Cinnamic acid

Cinnamic Acid: A Comprehensive Reference

1. Identity, Chemical Characterization, and Natural Sources

Chemical Identity

Cinnamic acid, also known as 3-phenylpropenoic acid, is an organic compound with the chemical formula C₆H₅CH=CHCOOH that is found in nature and commercially available. It is an aromatic carboxylic acid characterized by a benzene ring conjugated with a propenoic acid side chain. Its IUPAC chemical name is 2-propenoic acid, 3-phenyl-, and it carries the molecular formula C₉H₈O₂ with a molecular weight of 148.16 g/mol.

It exists in two isomeric forms: cis and trans. The trans isomer is more stable and is the predominant form found naturally and commercially. Both cis- and trans-configurations of cinnamic acids are known to exist in nature, although the cinnamic acids in chlorogenic acid are predominantly in the trans-configuration.

Its structure is composed of a benzene ring, an alkene double bond, and an acrylic acid functional group, making it possible to modify the aforementioned functionalities with a variety of compounds resulting in bioactive agents with enhanced efficacy.

Botanical Sources

Cinnamic acid is an organic acid naturally occurring in plants of the Cinnamomum genus. Its documented botanical sources include the bark of Cinnamomum cassia Presl (family Lauraceae), the velamen of Lycium chinense Mill, leaves of Foeniculum vulgare Mill (family Umbelliferae), and the resin of Myroxylon pereira (Royle) Klotzsch (family Leguminosae).

Cinnamic acids are also readily available from coffee beans, tea, maté, cocoa, apples and pears, berries, citrus, grape, brassica vegetables, spinach, beetroot, artichoke, potato, tomato, celery, faba beans, and cereals. They are widely distributed as organic conjugates in a variety of plant materials and are rarely found as free acids in unprocessed plant material. Besides cinnamic acid itself, the most common members of the cinnamic acid family are caffeic acid (3,4-dihydroxycinnamic acid), ferulic acid (3-methoxy-4-hydroxycinnamic acid), sinapic acid (3,5-dimethoxy-4-hydroxycinnamic acid), and p-coumaric acid (p-hydroxycinnamic acid).

Key Derivatives

Cinnamic acid serves as the fundamental scaffold for numerous derivatives, including methoxycinnamic acid, hydroxycinnamic acid, and caffeic acid phenethyl ester (CAPE), each bearing specific substituents on the aromatic ring or modifications on the aliphatic chain. The nature of the substituents incorporated into cinnamic acid has been found to play a huge role in either enhancing or decreasing the biological efficacy of the synthesized cinnamic acid derivatives.

Discovery and Isolation History

In 1780, cinnamic acid was isolated as crystals from cinnamon oil by Trommsdorf, who mistook it for benzoic acid. Later, in 1835, Dumas and Péligot correctly identified it. Bertagnini successfully synthesized cinnamic acid from benzaldehyde and acetyl chloride in 1856.

Common Forms and Preparations

Commercial synthesis of cinnamic acid almost always results in the trans isomer. The Perkin reaction is the traditional commercial method for producing cinnamic acid, achieved by condensation of benzaldehyde with acetic anhydride in the presence of a catalyst, typically sodium acetate; other catalysts such as potassium acetate, tertiary amines, potassium phosphate, and trimethyl borate can also be used. The compound is available in several forms relevant to dietary supplement and research contexts, including pure crystalline powder (the trans isomer), standardized plant extracts of cinnamon bark, encapsulated supplements, and topical preparations such as esters used in cosmetics and sunscreens.

2. Biosynthesis in Plants

The enzyme phenylalanine ammonia lyase (PAL; EC 4.3.1.24) catalyzes the conversion of L-phenylalanine to ammonia and trans-cinnamic acid. PAL is the first and committed step in the phenylpropanoid pathway and is therefore involved in the biosynthesis of polyphenol compounds such as flavonoids, phenylpropanoids, and lignin in plants. Phenylalanine ammonia lyase is found widely in plants, as well as in some bacteria, yeast, and fungi, with isoenzymes existing within many different species.

