acids and di- and tripeptides linked to various health benefits. A randomized, double-blind crossover clinical study was performed with healthy volunteers assessing the plasma concentration of free and peptide-bound hydroxyproline as well as selected peptides reported to be abundantly present in collagen derived from fish, porcine and bovine origin with different molecular weights. The pharmacokinetic endpoints demonstrated comparable uptake of free hydroxyproline from all collagen hydrolysates. A higher amount of total compared to free hydroxyproline indicated the uptake of substantial amounts of hydroxyproline-containing di- or tripeptides. Independently of source and molecular weight, all collagen hydrolysates yielded relevant plasma concentrations of the investigated metabolites.

Evidence strength: The evidence for collagen-derived peptide supplementation in skin health (elasticity, hydration) is preliminary to moderate, based on small randomized controlled trials. The specific contribution of hydroxylysine versus other peptide components (e.g., Pro-Hyp dipeptides) has not been established in isolation. No clinical trials have administered hydroxylysine alone for skin or wound healing outcomes.

4.4 Fibrosis and Cancer Biology

A substantial and growing body of basic and translational research implicates the lysyl hydroxylation pathway — and by extension hydroxylysine — in the pathological stiffening of tissues seen in fibrosis and cancer. This is mechanistic research and is not evidence of hydroxylysine supplementation as a therapeutic strategy; rather, it identifies the enzyme pathway as a potential drug target for inhibition.

High LH2 levels cause stable collagen cross-link accumulations that promote fibrosis and cancer progression. PLOD2 (LH2) is increased in cancer-associated fibroblasts, which leads to increased hydroxylation of key lysine residues leading to the formation of more stable pyridinoline-derived crosslinks, which are thought to contribute to the increased stiffness of tumor stroma.

Overhydroxylation of lysine residues and the subsequent formation of hydroxylysylpyridinoline (HP) and lysylpyridinoline (LP) cross-links underlie ECM stiffness and profoundly affect tumor progression. Hydroxylysine (Hyl) and HP cross-links are significantly higher in uterine fibroids compared to normal myometrial tissues, accompanied by increased expression of LH (LH2b) and LOX.

Malfunctions in the LH/PLOD and GLT25D/COLGALT enzyme families are linked to developmental pathologies and extracellular matrix alterations associated with enhanced aggressiveness of solid tumors.

Evidence strength: This area is supported by cell biology, animal models, and translational human tissue studies, but represents an early-stage therapeutic target area, not a basis for hydroxylysine supplementation. All evidence in this domain suggests that in pathological fibrosis and cancer, reducing excessive hydroxylysine-dependent crosslinking — not adding more hydroxylysine — is the therapeutic goal.

4.5 Oral Submucous Fibrosis

The upregulation of lysyl oxidase and lysyl hydroxylase has been shown to exhibit higher levels of the hydroxylysine aldehyde-derived cross-links in fibrosis and tumor stroma, promoting tumor cell survival, resistance, and invasion. Research in oral submucous fibrosis has investigated strategies to reverse this crosslinking, targeting the enzymes that generate hydroxylysine aldehyde-derived crosslinks as potential anti-fibrotic approaches. This research is in silico and preclinical; no clinical trials have been published using hydroxylysine modulation as a treatment for this condition.

5. Body Systems and Health Areas Associated with Hydroxylysine

5.1 Skeletal System and Bone

Hydroxylysine contributes to the stability of collagen, which forms the framework for skin, bones, tendons, and ligaments. In bone, the predominant glycosylated form of hydroxylysine is galactosyl hydroxylysine, which is distinct from the glucosyl-galactosyl form found more abundantly in soft tissues — a difference that underlies the tissue specificity of urinary GH as a bone biomarker. Increased levels of hydroxylysine are seen with connective and bone tissue breakdown.

5.2 Skin and Dermal Connective Tissue

Depending on the type of collagen, varying numbers of hydroxylysines are glycosylated (mostly having disaccharides attached). In the dermis, hydroxylysine-derived crosslinks provide structural integrity to the skin. Scurvy — the archetypal vitamin C deficiency disease — produces skin fragility, wound-healing failure, and perifollicular hemorrhage, all of which reflect impaired hydroxylysine formation in dermal collagen. Collagen is a primary structural protein in the human body, necessary for healthy blood vessels, muscle, skin, bone, cartilage, and other connective tissues. Defective connective tissue leads to fragile capillaries, resulting in abnormal bleeding, bruising, and internal hemorrhaging.

