Propionic acid

Propionic Acid

1. Identity

Chemical Names and Basic Chemistry

Propionic acid (also known as propanoic acid) is a naturally occurring carboxylic acid with the chemical formula CH₃CH₂CO₂H. Its name derives from the Greek words prōtos ("first") and píōn ("fat"), as it is the smallest fatty acid to exhibit fatty-acid properties. It is a naturally occurring short-chain fatty acid with the molecular formula C₃H₆O₂, presenting as a colorless liquid with a pungent odor, found both in nature and as a synthesized compound.

In biochemical contexts, propionic acid most frequently appears as its conjugate base, propionate (CH₃CH₂COO⁻), at physiological pH. For the purposes of this article, "propionic acid" and "propionate" are used interchangeably in accordance with their use in primary literature, depending on whether the ionized or non-ionized form is being discussed. In food regulatory and labeling contexts, it carries the European food additive code E 280, while its common salts — sodium propionate, calcium propionate, and potassium propionate — are designated E 281, E 282, and E 283, respectively.

Natural Sources

Propionic acid is found across a remarkable range of natural contexts. Propionate is among the most common short-chain fatty acids produced in the large intestine of humans by gut microbiota in response to indigestible carbohydrates (dietary fiber) in the diet. Propionic acid also naturally occurs in animals and in dairy products in small amounts.

The principal biological producers of propionic acid in food systems are bacteria of the genus Propionibacterium. Propionic acid bacteria (PAB) are used mainly as adjuncts in cheeses with eyes, which are also called Swiss-type cheeses, particularly Emmental, Jarlsberg, and Maasdam. Propionibacteria are non-spore-forming Gram-positive bacteria, strict anaerobic or aerotolerant, producing catalase, and fermenting lactate (lactic acid), resulting in propionic acid, acetic acid, and carbon dioxide (CO₂).

Since propionibacteria are heterofermentative, they are able to metabolize carbohydrates, polyols, and pyruvate to form propionate, acetate, succinate, and carbon dioxide. They can be isolated from the soil, from plants, and also from the digestive tract of ruminants. Four species are currently classified as dairy PAB: Propionibacterium acidipropionici, P. freudenreichii, Propionibacterium jensenii, and Propionibacterium thoenii, which are distinct from those that occur on the human skin surface.

Beyond dairy, propionic acid is also naturally present in certain fermented vegetables. Starter cultures consisting of PAB and lactic acid bacteria are utilized in vegetable pickle production. Their combination increases the speed of the fermentation process and protects the final product against mold and rot, while pickles obtained by this method are vitamin B12 enriched and possess better taste and dietetic properties.

Common Forms and Preparations

Propionic acid is encountered in several commercial and supplemental forms:

• Free propionic acid (E 280): A colorless liquid used as a food preservative and antimicrobial agent.
• Propionate salts: Propionic acid and its salts are largely used as mold inhibitors in baking, as esterifying agents in the production of thermoplastics, and in the manufacture of flavors and perfume bases. The sodium, calcium, and potassium salts (E 281–E 283) are the forms most commonly used in food manufacturing.
• Inulin-propionate ester (IPE): A novel delivery vehicle developed to ferry propionate specifically to the colon. Oral propionate supplementation has been investigated in experimental studies of appetite regulation in humans, but its poor organoleptic properties, short circulating half-life, and the fact that it is absorbed in the proximal small intestine limit its use as a food supplement targeting large intestinal FFAR2. IPE was designed to circumvent these limitations by esterifying propionate to an inulin backbone.
• Microbial/fermentation-derived powder: Personalized dietary supplements adapted to the individual needs of athletes and the elderly enriched with microbial propionic acid have been produced in the form of a powder, ready to be mixed in drinks such as orange juices.

Fermentation processes for propionic acid production have been described since 1923. The increasing consumer demand for biological products and the more efficient performance of new fermentation processes have revived research and industrial interest for biological propionic acid production.

2. Traditional and Historical Use

Fermented Dairy and Food Traditions

Propionic acid does not have a documented history as an intentionally isolated supplement or traditional herbal remedy. Rather, its traditional "use" is best understood through the ancient practice of fermenting dairy foods that incidentally and naturally produced propionate.