The enzymatic reaction produces cinnamic acid, which acts as a precursor of many secondary substances including lignins, flavonoid pigments, UV protectants, plant hormones, and phytoalexins; it is also a major source of total carbon in plants. Following PAL action, cinnamate 4-hydroxylase (C4H), a cytochrome P450-dependent monooxygenase (CYP73), catalyzes the conversion of cinnamic acid to p-coumaric acid.

The activity of PAL is induced dramatically in response to various stimuli such as tissue wounding, pathogenic attack, light, low temperatures, and hormones.

3. Traditional and Historical Use

Traditional Chinese Medicine

Cinnamic acid, as an organic acid naturally occurring in plants of the Cinnamomum genus, has been highly valued for its medicinal properties in numerous ancient Chinese texts. In traditional medicine, cinnamon is an important medicinal herb for warming and tonifying yang, with its clinical applications primarily manifested in three main aspects: alleviating cold-induced pain in the lower back and knees, dysmenorrhea, and cold-induced abdominal pain by warming the meridians and promoting blood circulation; treating symptoms of yang deficiency such as dizziness, ocular redness, and an imbalance of upper-body heat with lower-body cold; and relieving cold-induced chest and abdominal pain. It is important to note that in these traditions, the whole herb or bark was employed, and cinnamic acid as an isolated constituent was not identified or named by early practitioners.

Traditional Use of Source Plants Across Cultures

The term "cinnamic" derives from the spice cinnamon (Cinnamomum zeylanicum), which has been used since antiquity as a flavoring agent and for its stimulant, carminative, antiseptic, and insecticide properties. Antimicrobial natural preparations involving cinnamon, storax, and propolis have been long used topically for treating infections. Cinnamic acids and related molecules are partly responsible for the therapeutic effects observed in these preparations.

The bark of several species of Cinnamomum contains considerable amounts of (E)-cinnamaldehyde, a volatile aldehyde responsible for the pungent, sweet, and hot flavor of cinnamon. Storax resin from Liquidambar species — used in ancient Greek perfumery and early Middle Eastern incense — also yielded cinnamic acid derivatives, illustrating the historic overlap between fragrance, medicine, and food in pre-modern traditions.

4. Key Constituents, Active Compounds, and Established Mechanisms of Action

Structural Basis of Bioactivity

The structure of cinnamic acid includes a benzene ring, an alkene double bond, and an acrylic acid functional group. The biological and pharmacological properties of cinnamic acid itself and its derivatives may result from the presence of the α,β-unsaturated bond, which is responsible for their potential therapeutic effect in cancer treatment. The nature of the substituents incorporated into cinnamic acid has been found to play a huge role in either enhancing or decreasing the biological efficacy of the synthesized cinnamic acid derivatives.

Antimicrobial Mechanisms

Cinnamic acid has been shown to be effective against a variety of pathogens such as Staphylococcus aureus, Pseudomonas aeruginosa, and foodborne Pseudomonas species. It acts by disrupting cell membranes, inhibiting ATPase activity, and preventing biofilm formation, thereby demonstrating its ability to act as a natural antimicrobial agent.

Anti-inflammatory Mechanisms

In terms of anti-inflammatory properties, cinnamic acid and its derivatives have been shown to inhibit the activation of Toll-like receptor 4 (TLR4) and nuclear factor kappa B (NF-κB) signaling pathways, leading to reduced production of pro-inflammatory cytokines such as TNF-α and IL-6. Its anti-inflammatory properties are further demonstrated by improving oxidative stress and reducing inflammatory cell infiltration.

Antidiabetic Mechanisms

Based on several in vitro studies and animal models, cinnamic acid and its derivatives act on different mechanisms of action, including stimulation of insulin secretion, improvement of pancreatic β-cell functionality, inhibition of hepatic gluconeogenesis, enhanced glucose uptake, increased insulin signaling pathway activity, delay of carbohydrate digestion and glucose absorption, and inhibition of protein glycation and insulin fibrillation. In cell-based studies, cinnamic acid has been shown to ameliorate hyperglycemia-induced complications by lowering inflammation through inhibiting the expression of IL-6, NF-κB, and DPP4.

