Lactic Acid (DB04398): A Comprehensive Monograph on its Biochemical, Pharmacological, and Industrial Significance

Executive Summary

Lactic acid, a simple α-hydroxy carboxylic acid, occupies a remarkably complex and multifaceted position at the intersection of biochemistry, pharmacology, and industrial science. Long mischaracterized as a mere metabolic waste product of anaerobic respiration, contemporary research has fundamentally redefined its role as a central coordinator of whole-body metabolism, a dynamic energy substrate, and a potent signaling molecule. This monograph provides an exhaustive analysis of lactic acid (DrugBank ID: DB04398), examining its dual identity as both an essential endogenous metabolite and a versatile exogenous agent.

Biochemically, lactic acid is far from a metabolic dead end. It serves as a critical link between glycolytic and oxidative pathways through the "lactate shuttle" system, acting as a readily transportable fuel source for tissues such as the heart and brain.[1] Furthermore, it functions as a signaling molecule, or "lactormone," exerting immunomodulatory effects and influencing gene expression through specific receptors like the hydroxycarboxylic acid receptor 1 (HCA1).[2] The clinical significance of endogenous lactate is most profoundly observed in the context of lactic acidosis, a life-threatening state of metabolic derangement where lactate accumulation serves as a critical biomarker of severe tissue hypoperfusion and cellular dysfunction.[3]

The pharmacological applications of lactic acid are exceptionally diverse, with its therapeutic function being dictated entirely by its chemical form and pharmaceutical formulation. As the salt sodium lactate, it is administered intravenously as a systemic alkalizing agent to correct metabolic acidosis.[4] In topical dermatological preparations, it functions as a keratolytic and humectant, treating conditions like xerosis and serving as a key ingredient in cosmetic anti-aging and exfoliation products.[4] In its polymeric form, poly-L-lactic acid (PLLA), it is the active component of the biostimulatory dermal filler Sculptra®, which gradually restores facial volume by stimulating the body's own collagen production.[6] Most recently, it has been formulated with citric acid and potassium bitartrate into the non-hormonal, on-demand contraceptive gel Phexxi®, which acts by modulating vaginal pH to immobilize sperm.[7]

The safety profile of lactic acid is similarly context-dependent. While topical and controlled systemic administrations are generally well-tolerated, its accumulation in pathological states defines the severe clinical syndrome of lactic acidosis. Industrially, lactic acid is a cornerstone of both traditional and modern biotechnology. It is a widely used food preservative and acidulant (E270) and is the fundamental monomer for the production of polylactic acid (PLA), a leading biodegradable and biocompatible polymer that represents a sustainable alternative to petroleum-based plastics.[8] This report synthesizes the extensive body of knowledge on lactic acid, providing a nuanced understanding of this pivotal molecule's properties, functions, and applications.

Chemical Profile and Physical Properties

A thorough understanding of lactic acid's therapeutic and physiological functions begins with its fundamental chemical and physical characteristics. Its simple structure belies a stereochemical complexity that is critical to its biological activity and a set of physicochemical properties that enable its diverse applications.

Structure and Stereochemistry

Lactic acid is an organic acid with the chemical formula C3​H6​O3​.[4] Its systematic IUPAC name is 2-hydroxypropanoic acid, which describes a three-carbon propanoic acid backbone with a hydroxyl group (

−OH) attached to the alpha-carbon (the carbon atom adjacent to the carboxyl group, −COOH).[2] This bifunctional structure, containing both a carboxylic acid and an alcohol functional group, classifies it as an alpha-hydroxy acid (AHA) and is the basis for its chemical reactivity and biological roles.[2]

The most critical structural feature of lactic acid is its chirality. The alpha-carbon is an asymmetric center, as it is bonded to four different groups: a carboxyl group (−COOH), a hydroxyl group (−OH), a methyl group (−CH3​), and a hydrogen atom (−H).[11] This asymmetry gives rise to two distinct, non-superimposable mirror-image stereoisomers, or enantiomers [2]:

• L-(+)-lactic acid: Also known as (S)-lactic acid. This is the enantiomer predominantly produced and metabolized in animal tissues, including humans, during processes like glycolysis. It is sometimes referred to as "sarcolactic acid" from the Greek sarx for flesh.[2]
• D-(-)-lactic acid: Also known as (R)-lactic acid. This enantiomer is primarily produced by certain species of bacteria and is not significantly metabolized by mammals.[2]

An equimolar mixture of the L- and D-enantiomers is known as DL-lactic acid or racemic lactic acid.[2] The specific stereoisomer is of paramount importance in both biological systems and industrial applications, such as the production of the biopolymer poly-L-lactic acid.

Physicochemical Characteristics

Lactic acid's physical properties vary slightly between its racemic and enantiomerically pure forms. In its solid state, it appears as white or colorless to yellow crystals, while in its more common liquid state, it is a colorless to yellow, odorless, viscous, and syrupy liquid.[11] It is notably hygroscopic, readily absorbing moisture from the air.[2]

The acidity of lactic acid is a defining characteristic. With a pKa of approximately 3.86, it is a weak acid but is roughly ten times more acidic than acetic acid (pKa ~4.76).[2] This enhanced acidity is attributed to intramolecular hydrogen bonding between the alpha-hydroxyl group and the carboxylate group, which stabilizes the conjugate base (lactate) upon deprotonation.[2]

Key physicochemical data are summarized in Table 1.