Propionibacterium freudenreichii has been most renowned historically for its pivotal role in the fermentation of Swiss-type cheeses, such as Emmental, where its metabolic activities contribute to the development of characteristic flavor and the formation of "eyes" or holes. Beyond its culinary importance, traditional medicine recognized the value of fermented dairy products for promoting digestive health and overall well-being.

The characteristic taste and the typical eye formation of Swiss-type cheeses are the result of propionic acid fermentation. The fermentation is initiated by the addition of propionic acid bacterial cultures, composed of strains of the species Propionibacterium freudenreichii. The acetate, propionic acid, and proline produced contribute to the sweet and nutty flavor characteristic of the cheese.

Propionic acid generated contributes to preserving cheese by preventing multiplication of filamentous fungi (mold), but also provides other aromatic features when used for manufacturing. For this reason, it came to be used as an additive in other foods such as precooked foods.

Lactic propionibacteria, as Propionibacterium freudenreichii, are essential during the cheese ripening process, especially in Swiss cheese, and they have a history of use that helps to establish their safety for humans. P. freudenreichii has GRAS (Generally Recognized As Safe) status in accordance with a long and documented history of safe use in food. It is widely cultivated and consumed by humans in fermented dairy products such as Swiss-type cheese and in food probiotic supplements.

The use of propionic acid as an intentional food preservative — added externally rather than generated by fermentation — became systematized in the 20th century. Its use as a chemical preservative in bread, rolls, and certain dairy products now forms a major part of the global food industry's antimicrobial toolkit.

3. Key Constituents, Biochemistry, and Mechanisms of Action

Classification as a Short-Chain Fatty Acid

The gastrointestinal tract is home to trillions of diverse microorganisms collectively known as the gut microbiota, which play a pivotal role in breaking down undigested foods such as dietary fibers. Through the fermentation of these food components, short-chain fatty acids (SCFAs) such as acetate, propionate, and butyrate are produced, offering numerous health benefits to the host. The production and absorption of these SCFAs occur through various mechanisms within the human intestine, contingent upon the types of dietary fibers reaching the gut and the specific microorganisms engaged in fermentation.

Short-chain fatty acids such as acetate, butyrate, and propionate, which are produced by gut microbial fermentation of dietary fiber, are recognized as essential host energy sources and act as signal transduction molecules via G-protein coupled receptors (FFAR2, FFAR3, OLFR78, GPR109A) and as epigenetic regulators of gene expression by the inhibition of histone deacetylase (HDAC).

Receptor-Mediated Signaling

Propionate acts as a signalling molecule through FFAR2/FFAR3 receptors and modulates immunity, energy metabolism, and gut–brain communication. Of the major SCFAs, propionate has the highest affinity for FFAR2, and a lower binding affinity for FFAR3.

Propionate acts locally in the gut on enteroendocrine L-cells to stimulate release of the anorexigenic gut hormones PYY and GLP-1. Propionate is largely absorbed across the intestine and sequestrated primarily in the liver where it can be oxidised or used in gluconeogenesis.

The SCFA propionate stimulates free fatty acid receptors 2 and 3 (FFAR2, FFAR3) found on enteroendocrine L-cells and increases the release of the anorectic gut hormones glucagon-like peptide-1 (GLP-1) and peptide YY (PYY) in vitro.

Hepatic Metabolism and Gluconeogenesis

Once synthesized, propionic acid is absorbed from the colon via the portal vein, with concentrations typically ranging between 100 and 300 µM, with around 6 µM in peripheral blood. Most propionic acid is metabolized in the liver, leaving acetate as the most abundant SCFA in the periphery. Propionic acid is converted in the liver to propionyl-CoA, which can then be converted to pyruvate and used in energy metabolism and gluconeogenesis.

Propionate is hypothesized to increase insulin sensitivity by stimulating intestinal gluconeogenesis. It acts as a direct gluconeogenic substrate, and the resulting glucose is detected by a portal vein glucose sensor that transmits the signal to the brain and promotes beneficial effects on food intake and glucose metabolism.

Propionyl-CoA, a key precursor of propionic acid, is derived not only from the catabolism of amino acids (methionine, threonine, valine, and isoleucine) but also from the metabolism of odd-chain fatty acids and from propionate produced by intestinal bacterial activity.

