Adipic Acid

Adipic acid, also known as hexanedioic acid, is an organic dicarboxylic acid with the formula (CH2)4(COOH)2. It is a white crystalline powder, highly soluble in water and polar organic solvents. 

This industrially significant compound, with an annual production of approximately 2.5 billion kilograms, is primarily used in the manufacture of nylon.  Additionally, adipic acid is utilized in the production of polyurethanes, plasticizers for PVC, and as an additive in foods, where it serves as a gelling agent and acidity regulator.

In the pharmaceutical field, adipic acid is incorporated into controlled-release formulations to modulate drug release profiles and as a pore-forming agent in enteric coatings to enhance disintegration at intestinal pH.  Though rarely found in nature, it can occur in small amounts in beets and as a microbial metabolite.

Adipic acid can also serve as a biomarker for the consumption of gelatin-containing foods such as jello, as its presence in urine and blood is typically exogenous.  

Elevated levels of adipic acid in biological fluids may indicate metabolic disorders like lysinuric protein intolerance and glutaric aciduria type I, which impair amino acid metabolism and lead to the accumulation of adipic acid and related metabolites.  Testing for adipic acid in urine is common for diagnosing these metabolic disorders and assessing overall metabolic function.

What is Adipic Acid?  [1., 2., 4., 6.]

Adipic acid, also known as hexanedioic acid, is an organic dicarboxylic acid with the chemical formula (CH2)4(COOH)2, containing two carboxyl groups (-COOH).  

It is a white crystalline powder that is highly soluble in water and polar organic solvents, with a melting point of 152-154°C and a molecular weight of 146.14 g/mol.  The IUPAC name is hexanedioic acid, and the SMILES notation is OC(=O)CCCCC(O)=O. 

Adipic acid is an important industrial chemical, with around 2.5 billion kilograms produced annually, mainly as a precursor for nylon production via polycondensation with hexamethylene diamine.  It is also used in the production of polyurethanes, plasticizers for PVC, and as a gelling aid and acidulant in some foods. 

Adipic acid is approved for use as a food additive.  In the medical and pharmaceutical fields, adipic acid has been incorporated into controlled-release formulations and matrix tablets to modulate drug release profiles, and used as a pore-forming agent in enteric polymer coatings to improve disintegration at intestinal pH. 

While rarely found in nature, adipic acid can occur in small amounts in beets and as a microbial metabolite.  

In humans, adipic acid in urine and blood is typically exogenous and can be a biomarker for consumption of gelatin-containing foods like jello.  

Adipic acid is a mild skin irritant and has a median lethal dose of 3600 mg/kg for oral ingestion in rats, indicating moderate toxicity.

Adipic Acid in Disease

Pathological Implications of Adipic Acid in Humans  [4.] 

Elevated levels of adipic acid in biological fluids, such as urine or blood, can serve as indicators of underlying metabolic disorders.  [3.]

For example, certain inborn errors of metabolism, such as lysinuric protein intolerance and glutaric aciduria type I, are associated with impaired amino acid metabolism, leading to the accumulation of adipic acid and related metabolites.  [5.] 

Additionally, adipic aciduria, a rare metabolic disorder characterized by high levels of adipic acid in urine, can result from deficiencies in enzymes involved in lysine catabolism.  [3.] 

Disorders involving elevated levels of adipic acid are characterized by increased excretion of these acids in the urine, sometimes accompanied by alpha-hydroxyadipic acid.  

Clinical presentations can vary widely and may include symptoms such as hypotonia, developmental delay, intellectual disability ranging from mild to severe, ataxia, epilepsy, and behavioral disorders like attention deficit hyperactivity disorder. 

However, many individuals with this biochemical phenotype remain asymptomatic, leading to questions about the clinical significance of the condition.

Lab Testing for Adipic Acid

Test Information, Sampling Methods and Preparation

Laboratory testing for organic acids including adipic scid is typically done in urine, although it can also be tested in blood.  Testing may be ordered to diagnose an inborn metabolic disorder, or to assess metabolic function and gastrointestinal health in a functional medicine setting.  

Urine samples may be collected in a clinical setting; they can also be collected at home.  Some labs recommend or require a first morning void sample, to provide a concentrated sample.  

Interpreting Adipic Acid Results

Optimal Range for Adipic Acid Testing

Generally, falling within reference ranges for organic acids is recommended, although for many of these organic acids, a level towards the lower end of the reference range is considered optimal.  

It is essential to consult with the laboratory company used for their recommended reference range for Adipic Acid.  

One company reports the following reference range for Adipic Acid:  <4.42 mcg/mg creatinine  [7.]

Clinical Significance of Elevated Levels of Adipic Acid

Elevated levels of Adipic Acid may indicate an underlying disorder of amino acid metabolism.  [3., 4., 5.]

Additional testing for organic acid levels may provide increased awareness of specific metabolic alterations or disorders.  

Clinical Significance of Low Levels of Adipic Acid

Low levels of Adipic Acid are not considered clinically relevant. 

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