Table 1: Key Chemical and Physical Properties of Lactic Acid

Property	Value	Source(s)
Molecular Formula	C3​H6​O3​	4
Average Molecular Weight	90.08 g/mol	4
Monoisotopic Mass	90.031694058 Da	4
Appearance	Colorless to yellow syrupy liquid or white crystalline solid	11
Melting Point	16-18 °C (DL-lactic acid)	2
	53 °C (L-lactic acid)	15
Boiling Point	122 °C at 15 mmHg	2
Solubility	Miscible with water, ethanol, glycerol	2
	Insoluble in chloroform, petroleum ether, carbon disulfide	15
Acidity (pKa)	~3.86	2
Specific Gravity	~1.21 (20/20)	18

Identification and Nomenclature

For regulatory, scientific, and commercial purposes, lactic acid is identified by numerous names and codes. The CAS Registry Number 50-21-5 specifically refers to the racemic mixture, DL-lactic acid.[10] Other enantiomer-specific CAS numbers also exist (e.g., 79-33-4 for L-lactic acid).[19]

• IUPAC Name: 2-hydroxypropanoic acid [10]
• Common Synonyms: Milk acid, acidum lacticum, (±)-2-hydroxypropanoic acid, 2-hydroxypropionic acid, lactovagan, tonsillosan [4]
• DrugBank Accession Number: DB04398 [4]
• CAS Number (Racemic): 50-21-5 [10]
• European Food Additive Code: E270 [4]
• UN Number (for transport): 3265 [15]

Endogenous Lactic Acid: A Central Role in Metabolism and Signaling

The perception of lactic acid within human physiology has undergone a profound transformation. Once dismissed as a toxic waste product responsible for muscle fatigue, it is now recognized as a pivotal metabolic fuel and a sophisticated signaling molecule that coordinates metabolic processes across the entire body. This modern understanding reframes lactate not as a metabolic dead-end but as a dynamic and essential intermediary.

Metabolic Pathways

Lactic acid, or more accurately its conjugate base lactate at physiological pH, is a central player in cellular energy metabolism, linking anaerobic and aerobic pathways.

Production and the Role in Anaerobic Glycolysis

Under conditions of high energy demand that outstrip oxygen supply, such as intense exercise, or in pathological states of tissue hypoxia like shock or sepsis, cells rely heavily on anaerobic glycolysis for rapid ATP production.[2] In this pathway, glucose is broken down to pyruvate. To sustain this high glycolytic flux, the cell must regenerate the oxidized coenzyme nicotinamide adenine dinucleotide (

NAD+), which is consumed during the conversion of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate. The enzyme lactate dehydrogenase (LDH) facilitates this by catalyzing the reduction of pyruvate to lactate, a reaction that simultaneously oxidizes NADH to NAD+.[2] This step is crucial; without the regeneration of

NAD+, glycolysis would halt, and rapid energy production would cease.[22] Thus, lactate production is not a sign of metabolic failure but a vital mechanism for sustaining energy generation under anaerobic conditions.

The Cori Cycle and Lactate as a Gluconeogenic Precursor

Once produced, lactate is not trapped within the muscle. It is transported out of the cells and into the bloodstream, from where it can be taken up by the liver. In the liver, lactate is converted back to pyruvate and then, through the process of gluconeogenesis, is used to synthesize new glucose.[21] This newly formed glucose can then be released back into the circulation to be used as fuel by other tissues, including the muscles from which the lactate originated. This inter-organ metabolic loop is known as the Cori cycle. It serves as a mechanism to recycle a valuable carbon source, maintain blood glucose levels during prolonged exertion, and shift part of the metabolic burden from the muscles to the liver.[21]

The Lactate Shuttle Concept and Direct Fuel Utilization

Beyond its role in the Cori cycle, lactate itself is a primary and often preferred energy substrate for many tissues. The "lactate shuttle" concept describes the transport of lactate between different cells and even between compartments within the same cell to serve as fuel.[1] For instance, lactate produced in highly glycolytic "fast-twitch" muscle fibers can be shuttled to adjacent "slow-twitch" oxidative fibers, where it is converted back to pyruvate and enters the tricarboxylic acid (TCA) cycle for complete oxidation. Similarly, tissues like the heart and brain readily take up lactate from the blood for use as an oxidative fuel.[1] During exercise, the brain's reliance on lactate for energy can increase from a baseline of 7% to as high as 25%.[1] This demonstrates that lactate is a key component of a sophisticated system for distributing energy throughout the body, bridging glycolysis and oxidative phosphorylation between different cellular and tissue compartments.[1]

Signaling and Regulatory Functions

The contemporary view of lactate extends beyond its metabolic roles to encompass its function as a signaling molecule, capable of influencing cellular processes and communication between tissues. This has led to the conceptualization of lactate as a "lactormone"—a hormone-like substance that coordinates systemic responses.

Receptor-Mediated Signaling

L-lactic acid is the primary endogenous agonist for the hydroxycarboxylic acid receptor 1 (HCA1), also known as GPR81.[2] HCA1 is a

Gi/o​-coupled G protein-coupled receptor (GPCR) expressed in various tissues, including adipocytes. The activation of this receptor by lactate can initiate downstream signaling cascades. One of the well-characterized effects of HCA1 activation is the inhibition of lipolysis in fat cells, which can have significant implications for whole-body fatty acid metabolism.[1] The existence of a specific receptor elevates lactate from a simple metabolite to a true signaling molecule, capable of eliciting specific cellular responses.

Immunomodulation

The role of lactate in the immune system is complex and highly context-dependent. In the microenvironment of tumors, chronic inflammation, or sepsis, high concentrations of lactate can exert a potent immunosuppressive effect.[1] Lactate has been shown to suppress the proliferation and cytokine production of cytotoxic T lymphocytes and to promote the differentiation of immunosuppressive cell types, such as regulatory T cells (Tregs) and M2-like tumor-associated macrophages (TAMs).[1] This function is particularly relevant in oncology, where the lactate-rich tumor microenvironment helps cancer cells evade immune surveillance.

Influence on Gene Expression

Lactate also acts as a signaling molecule that can influence the expression of genes involved in cellular adaptation. It has been shown to affect processes such as angiogenesis (the formation of new blood vessels) and mitochondrial biogenesis (the production of new mitochondria).[22] These adaptations are critical for improving endurance and the body's capacity to perform under conditions of high metabolic demand, representing another layer of lactate's role in coordinating physiological responses to exercise and stress.[22]

Comprehensive Pharmacology

The pharmacological profile of lactic acid is exceptionally broad, a direct consequence of its versatile chemical nature. A critical principle in understanding its pharmacology is that the formulation dictates the function. The same core molecule can act as a systemic metabolic regulator, a topical skin-resurfacing agent, a localized pH modulator, or a long-acting tissue stimulant, depending entirely on how it is prepared and administered.