Epigenetic Regulation: Histone Deacetylase Inhibition

As a member of the SCFA class, propionate shares with butyrate the capacity to inhibit histone deacetylase (HDAC) enzymes. SCFAs act as epigenetic regulators of gene expression by the inhibition of histone deacetylase (HDAC), thereby influencing gene expression patterns related to cell proliferation, inflammation, and metabolism. This mechanism is relevant to propionate's documented anti-inflammatory effects in adipose tissue and its potential relevance in intestinal epithelial homeostasis, though the HDAC-inhibitory potency of propionate at physiological concentrations in humans is considerably lower than that of pharmaceutical HDAC inhibitors.

Anti-Inflammatory and Immunomodulatory Properties

Both in vitro and in vivo studies published between 2011 and 2020 confirmed the ability of propionate to inhibit the growth of several cellular pathogens, including Gram-positive and Gram-negative multi-drug resistant bacteria and fungi. In addition, heterogeneous immune-modulatory and in particular anti-inflammatory effects of propionate were assessed, involving a diverse signaling network. The literature survey provides evidence that propionate displays a plethora of health-beneficial effects, including antimicrobial and immune-modulatory effects.

At the tissue level, treatment of human subcutaneous adipose tissue with propionic acid results in a significant downregulation of inflammatory parameters (e.g., TNF-α and IP-10) and macrophage markers (e.g., CD163 and MMP-9). Propionic acid also significantly upregulated the expression of lipoprotein lipase (LPL), sterol regulatory-element-binding protein-1c (SREBP-1c), and glucose transporter 4 (GLUT-4), which are associated with lipogenesis and glucose uptake.

Inhibition of Cholesterol Synthesis

The short-chain fatty acid propionic acid influences gut-mediated immune regulation, reduces lipogenesis, and inhibits serum cholesterol synthesis. The mechanism proposed for propionate's lipogenic and cholesterol effects relates to its role as a substrate competing with acetyl-CoA in hepatic metabolic pathways, effectively reducing the pool available for de novo lipid synthesis. Elevated acetic acid-to-propionic acid ratios have been associated with obesity and insulin resistance. This is because acetic acid promotes lipogenesis and fat accumulation, while propionic acid acts as a counter-regulator, inhibiting fat synthesis and promoting glucose homeostasis.

4. Scientific Evidence by Area of Use

4.1 Appetite Regulation and Body Weight

This is the area with the most developed clinical evidence for propionic acid supplementation. The research program centered on IPE has produced a series of controlled human trials.

Acute appetite effects (crossover RCT, n=14): Fourteen healthy men and women (mean age 31 ± 2 years; mean BMI 24.0 ± 0.9 kg/m²) were recruited and completed two study visits in a randomized, double-blind, crossover manner. On study days, subjects attended having fasted overnight and were given 10 g inulin propionate ester or 10 g inulin control in a standard breakfast. The trial found that IPE significantly increased feelings of satiety and reduced appetite, and confirmed hormonal effects consistent with FFAR activation.

24-week weight maintenance study (RCT, n=60): The long-term effects of inulin-propionate ester on weight gain were assessed in a randomised, controlled 24-week study involving 60 overweight adults. Propionate significantly stimulated the release of PYY and GLP-1 from human colonic cells. Acute ingestion of 10 g inulin-propionate ester significantly increased postprandial plasma PYY and GLP-1 and reduced energy intake. Over 24 weeks, 10 g/day inulin-propionate ester supplementation significantly reduced weight gain, intra-abdominal adipose tissue distribution, intrahepatocellular lipid content, and prevented the deterioration in insulin sensitivity observed in the inulin-control group. These data demonstrated for the first time that increasing colonic propionate prevents weight gain in overweight adult humans.

Food product incorporation study (crossover RCT, n=21): Supplementation with inulin-propionate ester (IPE), which delivers propionate to the colon, suppresses ad libitum energy intake and stimulates the release of satiety hormones acutely in humans, and prevents weight gain. A bread roll and fruit smoothie were produced. Twenty-one healthy overweight and obese humans participated, consuming 10 g/day inulin or IPE for six days in a randomised crossover design.

