Fatty Acid Oxidation Disorders

Fatty acid oxidation disorders are inherited defects in the mitochondrial breakdown of fatty acids, the process that supplies energy during fasting and prolonged exertion when glucose is scarce. When this pathway fails, the body cannot mobilise fat for fuel, leading to energy deficiency and the toxic accumulation of partially oxidised fatty acid intermediates, often unmasked by fasting or illness. Medium-chain acyl-CoA dehydrogenase deficiency is the prototype and one of the most common disorders detected by newborn screening.

Definition

A fatty acid oxidation disorder is an inherited defect in the mitochondrial import or beta-oxidation of fatty acids, impairing energy production from fat and causing accumulation of acyl-CoA and acylcarnitine intermediates, typically with disease provoked by fasting or catabolic stress.

Scope

The entry covers the role of mitochondrial beta-oxidation in energy supply, the carnitine shuttle that imports fatty acids, the chain-length-specific enzyme defects, and the fasting-triggered, energy-deficiency pattern of disease. It is a reference overview that uses specific defects only as illustrations and does not give management advice.

Key concepts

• Mitochondrial beta-oxidation
• Carnitine shuttle and the carnitine cycle
• Chain-length-specific acyl-CoA dehydrogenases
• Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency
• Long-chain defects (VLCAD, LCHAD, trifunctional protein)
• Hypoketotic hypoglycaemia
• Acylcarnitine profiling in newborn screening
• Fasting and illness as metabolic triggers

Mechanisms

During fasting, fatty acids are released from adipose tissue and transported into mitochondria by the carnitine shuttle, then broken down by repeated cycles of beta-oxidation that shorten the chain and yield acetyl-CoA for energy and ketone production. A defect in carnitine transport or in one of the chain-length-specific enzymes interrupts this flux, so tissues dependent on fat-derived energy - liver, heart, and skeletal muscle - run short of fuel, and ketone production fails, producing the characteristic hypoketotic hypoglycaemia. Partially oxidised intermediates accumulate as acyl-CoA species and their corresponding acylcarnitines, which give each disorder a distinctive acylcarnitine signature in blood; this is the basis for their detection by newborn screening, as described by Spiekerkoetter and reviewed by Rinaldo and colleagues. Because the pathway is mainly called upon during fasting or illness, symptoms are often episodic and stress-triggered.

Clinical relevance

Fatty acid oxidation disorders exemplify energy-deficiency-type metabolic disease and explain why fasting or intercurrent illness can precipitate metabolic crises. Their recognisable acylcarnitine signatures make them important targets of newborn screening. This entry summarises the biochemical and evidentiary landscape for reference and is not a basis for individual diagnosis or treatment.

Epidemiology

As a group these disorders are an important component of expanded newborn screening; medium-chain acyl-CoA dehydrogenase deficiency is among the most frequently detected inborn errors of metabolism in many populations, while the long-chain defects are individually rarer. Spiekerkoetter's work documents how screening has altered their recognised clinical spectrum.

History

Mitochondrial fatty acid oxidation was characterised biochemically through the mid-twentieth century, and specific human deficiencies were defined from the 1970s onward as enzymology and metabolite analysis matured. The development of tandem mass spectrometry made acylcarnitine profiling practical, and the inclusion of medium-chain acyl-CoA dehydrogenase deficiency and related defects in newborn screening transformed their detection from symptomatic presentation to presymptomatic identification, broadening the recognised phenotypic range as Spiekerkoetter describes.

Key figures

• Ute Spiekerkoetter
• Piero Rinaldo
• Michael Bennett
• Charles Roe

Seminal works

• rinaldo-2002
• spiekerkoetter-2010

Frequently asked questions

Why are symptoms of fatty acid oxidation disorders often triggered by fasting or illness?

The fatty acid oxidation pathway is mainly needed when glucose runs low - during fasting, prolonged exercise, or illness. When the pathway is defective, the body cannot switch to burning fat for energy at those times, so crises tend to occur during catabolic stress.

What is hypoketotic hypoglycaemia and why does it occur?

It is low blood sugar accompanied by inappropriately low ketone levels. In fatty acid oxidation disorders the impaired pathway cannot generate ketones from fat, so the usual ketone fuel is missing even as glucose falls.