Mechanisms of Action (Context-Dependent)

Systemic Alkalizing Agent (as Sodium Lactate)

When administered intravenously as sodium lactate, the lactate anion (CH3​CH(OH)COO−) serves as a metabolic precursor to bicarbonate (HCO3−​).[23] This conversion occurs primarily in the liver and, to a lesser extent, the kidneys. The metabolic process consumes hydrogen cations (

H+), ultimately leading to the formation of carbon dioxide and water.[4] The net effect is an increase in plasma bicarbonate concentration and a rise in blood pH, producing a systemic alkalinizing effect.[4] This mechanism is effective for correcting mild to moderate metabolic acidosis. However, it is dependent on intact cellular oxidative processes and is relatively slow, requiring one to two hours for the full conversion to occur.[23]

Topical Keratolytic and Humectant (as Lactic Acid)

When applied topically to the skin, lactic acid functions as a classic alpha-hydroxy acid (AHA). Its primary mechanism of action is keratolytic; it works by dissolving the bonds (corneodesmosomes) that hold corneocytes (dead skin cells) together in the stratum corneum, the outermost layer of the epidermis.[5] This action promotes the shedding of these dead cells, a process known as exfoliation, which results in smoother, brighter skin.[26] In addition to its keratolytic properties, lactic acid is a natural humectant. It is a key component of the skin's own natural moisturizing factor (NMF), a collection of water-soluble compounds that maintain hydration. Its structure allows it to bind water within the skin, thereby increasing hydration, improving skin barrier function, and promoting firmness.[27]

Vaginal pH Modulator (in Phexxi®)

As the primary active ingredient in the contraceptive gel Phexxi®, lactic acid, in combination with citric acid and potassium bitartrate, exerts its effect by modulating the vaginal environment. The gel is formulated to maintain an acidic vaginal pH (in the range of 3.5 to 4.5).[29] Normal vaginal pH is acidic due to endogenous lactic acid production by lactobacilli. However, the introduction of semen, which is alkaline, temporarily raises the vaginal pH. Phexxi®'s buffering capacity counteracts this change, keeping the environment acidic. This acidic milieu is hostile to sperm, causing a rapid reduction in their motility and viability, thereby preventing them from reaching and fertilizing an egg.[29] This mechanism is distinct from traditional spermicides that kill sperm via a detergent-like action.

Biostimulatory Agent (as Poly-L-Lactic Acid in Sculptra®)

In its polymeric form, poly-L-lactic acid (PLLA), the mechanism is entirely different. Sculptra® is an injectable suspension of PLLA microparticles. Following injection into the deep dermis or subcutaneous tissue, these microparticles elicit a controlled, subclinical inflammatory response.[6] Macrophages and fibroblasts are recruited to the site, where they begin to encapsulate and break down the PLLA particles through phagocytosis. This process stimulates the fibroblasts to synthesize and deposit new type I collagen.[6] Over a period of weeks to months, as the PLLA is gradually resorbed, a new collagen framework is built in its place. This results in a gradual and natural-looking restoration of tissue volume and improvement in skin structure, a mechanism known as biostimulation.[32]

Pharmacodynamics

The observable effects of lactic acid on the body are directly linked to its specific mechanism of action in each context.

• Systemic: The primary pharmacodynamic effect of intravenous sodium lactate is a dose-dependent increase in plasma bicarbonate concentration and blood pH, which serves to correct metabolic acidosis.[23] Excessive administration can overwhelm the body's buffering capacity, leading to the opposite effect: metabolic alkalosis.[24]
• Topical: The pharmacodynamic effects on the skin are multifaceted and concentration-dependent. Low concentrations (<10%) lead to increased hydration, improved skin texture, and a brighter complexion.[15] Higher concentrations (e.g., 12% and above) can stimulate an increase in the thickness of the dermis and epidermis, promoting firmness and reducing the appearance of fine lines and wrinkles.[35] It can also help fade hyperpigmentation by accelerating the turnover of pigmented surface cells.[27]
• Vaginal: The key pharmacodynamic effect is the immediate and sustained maintenance of a low vaginal pH upon coitus, resulting in the immobilization of sperm.[29]
• Intradermal (PLLA): The effect is not immediate. The pharmacodynamic response is a gradual, progressive increase in skin thickness and volume at the treatment site, which becomes noticeable over several weeks and continues to improve for several months as new collagen is deposited.[6] The peak effect is typically seen after a series of treatments.

Pharmacokinetics

The absorption, distribution, metabolism, and elimination (ADME) of lactic acid depend on the route of administration and its chemical form.

• Absorption: When administered intravenously, lactate is 100% bioavailable. Following topical application to the skin, absorption into the systemic circulation is generally limited, though this can increase if applied to large surface areas or compromised skin. Vaginal absorption of the components in Phexxi® is expected to be minimal, and systemic exposures are not anticipated to pose safety concerns.[36] PLLA microparticles from Sculptra® are not absorbed systemically; they remain localized at the injection site until they are fully metabolized.
• Distribution: Endogenous lactate is distributed throughout total body water. Normal plasma lactate concentrations are typically below 2 mmol/L (or 19.8 mg/dL) at rest but can increase substantially during strenuous exercise.[23]
• Metabolism: Lactate is a key metabolic intermediate. The liver is the primary site of lactate clearance, converting about 60% of it to glucose (via the Cori cycle) and oxidizing the remainder.[22] The kidneys also play a significant role, clearing about 30% of the lactate load. Other tissues, like the heart and skeletal muscle, can also take up and oxidize lactate directly for energy.[1] The metabolism of PLLA occurs locally at the injection site, where it is hydrolyzed into lactic acid monomers, which then enter the body's normal metabolic pathways.[6]
• Elimination: Under normal physiological conditions, the body's capacity to clear lactate is very high. The elimination half-life of an intravenously infused bolus of lactic acid in healthy, normoventilating individuals is short, ranging from approximately 4.5 to 12 minutes.[40] However, this clearance can be significantly impaired by factors that reduce hepatic or renal perfusion (e.g., shock), cause liver disease, or alter blood pH (e.g., respiratory alkalosis can decrease clearance by 40%).[39]

Therapeutic Applications and Clinical Evidence

The therapeutic utility of lactic acid is remarkably broad, spanning critical care medicine, dermatology, and reproductive health. Its applications are highly specific to its formulation and intended route of administration, ranging from life-sustaining intravenous fluids to cosmetic injectables.