Antitumor Mechanisms

Cinnamic acid and its derivatives have antitumor effects, which encompass inhibiting cell proliferation, arresting the cell cycle, inducing apoptosis, limiting cell migration and invasion, and suppressing angiogenesis.

Neuroprotective Mechanisms

Research has identified that cinnamic acid activates PPARα (peroxisome proliferator-activated receptor alpha) to stimulate lysosomal biogenesis. Studies have reported that cinnamic acid exhibits antioxidant, antimicrobial, anticancer, neuroprotective, anti-inflammatory, and antidiabetic properties. Cinnamic acid has a cytoprotective effect on nerves in neurodegenerative diseases and an anti-inflammatory effect.

Antioxidant Mechanisms

Cinnamic acids have been identified as interesting compounds with antioxidant, anti-inflammatory, and cytotoxic properties. Particularly, p-coumaric acid or 4-hydroxy-trans-cinnamic acid presents antioxidant activity involving direct scavenging of reactive oxygen species (ROS) by minimizing the oxidation of low-density lipoprotein (LDL).

5. Scientific Evidence by Area of Use

5.1 Metabolic Health and Diabetes

The antidiabetic potential of cinnamic acid is the area with the largest body of preclinical evidence. Cinnamic acid and its derivatives occur naturally in high levels of plant-based foods, and among various biological activities, they are associated with a beneficial influence on diabetes and its complications.

Animal studies: Administration of cinnamic acid to diabetic rats at doses of 5 and 10 mg/kg of body weight improved glucose tolerance in a dose-dependent manner. The results obtained from the administration of 10 mg/kg cinnamic acid were comparable to that of glibenclamide (5 mg/kg) used as a standard drug.

Although the antidiabetic activity of cinnamic acid, a pure compound from cinnamon, has been reported, its mechanism(s) were not initially clear. Studies were designed to explore the possible mechanisms of antidiabetic activity in both in vitro and in vivo non-obese type 2 diabetic rat models. Such work concluded that cinnamic acid exerts antidiabetic activity by improving glucose tolerance in vivo and stimulating insulin secretion in vitro.

In both fasting and glucose-loading conditions, p-methoxycinnamic acid (10–100 mg/kg) lowered plasma glucose concentration with concomitant increased plasma insulin concentration in both normal and streptozotocin (STZ)-induced diabetic rats. The overall findings suggest that p-methoxycinnamic acid improves glucose tolerance without hypoglycemia, which may be beneficial to diabetic conditions that have defects in the response of insulin secretion to glucose stimulation.

In vitro studies: In TNF-α-induced insulin-resistant hepatocytes, cinnamic acid stimulated glucose uptake and alleviation of insulin resistance through promotion of insulin receptor tyrosyl phosphorylation and up-regulation of insulin signal-associated protein expression. Although there were high levels of cinnamic acid in aqueous cinnamon extract, cinnamic acid alone did not demonstrate any effect on glucose production in hepatoma cells, whereas procyanidin B2 and cinnamtannin B1 showed modest but reproducible inhibition of glucose production. This highlights that within complex extracts, different constituents contribute differentially to observed effects.

Cell-based (HepG2) studies: In HepG2 cells, cinnamic acid increased catalase and glutathione peroxidase activity and GSH production in a dose-dependent manner in the presence of high glucose concentrations, with the greatest effect seen at a concentration of 75 mg/ml. A major limitation of this line of research is that studies were conducted as in vitro assays, and the antidiabetic mechanism must be further investigated in human circumstances.

Evidence strength: Due to the limited intestinal absorption resulting from low bioavailability of cinnamic acid and its derivatives, further human clinical studies are needed to clarify the effects of cinnamic acid and its derivatives in diabetic patients. The evidence base for cinnamic acid specifically in humans remains preliminary; existing data are drawn primarily from cell cultures and rodent models.

5.2 Antimicrobial Activity

Most of the cinnamic acids, their esters, amides, aldehydes, and alcohols show significant growth inhibition against one or several bacterial and fungal species. Cinnamic acid and its derivatives demonstrate varying levels of antibacterial activity against Staphylococcus aureus, Pseudomonas aeruginosa, foodborne Pseudomonas species, Candida albicans, and other fungal strains.