5.3 Tendons, Ligaments, and Cartilage

The mechanical properties of tendons and ligaments are heavily dependent on the nature of collagen crosslinks, which are in turn determined by the degree of lysyl hydroxylation. The consequence of decreased lysyl hydroxylation is altered production of the stable intermolecular cross-links that provide tissues with tensile strength.

5.4 Cardiovascular System

Collagen type I and III are major structural components of the myocardium, arterial walls, and cardiac valves. The hydroxylysine crosslinking density of arterial collagen influences the stiffness and mechanical behavior of blood vessels. Individuals with PLOD1-kEDS (kyphoscoliotic Ehlers-Danlos syndrome) are at risk of life-threatening arterial ruptures and spontaneous dissections of medium-sized arteries, illustrating the critical structural role of adequate hydroxylysine crosslinking in vascular integrity.

5.5 The Kyphoscoliotic Ehlers-Danlos Syndrome (PLOD1-kEDS): A Disease of Hydroxylysine Deficiency

The most direct clinical evidence for hydroxylysine's physiological importance comes from rare inherited disorders caused by mutations in the genes encoding lysyl hydroxylases.

Defects of the lysyl hydroxylases LH1, LH2, and LH3 impair collagen secretion and stability in the extracellular matrix, and cause rare connective tissue disorders such as Ehlers-Danlos type-VI, Bruck syndrome, and skeletal dysplasia, respectively.

Ehlers-Danlos syndrome type VI (EDS VI) is a rare autosomal recessively inherited disease of connective tissue. The characteristic symptoms are hyperflexibility of joints and hyperelasticity of skin together with marked scoliosis, ocular manifestations and involvement of the vascular system. The underlying biochemical defect in EDS VI is a deficiency in lysyl hydroxylase (PLOD) activity resulting from mutations in the PLOD gene, causing a low hydroxylysine content in various tissues.

More than 30 mutations in the PLOD1 gene have been found to cause a form of Ehlers-Danlos syndrome called the kyphoscoliotic type. Ehlers-Danlos syndrome is a group of disorders that affect the connective tissues that support the skin, bones, blood vessels, and many other organs and tissues.

Deficiency of the enzyme procollagen-lysine, 2-oxoglutarate 5-dioxygenase 1 (PLOD1; also called lysyl hydroxylase or LH1) results in a deficiency in hydroxylysine-based pyridinoline cross-links in collagens. PLOD1-related kyphoscoliotic Ehlers-Danlos syndrome is characterized by hypotonia, generalized joint hypermobility, early-onset kyphoscoliosis, skin fragility, and ocular abnormality.

Biochemically, the condition is diagnosed by: detection of an increased ratio of deoxypyridinoline (Dpyr) to pyridinoline (Pyr) cross-links in urine quantitated by HPLC, a highly sensitive and specific test. The normal ratio of Dpyr:Pyr cross-links is approximately 0.2, whereas in PLOD1-kEDS the ratio is approximately 6.0.

The related condition, Bruck syndrome, involves mutations in PLOD2: Bruck syndrome is characterized by a pyridinoline deficiency in bone collagen. Analysis identified two missense mutations in exon 17 of PLOD2, identifying PLOD2 as a putative telopeptide lysyl hydroxylase.

6. Metabolism, Excretion, and Biomarker Applications

6.1 Metabolic Fate After Collagen Catabolism

Hydroxylysine is not reused for collagen biosynthesis. Glycosylation of hydroxylysine derivatives results in galactosyl hydroxylysine (GHL). During collagen breakdown, hydroxylysine and GHL are lost to the circulation and excreted in the urine.

Crucially, hydroxylysine is not affected by diet, a property that enhances its utility as a biomarker — unlike hydroxyproline, which can be elevated by collagen-rich food intake prior to sample collection and therefore requires dietary restriction before testing.

6.2 Urinary Hydroxylysine as a Diagnostic Marker

The bone resorption biomarkers include hydroxyproline, hydroxylysine, deoxypyridinoline, pyridinoline, bone sialoprotein, osteopontin, tartrate-resistant acid phosphatase 5b, carboxy-terminal crosslinked telopeptide of type 1 collagen, amino-terminal crosslinked telopeptide of type 1 collagen, and cathepsin K.

Assessment of bone resorption can be achieved with measurement of urinary hydroxylysine glycosides, urinary excretion of the collagen pyridinium cross-links, urinary excretion of type I collagen telopeptide breakdown products (cross-linked telopeptides), and urinary hydroxyproline.