Systemic Administration: Sodium Lactate and Lactated Ringer's in Intravenous Therapy

Lactated Ringer's solution, also known as Hartmann's solution, is a cornerstone of intravenous fluid therapy. This isotonic crystalloid solution contains sodium lactate along with sodium chloride, potassium chloride, and calcium chloride. Its primary applications include:

• Fluid and Electrolyte Resuscitation: It is widely used for restoring extracellular fluid volume after losses due to trauma, surgery, burns, or severe dehydration.[2]
• Treatment of Metabolic Acidosis: The sodium lactate component serves as a bicarbonate precursor, making the solution useful for the prevention or treatment of mild to moderate metabolic acidosis.[4] The metabolic conversion of lactate to bicarbonate helps to correct the acid-base imbalance.[23]

It is crucial to note that sodium lactate is not indicated for the treatment of severe acidosis that requires immediate correction, due to the 1-2 hour delay in its conversion to bicarbonate.[34] Furthermore, it is contraindicated in the management of lactic acidosis itself, as patients in this state have impaired lactate metabolism, and adding more lactate would be detrimental.[23]

Dosage and Administration: Sodium lactate is administered by intravenous infusion only after dilution. A common preparation involves diluting a concentrated 50 mEq vial to create a 1/6 molar isotonic solution.[24] The infusion rate and total volume are tailored to the individual patient's clinical condition, age, weight, and electrolyte status, with typical adult rates not exceeding 300 mL/hour.[34]

Dermatological and Topical Formulations

Lactic acid is a prominent ingredient in both prescription and over-the-counter (OTC) dermatological products due to its dual action as a keratolytic and a humectant.

• Approved Indications: Prescription-strength topical lactic acid (often 10-12%, sometimes formulated with ammonium hydroxide to form ammonium lactate) is indicated for the treatment of moderate to severe dry, scaly skin conditions, including xerosis (abnormally dry skin) and ichthyosis vulgaris (an inherited condition characterized by dry, thickened, scaly skin).[25] By exfoliating dead skin cells and increasing hydration, it helps to soften the skin, reduce scaling, and alleviate associated itching.[5]
• Off-label and Cosmetic Applications: The use of lactic acid in cosmetic skincare is widespread. It is favored for its efficacy and relatively mild profile compared to other AHAs like glycolic acid, making it suitable for sensitive skin.[35] Common applications include:
• Anti-Aging: Improving the appearance of fine lines and wrinkles.[27]
• Hyperpigmentation: Fading age spots, sun spots, and post-inflammatory hyperpigmentation from acne.[26]
• Acne: Helping to clear pores and reduce inflammatory lesions, often in low concentrations.[35]
• Chemical Peels: Used by dermatologists in higher concentrations (e.g., 30% or more) for professional skin resurfacing treatments.[15]

Table 2: Common Dermatological Formulations and Brand Names

Brand Name(s)	Active Ingredient(s)	Dosage Form	Primary Indication(s)
AmLactin®, Lactinol-E®, U-Lactin®	Lactic acid (often as ammonium lactate), Urea	Lotion, Cream	Xerosis (dry skin), Ichthyosis, Pityriasis rubra pilaris
CeraVe® SA Cream/Lotion for Rough & Bumpy Skin	Salicylic acid, Lactic acid, Ceramides	Cream, Lotion	Rough and bumpy skin, Keratosis pilaris
Phexxi®	Lactic acid, Citric acid, Potassium bitartrate	Vaginal Gel	Contraception
SLT®	Coal tar, Lactic acid, Salicylic acid	Topical	Not specified
Sculptra®	Poly-L-lactic acid	Injectable Suspension	Facial lipoatrophy, Wrinkle correction

Sources: [4]

Specialized Pharmaceutical Products: In-Depth Analysis

Beyond general dermatological use, lactic acid is a key component in highly specialized, innovative pharmaceutical products.

Phexxi® (Lactic acid/Citric acid/Potassium bitartrate)

Phexxi® represents a novel category of contraception: a non-hormonal, on-demand, prescription vaginal pH modulator.[7]

• Indication: For the prevention of pregnancy in females of reproductive potential.[37]
• Mechanism of Action: Phexxi works by maintaining the vagina's natural acidic pH in the presence of alkaline semen. Each 5-gram dose contains 90 mg of lactic acid, 50 mg of citric acid, and 20 mg of potassium bitartrate.[7] This formulation's buffering capacity prevents the transient pH rise that normally occurs after ejaculation, thereby immobilizing sperm and preventing fertilization.[29]
• Dosage and Administration: A single pre-filled applicator is administered vaginally immediately before or up to one hour before each act of intercourse. It is crucial that a new dose is applied for each subsequent act of intercourse, even if it occurs within the same hour.[29] The product is not effective if administered after intercourse.[7]
• Efficacy: In clinical trials, Phexxi demonstrated an efficacy rate of approximately 86% with typical use and 93% with perfect use.[30]
• Safety and Contraindications: The most common side effects are localized and include vulvovaginal burning, itching, and discomfort, which can also be experienced by male partners.[37] An increased incidence of urinary tract infections (UTIs), including cystitis and pyelonephritis, has been observed.[49] Consequently, Phexxi is contraindicated in women with a history of recurrent UTIs or known urinary tract abnormalities.[37]

Sculptra® (Poly-L-Lactic Acid - PLLA)

Sculptra® is not a traditional dermal filler but a biostimulatory injectable that provides gradual and long-lasting results.