Cinnamic acid acts by disrupting cell membranes, inhibiting ATPase activity, and preventing biofilm formation, thereby demonstrating its ability to act as a natural antimicrobial agent. Synthetic derivatives of cinnamic acid are often more effective in vitro than parent compounds due to stronger biological activities.

Evidence strength: The antimicrobial evidence for cinnamic acid itself is predominantly in vitro. There are no robust human clinical trials evaluating isolated cinnamic acid as an antimicrobial treatment. Evidence must be considered preliminary and preclinical.

5.3 Anti-inflammatory Activity

Through systematic integration of existing knowledge, it has been revealed that cinnamic acid has a wide range of pharmacological activities, including anti-tumor, antibacterial, anti-inflammatory, antidepressant, and hypoglycemic effects. In mechanistic terms, inhibition of NF-κB and TLR4 pathways, with downstream reduction of pro-inflammatory cytokines including TNF-α and IL-6, has been observed in cell-based studies.

Cinnamic acid has a cytoprotective effect on nerves in neurodegenerative diseases and an anti-inflammatory effect at 200 mg/kg/day in animal model studies. No controlled clinical trials in humans specifically using isolated cinnamic acid for inflammatory endpoints have been published in the reviewed literature, and evidence remains preclinical.

5.4 Anticancer Activity

Numerous studies have shown that cinnamic acid and its derivatives exhibit therapeutic effects against various types of cancer, including breast cancer, colon cancer, lung adenocarcinoma, prostate cancer, and chronic myelogenous leukemia. These findings are from in vitro and animal-model experiments. Some of the derivatives have been reported to be more effective when compared to standard drugs used to treat chronic or infectious diseases in vitro, making them very promising therapeutic agents.

Evidence strength: All anticancer evidence for cinnamic acid to date comes from cell-culture models and animal experiments. No human clinical trials have evaluated cinnamic acid as a standalone anticancer agent. The evidence is preliminary and cannot be extrapolated to human therapeutic use without further research.

5.5 Neuroprotective Activity

Cinnamic acid has been shown to activate PPARα to stimulate lysosomal biogenesis and lower amyloid plaque pathology in an Alzheimer's disease mouse model. Emerging application areas include antidepressant, neuroprotective, and other neurological effects.

Evidence strength: Neuroprotective findings for cinnamic acid are based exclusively on in vitro and animal (murine) models, including the Alzheimer's disease mouse model. No human or clinical trial data are available; the evidence is very preliminary.

5.6 Cardiovascular and Antihypertensive Activity

In animal studies, cinnamic acid significantly reduces body weight in obese rats and acts as an antihypertensive agent by inhibiting angiotensin serum converting enzyme (ACE) activity at 30 mg/kg/day. It also shows a cardioprotective profile by preventing vasoconstriction and reducing the risk of complications of hypertension. These findings emerge from animal and cell-based research; human clinical confirmation is lacking.

5.7 Antioxidant Activity

Cinnamic acids have been identified as interesting compounds with antioxidant, anti-inflammatory, and cytotoxic properties. Antioxidant properties have been evaluated in multiple in vitro assays, demonstrating free-radical scavenging activity. In hyperglycemia models, cinnamic acid increased catalase and glutathione peroxidase activity in a dose-dependent manner. Evidence for meaningful antioxidant effects in humans from cinnamic acid as an isolated supplement is not established.

6. Pharmacokinetics, Bioavailability, and Metabolism

Cinnamic acid is a compound of low toxicity which is converted in the mammalian body primarily to hippuric acid. The metabolism of [¹⁴C/phenyl-²H₅] cinnamic acid has been investigated in rats and mice at a dose level of 2.5 mmol/kg body weight. Recoveries of the ¹⁴C dose were between 92 and 98%, with most (82–90%) present in the 0–24-hour urine samples. Urinary metabolites were identified by chromatographic properties and mass spectra. In both species the major metabolite was hippuric acid, which is also an endogenous urinary component.

Several minor metabolites — 3-hydroxy-3-phenylpropionic acid, benzoic acid, and benzoyl glucuronide — were found in both species. Two additional metabolites, acetophenone and cinnamoylglycine (the glycine conjugate of cinnamic acid), could be positively identified only in mouse urine.