The hydroxylysine/creatinine ratio in a 24-hour urine collection (LOINC code 56684-4) is an established standardized clinical chemistry measurement used in the investigation of metabolic bone diseases.

6.3 Tissue Specificity of Glycosylated Forms

The relative proportion and total content of galactosyl hydroxylysine and glucosyl-galactosyl hydroxylysine vary in bone and soft tissues, with a higher content of galactosyl hydroxylysine in bone, suggesting that its urinary excretion might be a more sensitive marker of bone resorption than urinary hydroxyproline. Because the only source of GHL is bone, this marker is more specific than hydroxyproline.

The method for measuring hydroxylysine glycosides is complex, based on HPLC separation of the different forms, which contributes to the limitations of this marker for routine clinical applications.

7. Oral Bioavailability of Hydroxylysine from Dietary Sources

An important practical question is whether hydroxylysine consumed in the diet (as part of collagen hydrolysates, gelatin, or bone broth) survives digestion and reaches systemic circulation in biologically meaningful concentrations. Collagen hydrolysates in functional foods and supplements are dietary sources of amino acids and di- and tripeptides linked to various health benefits. A study investigated the single-dose bioavailability of skin- and hide-derived collagen hydrolysates from fish, porcine and bovine origin with different molecular weights. A randomized, double-blind crossover clinical study was performed with healthy volunteers assessing the plasma concentration of free and peptide-bound hydroxyproline and selected peptides. The pharmacokinetic endpoints demonstrated comparable uptake of free hydroxyproline from all collagen hydrolysates. A higher amount of total compared to free hydroxyproline indicated the uptake of substantial amounts of hydroxyproline-containing di- or tripeptides. Independently of source and molecular weight, all collagen hydrolysates yielded relevant plasma concentrations of the investigated metabolites.

These studies have characterized hydroxyproline absorption in greater detail than hydroxylysine specifically. While hydroxylysine is present in collagen hydrolysate products, the dominant collagen-characteristic amino acids tracked in absorption studies are hydroxyproline-containing peptides (e.g., Pro-Hyp). The specific pharmacokinetics of dietary hydroxylysine-containing peptides post-ingestion remain less well characterized in the peer-reviewed clinical literature.

It bears emphasis that the body does not directly obtain hydroxylysine from the diet; instead, it synthesizes it from the amino acid lysine. This means the body's primary route to maintaining hydroxylysine content in collagen is via endogenous synthesis (dependent on adequate lysine, vitamin C, iron, and alpha-ketoglutarate availability), not direct dietary hydroxylysine intake.

8. Dosage Forms and Dosages Reported in Studies

Because hydroxylysine has not been studied as a standalone oral supplement in human clinical trials, there are no established or recommended clinical dosages for isolated hydroxylysine administration. The following represents what the scientific literature does report:

As a collagen hydrolysate component: Studies have investigated the single-dose bioavailability of skin- and hide-derived collagen hydrolysates from fish, porcine, and bovine origin with different molecular weights (bovine 2,000 and 5,000 Da). The collagen hydrolysate doses used in absorption studies and clinical skin trials typically range from approximately 2.5 g to 10 g per day, within which hydroxylysine is one constituent amino acid among many; its proportion varies by preparation type.
As a urinary biomarker: Hydroxylysine and galactosyl hydroxylysine are measured in 24-hour urine collections or corrected to creatinine concentration in spot samples. In biochemical marker studies, serum concentrations of free galactosyl hydroxylysine have been measured at a mean value of approximately 48 nmol/L in healthy subjects.
As a free amino acid research chemical: No clinical trial dosing for free L-hydroxylysine as an oral supplement has been identified in the peer-reviewed literature.

9. Safety Considerations and Notable Interactions

9.1 Safety of Endogenous Hydroxylysine

Hydroxylysine is a normal endogenous metabolite of human physiology and is present in blood and urine of all healthy individuals as a product of routine collagen catabolism. At the concentrations found physiologically, it presents no known toxicity. No regulatory body (FDA, EFSA, WHO) has issued safety warnings or adverse effect notices specifically for hydroxylysine as a dietary supplement.

9.2 Cadmium and Heavy Metal Interaction

Changes in hydroxylysine excretion with cadmium loading in experimental animals indicate that cadmium interferes with collagen synthesis. This is an important mechanistic finding from toxicology: heavy metal exposure (particularly cadmium, as seen in occupational or environmental nephropathy) can alter hydroxylysine metabolism, and measurement of urinary collagen metabolites can therefore serve as an indicator of collagen synthesis disruption in heavy metal exposure.