• Indications: It is FDA-approved for two primary uses: 1) the correction of facial fat loss (lipoatrophy) in people with HIV, and 2) the correction of shallow to deep nasolabial fold contour deficiencies and other facial wrinkles in immunocompetent individuals.[6]
• Mechanism of Action: As a biostimulatory agent, PLLA microparticles stimulate the patient's own fibroblasts to produce new collagen, gradually rebuilding the skin's underlying structure and restoring volume.[6]
• Administration: Sculptra is a sterile dry powder that must be reconstituted with sterile water for injection (and optionally, lidocaine for anesthesia) to form a suspension.[31] It is injected into the deep dermis or subcutaneous layer by a trained healthcare professional. A treatment regimen typically consists of a series of 2-4 injection sessions spaced at least 3 weeks apart to achieve the desired effect.[31] The results are gradual, appearing over several months, and can last for two years or more.[32]
• Off-Label Uses: Due to its volumizing and skin-tightening effects, Sculptra is widely used off-label for non-facial rejuvenation, including augmenting the buttocks (the "Sculptra Butt Lift"), enhancing cheek and jawline contours, treating skin laxity on the neck and décolleté, and improving the appearance of cellulite.[6]
• Safety and Contraindications: It is contraindicated in patients with known hypersensitivity to any of its components or with a history of or susceptibility to keloid formation or hypertrophic scarring.[58] The most common side effects are injection site reactions like bruising, pain, swelling, and redness.[63] A key risk specific to PLLA is the delayed formation of subcutaneous papules and nodules (small lumps under the skin), which are typically non-visible but palpable. While most resolve spontaneously, some may require treatment.[58] Intravascular injection is a rare but serious adverse event associated with all dermal fillers, which can lead to vascular occlusion and tissue necrosis.[58]

Summary of Clinical Trial Data

Lactic acid, primarily as a component of balanced electrolyte solutions, has been investigated in various clinical settings. While it is a standard component of care, specific trials have evaluated its role in different contexts.

Table 3: Overview of Key Clinical Trials for Lactic Acid-Containing Products

ClinicalTrials.gov ID	Indication(s)	Product/Combination	Phase	Status	Key Focus/Findings
NCT03340805	Septic Shock (Pediatric)	Balanced fluid (containing lactate) vs. Normal Saline	Phase 1	Completed	A pilot feasibility study comparing fluid types in pediatric sepsis, where lactate-containing solutions are considered more physiologically balanced.
NCT05391607	Blood Loss During Surgery, Body Fluid Retention	Albumin with Lactated Ringer's solution	Phase 4	Completed	Compared hyperoncotic vs. isooncotic albumin to support blood loss replacement, using a lactate-containing solution as the crystalloid base.
NCT03764085	Sepsis, Intoxication	Rheosorbilact® (a solution containing lactate)	Phase 4	Completed	Evaluated the efficacy and safety of a lactate-containing infusion solution as part of a complex therapy for sepsis.

Sources: [65]

Safety Profile and Toxicology

The safety profile of lactic acid is highly dependent on its form, concentration, and route of administration. While topical applications and controlled intravenous infusions are generally safe, the endogenous accumulation of lactate leads to the severe and life-threatening condition of lactic acidosis.

Adverse Effects and Contraindications

Topical Administration

When used in dermatological products, the most common adverse effects are localized to the site of application. These are typically mild and transient, and include [5]:

• Burning or stinging sensations
• Itching (pruritus)
• Redness (erythema)
• Skin irritation and peeling

These effects are more likely to occur on sensitive skin, such as the face, or when applied to broken or irritated skin.[28] A significant precaution with all AHAs, including lactic acid, is photosensitivity. Lactic acid increases the skin's sensitivity to ultraviolet (UV) radiation, heightening the risk of sunburn. Patients using topical lactic acid products should be advised to minimize sun exposure and use broad-spectrum sunscreen.[28] The primary contraindication for topical use is a known hypersensitivity to lactic acid or any other ingredient in the formulation.[25]

Systemic Administration (IV Sodium Lactate)

The risks associated with intravenous sodium lactate are primarily related to improper administration, specifically overdosage or too-rapid infusion. Potential adverse effects include [24]:

• Fluid and Solute Overload: Leading to dilution of serum electrolytes, overhydration, congested states, or pulmonary edema.
• Hypernatremia: An excess of sodium in the blood, particularly risky for patients with congestive heart failure, renal insufficiency, or other sodium-retaining states.
• Metabolic Alkalosis: Excessive administration of lactate can overcorrect acidosis and lead to an alkaline shift in blood pH.

Systemic sodium lactate is contraindicated in patients with hypernatremia, fluid retention, or conditions associated with elevated lactate levels and impaired lactate utilization, such as shock, anoxia, or severe liver disease.[42] It is explicitly not for use in treating lactic acidosis.[42]

The Clinical Syndrome of Lactic Acidosis

Lactic acidosis is not a side effect of administered lactic acid but rather a severe pathological state of metabolic dysregulation characterized by the body's own overproduction and/or underutilization of lactate. It is a medical emergency and a powerful indicator of critical illness.

• Definition and Pathophysiology: Lactic acidosis is defined by a serum lactate concentration above 4 mmol/L accompanied by a blood pH below 7.35.[3] It represents a critical failure of cellular aerobic metabolism, forcing a massive shift to anaerobic glycolysis. This results in lactate production overwhelming the clearance capacity of the liver and kidneys, leading to systemic acidosis.[3] The condition has profound hemodynamic consequences and is associated with high mortality.[3]
• Etiology and Classification: The causes of lactic acidosis are broadly classified into two main types, as detailed in Table 4.

Table 4: Classification and Principal Causes of Lactic Acidosis

Acidosis Type	Mechanism	Specific Causes and Examples
Type A	Tissue Hypoxia / Hypoperfusion	Shock States: Septic, cardiogenic, hypovolemic, obstructive shock. Regional Ischemia: Mesenteric ischemia, limb ischemia. Other: Cardiopulmonary arrest, severe anemia, respiratory failure, carbon monoxide poisoning.
Type B1	Underlying Systemic Disease	Liver failure, renal failure, malignancy (especially leukemia and lymphoma), thiamine deficiency, inborn errors of metabolism.
Type B2	Drug- or Toxin-Induced	Metformin, propofol, linezolid, nucleoside reverse transcriptase inhibitors (NRTIs), cyanide, ethanol, methanol, ethylene glycol.
Type B3	Altered Lactate Metabolism	Seizures, strenuous exercise (usually transient).
D-Lactic Acidosis	Bacterial Overproduction of D-isomer	Short bowel syndrome, other forms of gastrointestinal malabsorption leading to bacterial overgrowth and fermentation of undigested carbohydrates.