Limitations: Direct oral supplementation of free propionic acid as a dietary supplement faces pharmacokinetic limitations. Oral propionate supplementation is another method, but unfortunately its short plasma half-life, poor palatability, and the fact that its main absorption happens in the small intestine limit its use as a food supplement. The IPE delivery system was developed specifically to overcome this barrier, but it is a patented novel food rather than a simple "propionic acid supplement."

4.2 Insulin Sensitivity and Glucose Metabolism

Randomized crossover trial (n=12): To investigate the underlying mechanisms behind changes in glucose homeostasis with delivery of propionate to the human colon, twelve non-diabetic adults with overweight and obesity received 20 g/day of inulin-propionate ester (IPE), designed to selectively deliver propionate to the colon, alongside a high-fermentable fibre control (inulin) and a low-fermentable fibre control (cellulose) in a randomised, double-blind, placebo-controlled, cross-over design. Outcome measurements of metabolic responses, inflammatory markers and gut bacterial composition were analysed at the end of each 42-day supplementation period. These data demonstrate a distinctive physiological impact of raising colonic propionate delivery in humans, as improvements in insulin sensitivity promoted by IPE and inulin were accompanied with different effects on the plasma metabolome, gut bacterial populations, and markers of systemic inflammation.

Short-chain fatty acids (SCFAs), derived from fermentation of dietary fibre by the gut microbiota, have been shown to improve host insulin sensitivity. Long-term ingestion of 10 g/day IPE has been observed to ameliorate body weight gain and the development of abdominal visceral adipose tissue in overweight human adults.

Animal evidence: Insulin secretion after oral glucose load was reduced in high-fat diet animals receiving high-propionate diet after 15 and 30 minutes. This reduction was stronger with an increasing proportion of dietary propionate, demonstrating that the suppression of high-fat diet-induced insulin resistance appears to be mediated by propionate, which is in line with other animal studies. Evidence from human trials is promising but limited in scale; no large-scale, long-term clinical trials specifically targeting type 2 diabetes prevention using propionate supplementation have been reported in the searched literature.

4.3 Gut Microbiota and Gastrointestinal Health

This area of research focuses on propionate's effects on metabolism, inflammation, microbiota, and gastrointestinal diseases. SCFAs produced primarily from the microbial fermentation of dietary fibre appear to be key mediators of the beneficial effects elicited by the gut microbiome. Not only does dietary fibre fermentation regulate microbial activity in the gut, but SCFAs also directly modulate host health through a range of tissue-specific mechanisms related to gut barrier function, glucose homeostasis, immunomodulation, appetite regulation, and obesity.

Propionate has beneficial effects in metabolic disorders, inflammatory bowel disease (IBD), and alcohol-related liver disease (ALD). However, the clinical evidence for propionate specifically in IBD remains largely preclinical or derived from indirect observations in fiber-supplementation studies. Direct human interventional trials with propionate or IPE in IBD patients were not identified in the searched literature.

Propionibacterium freudenreichii is an Actinobacterium widely used in the dairy industry and responsible for aroma development and eye formation in Swiss-type cheeses. Some strains are also used as probiotics because they produce bifidogenic compounds, they are resistant to digestive stress, and they may be endowed with anti-inflammatory capabilities and could be used to prevent inflammatory bowel diseases.

4.4 Anti-Inflammatory Effects in Adipose Tissue

Ex vivo human tissue study: Human omental adipose tissue explants were obtained from overweight (mean BMI 28.8) gynaecological patients who underwent surgery. Explants were incubated for 24 hours with propionic acid. Treatment of adipose tissue explants with propionic acid resulted in a significant down-regulation of several inflammatory cytokines and chemokines such as TNF-α and CCL5. In addition, expression of lipoprotein lipase and GLUT4, associated with lipogenesis and glucose uptake, respectively, increased. Similar effects on cytokine and chemokine production by macrophages were observed. This suggests that propionic acid, normally produced in the colon, may have a direct beneficial effect on visceral adipose tissue, reducing obesity-associated inflammation and increasing lipogenesis and glucose uptake. Effects on adipose tissue as a whole are at least partially explained by effects on macrophages but likely also adipocytes are involved. This suggests that, in vivo, propionic acid and dietary fibres may have potential in preventing obesity-related inflammation and associated diseases.