Following an oral load of sodium cinnamate in human subjects, there is an increase in urinary hippuric acid largely due to the excretion of labeled hippuric acid. No major difference in rate of elimination was found between healthy subjects and those with phenylketonuria, although the amount of cinnamic acid converted was less in those with phenylketonuria, possibly reflecting reduced first-pass absorption by the liver.

In rat pharmacokinetic studies, cinnamic acid was quickly absorbed and then metabolized mainly into hippuric acid. The area under the curve (AUC) of cinnamic acid was higher when administered as part of a Ramulus Cinnamomi decoction compared to pure cinnamic acid, and the bioavailability of cinnamic acid from the decoction was higher than from the pure compound.

The stomach, intestine, and liver are the principal organs involved in the biological conversion of cinnamic acid, which is possibly related to the low bioavailability of oral administration. Due to the limited intestinal absorption resulting from low bioavailability of cinnamic acid and its derivatives, current improvement efforts involve entrapping the compounds into solid and liquid particles.

A secondary peak (C_max) of plasma cinnamic acid has been observed in human subjects following consumption of polyphenol-rich curry containing mixed spices. A similar secondary peak was observed in rats with oral administration of cinnamon preparations. Several reasons could account for this observation, including enterohepatic recycling, site-specific absorption, or delayed gastric emptying.

7. Dosage Forms and Reported Study Dosages

The following dosages reflect only those reported in the cited peer-reviewed sources; they describe experimental conditions and do not constitute recommendations.

• In diabetic rats, cinnamic acid was administered at doses of 5 mg/kg and 10 mg/kg of body weight to improve glucose tolerance in a dose-dependent manner; the 10 mg/kg dose produced results comparable to the standard drug glibenclamide at 5 mg/kg.
• In STZ-induced diabetic rat studies, p-methoxycinnamic acid was administered at 10–100 mg/kg to lower plasma glucose and raise plasma insulin concentrations.
• Anti-inflammatory effects in animal studies were noted at 200 mg/kg/day.
• Antihypertensive effects, including ACE inhibitory activity, have been studied in animals at 30 mg/kg/day.
• Antidiabetic activity by improving glucose tolerance in vivo was studied at 10 mg/kg/day in animal models.
• In a human maximization test for skin sensitization assessment, no reactions indicative of sensitization were observed at 4% (2760 μg/cm²) applied topically.
• In food applications, FEMA-reported typical exposure levels in foods include: soft drinks 31 mg/kg, cold drinks 40 mg/kg, confectionery 30 mg/kg, bakery products 36 mg/kg, and gum 10 mg/kg.

No standardized human therapeutic dosage for cinnamic acid as a dietary supplement has been established in clinical guidelines. The dosages cited above are from preclinical animal studies and should not be interpreted as applicable to humans.

8. Body Systems and Health Areas Associated with Cinnamic Acid

• Metabolic/Endocrine System: Cinnamic acid enhances metabolic health by improving glucose uptake and insulin sensitivity, showing promising results in improving metabolic health in patients with diabetes and its complications.
• Immune and Inflammatory Systems: Cinnamic acid and its derivatives inhibit TLR4 and NF-κB signaling pathways, reducing production of pro-inflammatory cytokines TNF-α and IL-6.
• Microbial Defense: Cinnamic acid acts by disrupting cell membranes, inhibiting ATPase activity, and preventing biofilm formation against bacterial and fungal pathogens.
• Cardiovascular System: Cinnamic acid shows a cardioprotective profile by preventing vasoconstriction and reducing the risk of complications of hypertension in preclinical models.
• Nervous System: Cinnamic acid has been studied for activation of PPARα to stimulate lysosomal biogenesis and lower amyloid plaque pathology in Alzheimer's disease mouse models.
• Oncology (preclinical): Cinnamic acid and derivatives have shown effects against breast cancer, colon cancer, lung adenocarcinoma, prostate cancer, and chronic myelogenous leukemia in laboratory studies.
• Antioxidant/Cellular Protective: Involvement in ROS scavenging and upregulation of endogenous antioxidant enzymes including catalase and glutathione peroxidase, demonstrated in cell-based models.