Sources: [3]

• Symptoms and Management: The clinical presentation is often non-specific and reflects the underlying cause. Symptoms can include nausea, vomiting, rapid breathing (Kussmaul respirations), weakness, abdominal pain, and altered mental status.[39] Management is urgently focused on treating the root cause—for example, aggressive fluid resuscitation and vasopressors for shock, broad-spectrum antibiotics for sepsis, or discontinuing the offending drug. Bicarbonate therapy is controversial and generally reserved for cases of severe acidemia (pH < 7.1).[3]

Drug and Food Interactions

Transporter-Mediated Drug Interactions

Lactic acid's transport across cell membranes is not passive; it is mediated by a family of proton-linked monocarboxylate transporters (MCTs) and solute carrier organic anion transporters (OATs).[74] Lactic acid is both a substrate for and an inhibitor of several of these transporters, creating a significant potential for drug-drug interactions through competitive inhibition. For example, drugs that are also substrates for MCT1, such as salicylic acid, methotrexate, and valproic acid, may compete with lactate for transport, potentially altering their pharmacokinetics.[74] Conversely, inhibitors of these transporters, such as probenecid, can affect lactate clearance.[74] This is particularly relevant in the central nervous system and at the blood-brain barrier, where MCTs are highly expressed. A summary of these interactions is provided in Table 5.

Table 5: Lactic Acid Transporter Interactions and Associated Drugs

Transporter	Role of Lactic Acid	Interacting Drugs/Compounds	Potential Clinical Implication
Monocarboxylate Transporter 1 (MCT1)	Substrate, Inhibitor	Substrates: Salicylic acid, Valproic acid, Methotrexate, Niacin, Pravastatin, GHB. Inhibitors: Quercetin, Probenecid.	Competition for transport could alter drug distribution, particularly into the brain, or affect lactate clearance.
Monocarboxylate Transporter 2 (MCT2)	Substrate, Inhibitor	Substrates: Pyruvic acid. Inhibitors: GHB, Niflumic acid.	Primarily involved in neuronal lactate uptake; interactions could affect brain energy metabolism.
Solute Carrier Organic Anion Transporter 2B1 (OAT2B1)	Substrate, Inhibitor	Substrates: Atorvastatin, Pravastatin, Fexofenadine, Glyburide. Inhibitors: Gemfibrozil, Itraconazole.	Potential for altered pharmacokinetics of various statins, antihistamines, and antidiabetic drugs.
Solute Carrier Organic Anion Transporter 2A1 (OAT2A1)	Inhibitor	Substrates: Prostaglandins (Alprostadil, Dinoprostone), Latanoprost. Inhibitors: Furosemide, Rifamycin.	Inhibition by lactate could reduce the clearance of prostaglandins.

Source: [74]

A clinically critical drug interaction involves metformin. Metformin is a known, albeit rare, cause of Type B lactic acidosis, particularly in patients with renal impairment. Ribociclib, a CDK4/6 inhibitor used in cancer therapy, is an inhibitor of organic cation transporters (OCT1, OCT2) that are responsible for metformin's renal clearance.[75] Co-administration of these two drugs can lead to increased metformin plasma levels, precipitating severe and potentially fatal lactic acidosis.[75]

Food Interactions

Lactic acid is a natural component of many fermented foods, including yogurt, kefir, sauerkraut, kimchi, and sourdough bread.[77] This dietary intake is generally recognized as safe (GRAS) and contributes to the gut microbiome. There are no significant interactions between dietary lactic acid and medications, though consumption of hot beverages or foods can exacerbate the cutaneous flushing sometimes associated with niacin, which may be co-formulated with topical lactic acid products.[79]

Formal Toxicological Assessment

Formal toxicological studies have established a safety profile for lactic acid that distinguishes between the molecule itself and the biological systems that produce it.

• Acute Toxicity: Lactic acid exhibits moderate acute oral toxicity. The oral median lethal dose (LD50) in rats is reported as 3543-3730 mg/kg, and in mice as 4875 mg/kg.[80] The dermal LD50 in rabbits is greater than 2000 mg/kg, indicating low toxicity via skin exposure.[82]
• Carcinogenicity: Lactic acid is not considered carcinogenic. Animal studies have shown no evidence of carcinogenicity.[83] Studies using its salt, calcium lactate, as a surrogate in long-term feeding studies in rats also found no evidence of carcinogenic activity.[84]
• Mutagenicity: Lactic acid is not mutagenic. It has tested negative in standard genotoxicity assays, including the Ames test with Salmonella typhimurium.[85] In fact, a fascinating dichotomy exists where the molecule itself is non-mutagenic, but the biological systems that produce it—lactic acid bacteria (LAB)—can exhibit protective, anti-mutagenic properties. Studies have shown that milk cultured with various LAB strains can inhibit the mutagenicity of known chemical mutagens like N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and fecal mutagens.[87] This protective effect is thought to arise from the ability of the bacterial cells to bind to dietary carcinogens and mutagens in the gut, thereby reducing their bioavailability and preventing them from damaging host DNA.[89] This distinction is crucial: the safety of the chemical should not be conflated with the complex, often beneficial, biological activities of the microorganisms that produce it.
• Reproductive and Developmental Toxicity: Lactic acid is a natural and essential component of the female reproductive tract. A healthy vaginal microbiome is dominated by lactobacilli, which produce high levels of lactic acid, creating an acidic environment (pH < 4.5) that is protective against pathogens like Chlamydia trachomatis and those that cause bacterial vaginosis (BV).[93] Disruptions in this acidic environment are linked to adverse reproductive outcomes, including increased risk of STIs, pelvic inflammatory disease, and preterm birth.[93] Given its natural occurrence and essential role in metabolism, the U.S. Environmental Protection Agency (EPA) waived the requirement for developmental toxicity studies for certain uses.[97] While one source noted potential adverse reproductive effects in animal tests, no corroborating human data was found.[81]

Considerations in Specific Populations

The physiological role and therapeutic use of lactic acid require special consideration in distinct patient populations, including pregnant and lactating individuals, children, and the elderly.

