Acylcarnitines are molecules that serve as metabolic intermediaries, reflecting the body’s ability to process fats for energy. These compounds are formed when an acyl group, derived from fatty acids, attaches to L-carnitine, a naturally occurring substance. L-carnitine functions as a carrier molecule, and the resulting acylcarnitine is the cargo-loaded version. The concentration and specific types of acylcarnitines circulating in the bloodstream offer a snapshot of a person’s mitochondrial function and overall metabolic health. Historically, these molecules have been used as diagnostic markers for inherited disorders, but they are increasingly recognized as broader indicators of metabolic status.
The Role of Acylcarnitines in Energy Production
The core function of acylcarnitines is facilitating the breakdown of fatty acids, known as beta-oxidation, which occurs primarily within the mitochondria. Long-chain fatty acids (14 or more carbon atoms) cannot directly pass through the inner mitochondrial membrane, necessitating a transport mechanism called the carnitine shuttle.
The carnitine shuttle involves three main steps. First, the fatty acid is linked to Coenzyme A (CoA) in the cytoplasm, and the enzyme carnitine palmitoyltransferase 1 (CPT1) converts the fatty acyl-CoA into an acylcarnitine. This acylcarnitine is then transported into the mitochondrial matrix by the carnitine-acylcarnitine translocase (CACT).
Once inside the matrix, carnitine palmitoyltransferase 2 (CPT2) removes the carnitine, reforming the fatty acyl-CoA for beta-oxidation. The carnitine is then recycled back out of the mitochondria by the translocase to pick up another fatty acid. This highly regulated shuttle system transports the majority of fat-derived fuel for energy production.
Acylcarnitines are classified based on the length of the attached fatty acid chain: short-chain (C2-C5), medium-chain (C6-C12), and long-chain (C14-C20). The presence of these intermediate acylcarnitines in the blood reflects the current flux of fatty acid metabolism. They are essentially an overflow product, signaling that the supply of fatty acids has exceeded the capacity of the mitochondrial machinery.
Clinical Measurement and Newborn Screening
Acylcarnitine levels are routinely measured in clinical settings as they provide a specific and non-invasive window into mitochondrial function. The most widespread application is in expanded newborn screening (NBS) programs, which test infants shortly after birth for metabolic disorders. Detecting these conditions early allows for immediate medical intervention and significantly improves long-term health outcomes.
The technology enabling this screening is Tandem Mass Spectrometry (MS/MS). This analytical method requires only a small dried blood spot sample and simultaneously measures the concentrations of many different acylcarnitine species. The ability to profile these metabolites with high sensitivity revolutionized the early diagnosis of conditions like Fatty Acid Oxidation Disorders (FAODs).
The goal of NBS is to identify specific enzyme deficiencies that impair the body’s ability to generate energy from fat. A defect in an enzyme causes the metabolites before the blocked step to accumulate. By measuring the pattern of these accumulating acylcarnitines, the screening pinpoints the specific location of the metabolic blockage.
Understanding Abnormal Acylcarnitine Profiles
Interpreting an acylcarnitine profile involves analyzing the distinct pattern of individual species, abbreviated as ‘C’ followed by the number of carbon atoms. A dysfunctional enzyme causes its specific substrate to build up in the cell, which is then converted into the corresponding acylcarnitine for removal. This unique accumulation pattern serves as a biochemical fingerprint for a specific metabolic disorder.
For example, a high level of octanoylcarnitine (C8), often with lesser elevations of hexanoylcarnitine (C6) and decanoylcarnitine (C10), strongly indicates Medium-Chain Acyl-CoA Dehydrogenase Deficiency (MCADD). Since the enzyme MCAD processes fatty acids with 6 to 12 carbons, C8 becomes the most prominent signal in the blood when the enzyme fails. Clinicians also examine specific ratios, such as C8/C10 or C8/C2, to increase diagnostic certainty by comparing the accumulating molecule to other carnitines.
Conversely, an elevation in long-chain acylcarnitines, like C16 or C18, suggests a defect in later stages of fat processing, such as a deficiency in Very Long-Chain Acyl-CoA Dehydrogenase (VLCAD) or Carnitine Palmitoyltransferase 2 (CPT2). This profile reflects a problem with the longest fatty acid chains, which are the primary fuel source for the heart and muscle. An abnormal profile can also indicate a secondary issue, such as a low total carnitine level, which results when the body uses its free carnitine pool to excrete accumulating acyl groups. The acylcarnitine profile guides further confirmatory genetic or enzymatic testing.
Diet, Exercise, and Acylcarnitine Levels
In individuals without an inherited enzyme deficiency, acylcarnitine levels are highly responsive to lifestyle factors, particularly diet and physical activity. When the body shifts its metabolism to burn fat for fuel, the production of these intermediates naturally increases. For instance, during fasting, the body ramps up fatty acid oxidation, leading to a transient rise in plasma acylcarnitines.
Similarly, intense exercise, which relies heavily on fat stores for sustained energy, causes a temporary increase in acylcarnitine species. Muscle tissue is a major contributor to this elevation, with short-chain acylcarnitines like C2 often increasing notably during and immediately after a workout. This fluctuation reflects the body’s metabolic flexibility, demonstrating its ability to switch between using carbohydrates and fats as fuel sources.
High-fat or ketogenic diets, which restrict carbohydrate intake to induce a fat-burning state, also lead to persistently altered acylcarnitine profiles. The continuous reliance on fat results in a high flux through the beta-oxidation pathway and elevated levels of various acylcarnitines. Furthermore, L-carnitine supplements increase the available pool of the carrier molecule, which enhances the conversion of acyl groups into acylcarnitines and alters the measured profile.