Evidence strength: These findings are from ex vivo tissue preparations rather than a clinical trial with patient outcomes. They are mechanistically important but cannot be directly translated to clinical benefit without confirmatory in vivo human studies.

4.5 Cardiovascular Risk Markers

Evidence links SCFAs with host metabolic health and cardiovascular disease (CVD) risk. SCFAs have a range of effects locally in the gut and at both splanchnic and peripheral tissues which together appear to induce improved metabolic regulation and have direct and indirect effects on markers of CVD risk.

The mechanism by which propionate may benefit cardiovascular risk is primarily through its role in inhibiting hepatic cholesterol synthesis and reducing lipogenesis, and secondarily through its effects on insulin sensitivity, adipose tissue inflammation, and body weight. However, direct human clinical trials measuring hard cardiovascular endpoints (myocardial infarction, stroke) attributable to propionate supplementation were not identified in the searched literature. The cardiovascular evidence is therefore mechanistic and indirect rather than based on outcome trials.

4.6 Antimicrobial Properties

Propionic acid has well-established antimicrobial activity that is the basis of its regulatory approval as a food preservative. The association of propionic acid with lactic and acetic acids has been recommended for the preservation of foods. Propionic acid generated during cheese fermentation contributes to preserving cheese by preventing multiplication of filamentous fungi (mold). The mechanism of antimicrobial action is understood to involve intracellular acidification: the undissociated acid permeates microbial cell membranes and releases protons inside the cell, lowering intracellular pH and disrupting enzyme function and nutrient transport. This activity is strongest at lower external pH values where a greater proportion of the acid is in its undissociated form.

4.7 Neuroscience: Gut–Brain Axis, ASD, and Neurotoxicity

Propionic acid's relationship to neuroscience is dual-edged and scientifically complex. On one hand, it is a normal metabolite of the gut–brain axis. The role of the gut microbiota and their metabolites, including propionate, in mediating brain function has been reviewed. On the other hand, at elevated or abnormal concentrations, propionic acid has been linked to neurotoxic effects in experimental models.

Propionic acid is a short-chain fatty acid and an important intermediate of cellular metabolism. Although propionic acid has several beneficial biological effects, its accumulation is neurotoxic.

Autism spectrum disorder (ASD) is a neurodevelopmental disorder with complex etiology, involving many environmental and genetic risk factors, characterized by deficits in communication and social interaction, restricted interests, and repetitive behaviors. ASD is associated with "gut dysbiosis," a disruption in microflora homeostasis in the GI tract. Several lines of animal research suggest a mechanistic link between elevated propionic acid and ASD-like behaviors.

Propionic acid, produced by enteric gut bacteria, crosses both the gut-blood and the blood-brain barriers. Previous research has demonstrated that repeated intracerebroventricular (ICV) infusions of PPA in adult rats produce behavioural and neuropathological changes similar to those seen in ASD patients.

Propionic acid can readily cross the blood-brain barrier, accumulate in neural tissue, and disrupt neurophysiological processes such as neurotransmitter release, gene expression, mitochondrial function, and immune modulation, precipitating behavioral abnormalities relevant to ASD.

The effect of propionic acid, a product of dysbiotic ASD gut, on human neural stem cells (hNSCs) proliferation, differentiation, and inflammation has been investigated. High levels of propionic acid can induce oxidative stress and glutathione depletion in various brain regions such as cortex, hippocampus, thalamus, and striatum of rats infused with propionic acid.

Important caveat: Excessive accumulation of propionate is linked to neurotoxicity, autism spectrum disorder (ASD), and mitochondrial dysfunction. Its effects are dose-dependent and tissue-specific, with both protective and harmful potentials depending on the context. The animal model studies typically employed doses (e.g., 250 mg/kg/day administered directly) far exceeding physiological levels achievable through dietary means or normal gut fermentation. These findings should not be conflated with the safety profile of propionate at normal dietary or supplemental concentrations.