9. Safety Considerations and Notable Interactions

General Toxicity Profile

Cinnamic acid is a compound of low toxicity based on mammalian pharmacokinetic studies. The acute oral LD₅₀ in rats is 2.5 g/kg, and the acute dermal LD₅₀ in rabbits exceeds 5 g/kg.

Regulatory Status

In the United States, the regulatory status of cinnamic acid-related materials includes approval by the Food and Drug Administration (FDA) under 21 CFR 172.515 and recognition by the Flavor and Extract Manufacturers' Association (FEMA) as Generally Recognized as Safe (GRAS) as flavor ingredients. Cinnamic acid is regulated under FDA § 172.515 with moderation as the limit standard.

Skin Sensitization

In a human maximization test, no reactions indicative of sensitization were observed at 4% (2760 μg/cm²). The Expert Panel for Fragrance Safety reviewed the available data on cinnamic acid and concluded that it does not present a concern for skin sensitization in humans. Based on the weight of evidence from structural analysis, in vitro experiments, and animal and human studies, cinnamic acid does not present a concern for skin sensitization under the current, declared use conditions.

This is in contrast to closely related compounds: cinnamaldehyde, derived from the bark of the cinnamon tree and used as a fragrance, is a moderate skin sensitizer. It is important to distinguish between cinnamic acid and cinnamaldehyde, which have different sensitization profiles.

Dermal Absorption

There are limited data on the absorption of cinnamic acid through the skin, but the data that exist suggest there is significant absorption. A conservative estimate from in vitro studies on human skin is that approximately 61% of cinnamic acid is absorbed through the skin.

Carcinogenicity and Reproductive Toxicity

No ingredient of cinnamic acid present at levels greater than or equal to 0.1% is identified as a probable, possible, or confirmed human carcinogen by the IARC. Based on available data, cinnamic acid is not classified as a germ cell mutagen or reproductive toxicant in standard regulatory assessments.

Phenylketonuria (PKU) Relevance

The enzyme phenylalanine ammonia lyase (PAL) mediates the conversion of phenylalanine to cinnamic acid, and its possible clinical future in the management of the genetic disease phenylketonuria necessitated detailed study. Cinnamic acid is a compound of low toxicity which is converted in the mammalian body primarily to hippuric acid. Research has explored PAL-based enzyme substitution therapy for PKU, in which cinnamic acid is a direct metabolic product; this context has provided some of the most detailed human pharmacokinetic data on cinnamic acid metabolism.

Bioavailability Limitations

Due to limited intestinal absorption resulting from low bioavailability of cinnamic acid and its derivatives, current improvement efforts with entrapping into solid and liquid particles are highlighted in the pharmaceutical literature as a key hurdle for therapeutic development.

Gaps and Need for Further Research

The systematic approach in recent reviews highlights the need for further investigation of the mechanisms and safety of cinnamic acid to substantiate its use as a basis for new drug development. The antidiabetic mechanism of cinnamic acid must be further investigated in human body circumstances. Further human clinical studies are needed to clarify the effects of cinnamic acid and its derivatives in diabetic patients.

10. Summary of Evidence Levels

• Antidiabetic/Hypoglycemic: Moderate preclinical (animal and cell) evidence; no adequately powered human clinical trials for isolated cinnamic acid.
• Antimicrobial: Well-documented in vitro; no human clinical trial data for isolated compound.
• Anti-inflammatory: Established mechanistic pathways (NF-κB, TLR4) in cell studies; animal-level evidence; no human trials.
• Anticancer: Preliminary in vitro and animal evidence across multiple cancer types; no human data.
• Neuroprotective: Early-stage animal (Alzheimer's mouse model) and cell-based data; no human trials.
• Antioxidant: Demonstrated in vitro; significance in human supplementation context unestablished.
• Cardiovascular: Animal model evidence for ACE inhibition and cardioprotection; no human data.
• Safety: Low acute toxicity demonstrated in rodent studies; GRAS status for food use; not a skin sensitizer at declared fragrance-use levels; not classified as carcinogenic or mutagenic by IARC.

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