Pregnancy and Lactation

• Physiological Context: During early pregnancy, the placenta and developing embryo exhibit high rates of aerobic glycolysis (the Warburg effect), leading to significant local production of lactic acid. This lactate-rich microenvironment at the maternal-fetal interface is thought to play a crucial role in modulating maternal immune tolerance and promoting angiogenesis, similar to its function in the tumor microenvironment.[98] During labor, uterine contractions can cause a physiological rise in serum lactate levels. However, studies confirm that serum lactate remains a valid and important biomarker for diagnosing sepsis in pregnant patients, as septic individuals exhibit significantly higher levels independent of labor duration.[99]
• Therapeutic Use:
• Systemic: Intravenous solutions containing sodium lactate, such as Compound Sodium Lactate (Hartmann's solution), are considered safe for use during pregnancy and lactation for fluid and electrolyte replacement, provided that the patient's fluid balance and electrolyte levels are carefully monitored.[100]
• Topical: The use of topical lactic acid products is generally considered safe during breastfeeding.[102] Given its natural presence in the body and limited systemic absorption from skin application, risks to the infant are negligible.
• Phexxi®: As a contraceptive, Phexxi® has no indication for use during pregnancy and should be discontinued if pregnancy occurs.[37] There are no data on its presence in human milk, but because its action is local and systemic absorption is expected to be minimal, it is considered a potential option for breastfeeding individuals seeking non-hormonal contraception.[36]
• Sculptra®: The safety of poly-L-lactic acid injections has not been evaluated in pregnant or lactating individuals. Therefore, its use is not recommended in these populations.[58]

Pediatric Use

• As a Biomarker: Lactate measurement is an essential diagnostic tool in pediatric critical care. Elevated lactate levels are a key indicator of tissue hypoperfusion and metabolic distress in conditions such as sepsis, trauma, and hypoxic-ischemic injury.[104] Serial lactate measurements and lactate clearance rates are valuable prognostic indicators in critically ill children. It is important to use age-specific reference ranges for lactate, as normal levels are higher in infants compared to older children.[38]
• Congenital Lactic Acidosis: This term refers to a group of rare, inherited metabolic disorders affecting the enzymes of pyruvate metabolism or the mitochondrial respiratory chain. These disorders typically present in infancy or early childhood with symptoms such as hypotonia, lethargy, seizures, and developmental delay, accompanied by persistently high lactate levels.[105]
• Topical Use: Topical formulations containing lactic acid (often 10-12% as ammonium lactate) are used in pediatric patients to treat dry, scaly skin conditions like xerosis and ichthyosis.[25] Care must be taken to avoid application on open wounds, irritated skin, or sensitive areas like the face, as it can cause stinging and irritation.[107] While widely used, some prescription products note that safety and effectiveness in pediatric patients have not been formally established.[28]

Geriatric Use

• As a Biomarker: Serum lactate is an especially powerful prognostic tool in geriatric trauma patients. Elevated lactate levels (>2.5 mmol/L) are strongly associated with increased mortality, even in patients who present with normal vital signs (e.g., normal blood pressure).[109] This phenomenon, termed "occult hypoperfusion," highlights the diminished physiological reserve in older adults, who may not mount a typical tachycardic or hypotensive response to severe injury. An elevated lactate level in this population should be considered a critical warning sign warranting aggressive monitoring and intervention.[109]
• Physiological Changes: The aging process is associated with changes in skeletal muscle composition, including a shift towards type I muscle fibers, and a potential decline in aerobic enzyme activity. These changes may alter lactate metabolism during exercise, potentially lowering the maximal lactate steady-state (MLSS) exercise intensity in older individuals.[110]
• Systemic Use: When administering intravenous sodium lactate to elderly patients, dosing should be initiated cautiously. The higher prevalence of decreased cardiac, renal, and hepatic function in this population increases the risk of fluid overload, hypernatremia, and impaired lactate clearance.[24]
• Probiotic Applications: There is emerging research interest in the use of lactic acid bacteria (LAB) as probiotics to promote healthy aging. Proposed benefits include restoring a healthy gut microbiome, enhancing antioxidant defenses, mitigating chronic inflammation, and potentially improving cognitive function, though more extensive human trials are needed.[112]

Industrial and Commercial Landscape

Lactic acid is a major platform chemical in the bio-economy, with a vast and growing market driven by its utility in the food, pharmaceutical, cosmetic, and biopolymer industries. Its production has shifted predominantly towards sustainable, fermentation-based methods.

Manufacturing and Synthesis

There are two primary routes for the industrial production of lactic acid: microbial fermentation and chemical synthesis.

• Microbial Fermentation: This is the dominant manufacturing method, accounting for 70-90% of global production.[9] The process utilizes microorganisms, primarily lactic acid bacteria (LAB) from genera such as Lactobacillus, Lactococcus, and Bifidobacterium, or fungi like Rhizopus, to ferment carbohydrate-rich feedstocks.[115] Common substrates include corn starch, sugarcane or beet molasses, and whey.[118] The key advantage of fermentation is its ability to produce optically pure stereoisomers—either L(+)- or D(-)-lactic acid—by selecting specific microbial strains, which is critical for applications like PLA synthesis.[9] The downstream process typically involves neutralizing the fermentation broth with calcium hydroxide or calcium carbonate to form calcium lactate, which precipitates out. The calcium lactate is then filtered, re-acidified with sulfuric acid to yield free lactic acid and a gypsum ( CaSO4​) byproduct, and finally purified through steps like evaporation and distillation.[120]
• Chemical Synthesis: The traditional chemical route involves the hydrolysis of lactonitrile, which is produced from the reaction of acetaldehyde (a petrochemical derivative) and hydrogen cyanide.[2] This method invariably produces a racemic (50:50) mixture of D- and L-lactic acid, which is unsuitable for applications requiring stereochemical purity.[9] Due to its reliance on fossil fuel precursors, higher costs, and lack of stereocontrol, chemical synthesis has been largely supplanted by fermentation.[116]

Commercial Applications

Lactic acid's bifunctional nature makes it an exceptionally versatile chemical with a wide range of commercial uses.