4.8 Propionic Acidemia (Disease Context)

It is important to distinguish supplemental or dietary propionate from the pathological condition of propionic acidemia (PA). Propionic acidemia is an autosomal recessive inherited deficiency of propionyl-CoA carboxylase (PCC), which is involved in the catalytic breakdown of the amino acids valine, isoleucine, methionine, and threonine. Propionic acidemia is an inborn error metabolic disorder caused by mutations in the gene responsible for encoding propionyl-CoA carboxylase (PCC). Decreased PCC activity disrupts the conversion of propionyl-CoA into methylmalonyl-CoA and its subsequent entry into the tricarboxylic acid (TCA) cycle. This condition is not caused by dietary propionic acid intake in healthy individuals; rather, it represents a failure of propionyl-CoA metabolism resulting in toxic systemic accumulation.

5. Body Systems Associated with Propionic Acid

• Gastrointestinal system: Site of primary production (by gut microbiota), absorption, and local receptor signaling. Propionate modulates mucosal immunity and barrier function.
• Hepatic system: Primary site of systemic propionate metabolism. Propionate is extensively extracted by the liver and converted to propionyl-CoA for gluconeogenesis and energy metabolism.
• Endocrine/metabolic system: Propionate stimulates GLP-1 and PYY release, modulates insulin sensitivity, inhibits cholesterol synthesis, and acts as a gluconeogenic precursor.
• Adipose tissue: Propionate exerts anti-inflammatory effects on both visceral and subcutaneous adipose tissue, partly via macrophage-mediated mechanisms involving Gi/o protein-coupled receptors.
• Immune system: Propionate modulates both innate and adaptive immune responses, exerting anti-inflammatory effects, and shows in vitro antimicrobial activity against a range of pathogens.
• Nervous system: Propionate crosses the blood-brain barrier and is implicated in the gut–brain axis. At physiological concentrations it is a normal metabolite; at pathological concentrations it may impair neurological function.
• Cardiovascular system: Indirectly associated through effects on lipid metabolism, body weight, insulin resistance, and adipose inflammation — all risk factors for cardiovascular disease.

6. Dosage Forms and Reported Doses

No established recommended dietary allowance (RDA) or tolerable upper intake level for propionic acid as a dietary supplement has been set by any major regulatory body. The following doses are drawn directly from identified research studies:

• Inulin-propionate ester (IPE), acute appetite study: 10 g inulin-propionate ester given with a standard breakfast (641 kcal) in a randomized, double-blind crossover trial.
• IPE, 24-week weight management study: 10 g/day inulin-propionate ester supplementation over 24 weeks in overweight adults.
• IPE, insulin sensitivity study: 20 g/day of inulin-propionate ester in non-diabetic overweight and obese adults for each 42-day supplementation period.
• EFSA novel food safety evaluation (IPE): Despite the limitations of the provided human studies (primarily designed to investigate efficacy endpoints), doses up to 20 g/day for durations up to 12 months appear to be generally well-tolerated.
• Food preservative use (in baked goods): In food applications, propionic acid is used at low concentrations — typically 0.1–0.4% in bakery products.
• Microbial supplement product: Microbial propionic acid incorporated at 0.1 g/L has been evaluated as a postbiotic in a fruit beverage format.
• Animal neurotoxicity model (not applicable to humans): A neurotoxic dose of 250 mg/kg body weight/day for three days was used in rat studies — a dose with no clinical relevance to human supplementation but important to understanding safety boundaries.

7. Safety Considerations

Regulatory Safety Status

The US Food and Drug Administration (FDA) lists propionic acid, and the Na⁺, Ca²⁺, and K⁺ salts, as preservatives in their summary of Generally Recognised As Safe (GRAS) additives, and no upper limits are imposed except for bread, rolls, and cheeses (0.30–0.38%).

The Joint FAO/WHO Expert Committee on Food Additives (JECFA) recognizes propionic acid (INS: 280) with functional uses as a preservative, antimold and antirope agent, or flavoring agent. An acceptable daily intake (ADI) of "not limited" was set by JECFA in 1973.

EFSA stated there was no concern with respect to genotoxicity and carcinogenicity when propionic acid is used as a food additive. There is no safety concern from authorized maximum usage. However, an ADI was not allocated due to the available toxicity database.

Propionates are naturally occurring substances in the normal diet. The EFSA Panel considered that forestomach hyperplasia reported in long-term studies in rodents is not a relevant endpoint for humans because humans lack this organ.