9.3 Dependency on Cofactor Availability: Vitamin C and Iron

The endogenous production of hydroxylysine is critically dependent on the availability of ascorbic acid (vitamin C) and ferrous iron. In collagen biosynthesis, ascorbic acid is an essential cofactor of prolyl and lysyl hydroxylases for the hydroxylation of proline and lysine to form hydroxyproline and hydroxylysine. Humans fail to synthesize ascorbic acid in vivo because they lack l-gulonolactone oxidase. Therefore, humans develop the symptoms of scurvy when ingesting an ascorbic acid-free diet. This means that any condition impairing vitamin C availability — frank deficiency, malabsorption, extreme dietary restriction, or markedly elevated metabolic demand — will reduce hydroxylysine formation in newly synthesized collagen and impair connective tissue function.

9.4 Hydroxylysine in Disease States as a Safety Signal

High plasma levels of hydroxylysine are indicative of connective tissue breakdown. Clinically elevated urinary or serum hydroxylysine glycosides are therefore associated with accelerated bone resorption (as in postmenopausal osteoporosis, Paget's disease, metastatic bone disease), and their measurement may serve as a monitoring tool rather than a therapeutic one.

9.5 Considerations in Fibrosis and Cancer

High LH2 levels cause stable collagen cross-link accumulations that promote fibrosis and cancer progression. In pathological states characterized by excessive fibrosis (pulmonary fibrosis, hepatic fibrosis, uterine fibroids, tumor stroma stiffening), the accumulation of hydroxylysine-derived crosslinks contributes to disease severity. The deposited collagen shows an increase in pyridinoline cross-links, which are derived from hydroxylated lysine residues within the telopeptides. This change in cross-linking is related to irreversible accumulation of collagen in fibrotic tissues. These considerations are mechanistic observations; they do not establish that dietary hydroxylysine supplementation would worsen fibrosis, but they highlight the importance of understanding hydroxylysine metabolism in the context of these conditions.

9.6 Genetic Disorders of Lysyl Hydroxylase Activity

Individuals with known or suspected PLOD1/PLOD2/PLOD3-related disorders represent a population in whom the endogenous machinery for hydroxylysine synthesis is impaired. In fibroblasts from patients with a duplication mutation in the LH gene (associated with EDS VI), administration of hydralazine (an iron-chelating agent) and ascorbate (a cofactor for LH activity) stimulates LH activity and its mRNA significantly. This suggests that in certain genotypic backgrounds, optimizing cofactor availability may partially rescue enzyme activity, though this remains investigational.

10. Summary of Evidence Landscape

Hydroxylysine occupies a complex position in the landscape of dietary supplements and nutritional science. It is an indisputably essential structural component — without it, collagen cannot achieve the crosslink density required for normal tensile strength in bone, skin, tendon, blood vessel walls, and other connective tissues. Its roles in glycosylation-mediated collagen maturation and crosslink formation are mechanistically well-established, supported by decades of structural biochemistry and validated by clinical syndromes of lysyl hydroxylase deficiency.

As a biomarker, particularly in its glycosylated forms (galactosyl hydroxylysine and glucosylgalactosyl hydroxylysine) in urine and serum, hydroxylysine has demonstrated utility in clinical research on bone resorption, osteoporosis, Paget's disease, and metabolic bone disease, supported by multiple human studies. However, its analytical complexity has limited widespread clinical adoption compared to later-generation automated assays.

As a dietary supplement ingredient, hydroxylysine lacks independent clinical trial evidence. Its delivery occurs indirectly via collagen hydrolysate products, but the pharmacological activities attributed to those products cannot be assigned specifically to hydroxylysine in isolation versus the many other bioactive peptides and amino acids present. The body's primary strategy for maintaining hydroxylysine in collagen relies on endogenous enzymatic synthesis, not direct dietary intake.

In the domain of fibrosis and cancer biology, emerging translational research identifies the hydroxylysine crosslinking pathway as a pathological driver — representing a potential therapeutic target for inhibition, not supplementation.

References

Health Conditions

Health conditions that Hydroxylysine may help support.

Connective Tissue HealthScientific
Hydroxylysine is a post-translationally modified amino acid unique to collagen, formed from lysine by vitamin C-dependent lysyl hydroxylase in procollagen. It serves as the attachment point for carbohydrate groups and as the precursor for pyridinoline cross-links that give connective tissue its mechanical strength. It is a biomarker of collagen synthesis and connective tissue turnover.

Body Systems

Body systems that Hydroxylysine may help support.

No body systems available.