Food and Beverage Industry

This sector is a major consumer of lactic acid, where it is designated as food additive E270. Its functions are diverse [8]:

• Preservative and Antimicrobial Agent: It lowers the pH of foods, inhibiting the growth of spoilage and pathogenic bacteria, thereby extending shelf life.[77] It is used as a decontaminant in meat processing.[2]
• Acidulant and Flavor Enhancer: It imparts a mild, pleasant sour taste compared to other food acids, making it ideal for use in dairy products (yogurt, cheese), pickled vegetables (sauerkraut, kimchi, olives), sourdough bread, soft drinks, and confectionery.[2]
• pH Regulator and Curing Agent: It is used to control acidity during the production of cheese, beer, and wine (malolactic fermentation) and acts as a curing agent for products like salami.[2]

Biopolymers (Polylactic Acid - PLA)

The rapidly growing demand for sustainable materials has made PLA one of the most important applications of lactic acid.

• Production: PLA is a biodegradable and biocompatible thermoplastic polyester synthesized from lactic acid.[9] The industrial process typically involves the dimerization of lactic acid to form a cyclic intermediate called lactide, followed by ring-opening polymerization to produce high-molecular-weight PLA.[2]
• Properties and Advantages: As a bio-based polymer derived from renewable resources like corn, PLA offers significant environmental benefits. Its production consumes less energy and generates fewer greenhouse gas emissions compared to petroleum-based plastics like PET.[127] It is compostable under industrial composting conditions.[129]
• Applications: PLA is used extensively in applications where biodegradability is valued, including food packaging (containers, cups, films), disposable serviceware, agricultural films, and textiles.[127] Its biocompatibility also makes it a premier material for medical applications, such as resorbable sutures, orthopedic implants, drug delivery systems, and tissue engineering scaffolds.[2] It is also a very popular material for 3D printing filaments.[127]

Other Industrial and Pharmaceutical Uses

• Cosmetics and Personal Care: Used as a pH adjuster and for its exfoliating and moisturizing properties in a vast range of skincare products.[2]
• Pharmaceuticals: Employed to create water-soluble lactate salts from otherwise insoluble active pharmaceutical ingredients. It also serves as an excipient and has disinfectant properties.[2]
• Textile and Leather Industries: Used as a mordant (dye-fixing agent) in textile dyeing and for decalcifying hides in leather tanning.[13]
• Cleaning Products: Acts as a biodegradable and effective descaling agent for removing mineral deposits like limescale from industrial equipment and household appliances.[2]

Conclusion and Future Perspectives

Lactic acid (DB04398) is a molecule of profound duality. Its significance is not inherent in its simple chemical structure but is defined entirely by its context. Physiologically, it is a central hub of energy metabolism, not a waste product. Pathologically, its accumulation is a grave indicator of cellular distress. Pharmacologically, its function is dictated by its formulation, enabling it to be a systemic alkalizer, a topical exfoliant, a contraceptive agent, or a collagen stimulator. Industrially, it is both a traditional food preservative and a building block for the future of sustainable materials.

The synthesis of the available evidence reveals several key themes. First, the paradigm shift in understanding lactate's endogenous role from a metabolic culprit to a "lactormone"—a vital fuel and signaling molecule—is complete. This re-evaluation has critical implications for sports medicine, critical care, and our understanding of metabolic regulation. Second, the versatility of lactic acid as a therapeutic agent underscores the power of pharmaceutical formulation. The ability to transform the same core molecule into products as disparate as Lactated Ringer's, AmLactin®, Phexxi®, and Sculptra® is a testament to the sophistication of modern drug delivery and material science. Third, a nuanced view of its toxicology is essential, distinguishing between the benign nature of the molecule itself, the life-threatening syndrome of its endogenous accumulation (lactic acidosis), and the potentially protective, anti-mutagenic effects of the lactic acid bacteria that produce it in the gut.

Looking forward, several promising avenues for research and development emerge:

• Therapeutics: The role of lactate as a signaling molecule via the HCA1 receptor is an area ripe for exploration. Developing selective agonists or antagonists for this receptor could yield novel therapies for metabolic disorders, such as dyslipidemia, or inflammatory conditions. In regenerative medicine, further refinement of PLLA technology could lead to new applications in soft tissue augmentation and advanced drug delivery systems.
• Clinical Diagnostics: While lactate is an established biomarker, future research should focus on the clinical utility of lactate dynamics, such as clearance rates, particularly in refining prognostic models for pediatric and geriatric patients in critical care settings. This could enable earlier and more targeted interventions.
• Industrial Biotechnology: The future of lactic acid and PLA production lies in enhancing sustainability. A key goal is the development of more robust microbial strains and cost-effective enzymatic hydrolysis processes that can efficiently convert non-food, lignocellulosic biomass (e.g., agricultural waste) into lactic acid. This would decouple PLA production from food crops, lower its cost, and solidify its position as a truly green alternative to petroleum-based plastics.[115]
• Probiotics and Nutrition: The evidence suggesting anti-aging and anti-carcinogenic properties of specific lactic acid bacteria is compelling but requires validation in large-scale, rigorous human clinical trials. Identifying specific strains and elucidating their mechanisms of action could lead to the development of next-generation probiotics and functional foods aimed at promoting healthy longevity and preventing chronic disease.

In conclusion, lactic acid is a molecule that defies simple categorization. It is a fundamental component of life, a versatile tool of medicine, and a key enabler of a more sustainable industrial future. Continued investigation into its complex roles promises to yield further innovations across a remarkable spectrum of scientific and clinical fields.

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