In February 2002, EPA's Health Effects Division (HED) concluded that based on its low toxicity, limited use pattern, and affirmation as a GRAS chemical when used as a food additive, propionic acid is not expected to result in any adverse health effects via the food, drinking water, or residential exposure pathways.

Safety of IPE as a Novel Food

Taking into account the physicochemical properties of the novel food (inulin-propionate ester), the production process and metabolic fate, which do not raise safety concerns, and given that propionic acid and its salts were previously assessed by the EFSA ANS Panel (2014), as well as that a large body of safety data were available on inulin, the Panel considered that no genotoxicity and subchronic toxicological studies were required on the novel food. The Panel concluded that the novel food, inulin-propionate ester, is safe for the general population under the proposed conditions of use.

NOAEL and Dose-Related Safety Data

On the basis of a 90-day study in dogs, the EFSA ANS Panel identified a No Observed Adverse Effect Level (NOAEL) of 0.3% propionic acid in the diet, based on epithelial hyperplasia in the esophagus in the 1% group (which recovered thereafter). This concentration was equivalent to the highest maximum permitted level (MPL) of propionic acid–propionates (3000 mg/kg) in food, in the category of bread and rolls. The ANS Panel also noted that the concentration provoking site-of-contact effects in the 90-day study in dogs was a factor of three times higher than the concentration in food at the highest permitted level.

Occupational and Physical Irritation

Propionic acid is highly irritating to the eyes, and there is a possibility of eye and skin irritation to occupational handlers; the use of personal protective equipment is required. These hazards pertain to concentrated industrial-grade propionic acid and are not relevant to dietary consumption at food-additive levels.

Neurotoxicity at Supraphysiological Concentrations

As outlined above, excessive accumulation of propionate is linked to neurotoxicity, autism spectrum disorder (ASD), and mitochondrial dysfunction. Its effects are dose-dependent and tissue-specific, with both protective and harmful potentials depending on the context. Propionate use requires a personalized approach, considering the pathological context, host microbiota composition, and appropriate dosage to avoid adverse effects.

Propionic Acidemia: A Contraindicated Population

Individuals with confirmed propionic acidemia must restrict dietary intake of the amino acids that serve as propionyl-CoA precursors (valine, isoleucine, methionine, and threonine) and avoid additional sources of propionate. Nutritional management is a mainstay in the treatment of propionic acidemia, with the goal to reduce the accumulation of toxic metabolites by restricting dietary protein sources of the propiogenic amino acids and to prevent endogenous protein catabolism by providing sufficient energy.

Interactions and Pharmacological Considerations

No specific drug–drug interactions with supplemental propionic acid or IPE have been identified in the searched regulatory or clinical literature. However, given propionate's capacity to act as a partial HDAC inhibitor at higher concentrations, theoretical interactions with pharmaceutical HDAC inhibitors used in oncology cannot be ruled out. SCFAs act as epigenetic regulators of gene expression by the inhibition of histone deacetylase (HDAC), and combinations with pharmaceutical agents that share this mechanism might theoretically require monitoring. No clinical interaction data exist to substantiate this at dietary intake levels. The patented IPE delivery system may be subject to different regulatory considerations than ordinary dietary supplements in various jurisdictions.

8. Evidence Summary and Characterization

The scientific literature on propionic acid reflects a compound with a deeply established role in human physiology and food science, and an emerging — though still preliminary — evidence base as a supplemental or functional food ingredient. The strongest human evidence relates to colonic propionate delivery (via IPE) for appetite regulation, energy intake reduction, and prevention of weight gain in overweight adults. The insulin-sensitizing effects have been demonstrated in small, controlled crossover trials. Anti-inflammatory actions in adipose tissue have been shown in ex vivo human tissue studies. Evidence for cardiovascular benefit is mechanistic and indirect. The gut–brain axis and ASD data are derived predominantly from animal models using supraphysiological doses and do not translate directly to safety concerns at dietary levels. The condition of propionic acidemia is a distinct inborn error of metabolism unrelated to normal propionate dietary exposure.

Overall, the evidence base for propionic acid as a dietary supplement at this time is promising but limited in the scale and number of controlled human trials, particularly with regard to long-term outcomes. The compound is well-established as a safe food additive at levels used in food manufacturing, with GRAS status in the United States and an ADI of "not limited" from JECFA.

References

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