Introduction

The primary energy source shifts from glucose to fat during periods of fasting or starvation, when carbohydrate intake is markedly reduced. Adipose tissue releases stored triglycerides, which are hydrolyzed into free fatty acids (FFAs) and glycerol. FFAs are transported to the liver, where they undergo β-oxidation, generating large amounts of acetyl-CoA.

Accumulation occurs when acetyl-CoA production exceeds the liver’s oxidative capacity in the citric acid cycle. This imbalance is partly driven by decreased availability of oxaloacetate, which is diverted toward gluconeogenesis. Excess acetyl-CoA cannot be fully oxidized or redirected into fatty acid synthesis. The metabolic bottleneck channels acetyl-CoA into ketogenesis, in which the liver converts acetyl-CoA into ketone bodies, primarily acetoacetate and β-hydroxybutyrate (see Image. Ketogenesis Pathway).

Ketone bodies are released into the circulation and serve as an alternative energy substrate for peripheral tissues, particularly the brain, during prolonged carbohydrate deficiency. Plasma concentrations rise when the rate of ketone body production surpasses peripheral utilization, resulting in ketonemia. Progressive accumulation leads to urinary excretion, termed "ketonuria."

Marked ketonemia and ketonuria are most frequently associated with uncontrolled type 1 diabetes mellitus. In this setting, absolute insulin deficiency limits glucose uptake, promotes unregulated lipolysis, and accelerates ketogenesis. The resulting overproduction of acidic ketone bodies can exceed the buffering capacity of blood, causing metabolic acidosis. This condition, known as diabetic ketoacidosis (DKA), constitutes a serious and potentially life-threatening complication requiring prompt treatment.[1][2][3]

Mastery of the ketogenesis pathway enables clinicians to link biochemical findings with clinical presentations. This integrative approach enhances decision-making in metabolic, endocrine, and nutritional disorders.

Fundamentals

Excess acetyl-CoA is directed toward ketogenesis when its production in the liver surpasses the oxidative capacity of the citric acid cycle due to limited availability of oxaloacetate. Ketogenesis involves 3 key mitochondrial enzymes that convert 2 molecules of acetyl-CoA into acetoacetate, which can subsequently be reduced to D-β-hydroxybutyrate.

The 1st reaction is catalyzed by β-ketoacyl-CoA thiolase, which condenses 2 molecules of acetyl-CoA to form acetoacetyl-CoA. This enzyme also catalyzes the reverse reaction during step 4 of β-oxidation, cleaving acetoacetyl-CoA to release acetyl-CoA. Acetoacetyl-CoA is then converted to 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) by HMG-CoA synthase and subsequently cleaved by HMG-CoA lyase to generate acetoacetate.[4]

Acetoacetate can either be enzymatically reduced to D-β-hydroxybutyrate by D-β-hydroxybutyrate dehydrogenase or decarboxylated, through spontaneous or enzymatic processes, to yield acetone and carbon dioxide. Acetone is a minor byproduct and is largely exhaled. In contrast, D-β-hydroxybutyrate and acetoacetate are released into the circulation and serve as major alternative energy substrates for extrahepatic tissues, particularly the brain, heart, and skeletal muscle, during prolonged fasting or carbohydrate deprivation.[5]

Cellular Level

Ketogenesis is a critical survival mechanism that sustains energy production through high rates of fatty acid oxidation during carbohydrate depletion. However, dysregulation may result in severe pathological consequences. Under physiologic conditions, acetoacetate and D-β-hydroxybutyrate are produced in moderate amounts and efficiently utilized by peripheral tissues, including the brain, heart, and skeletal muscle. Circulating concentrations rise when production surpasses the rate of peripheral utilization, leading to ketoacidosis.[6][7]

Ketoacidosis is defined by a marked reduction in blood pH caused by the accumulation of acidic ketone bodies, which exceed the buffering capacity of plasma. This state is most frequently observed in individuals with undiagnosed or poorly controlled type 1 diabetes mellitus. Absolute or relative insulin deficiency promotes unrestrained lipolysis and accelerates hepatic ketogenesis. Blood and urine levels of acetoacetate and D-β-hydroxybutyrate can increase several orders of magnitude above baseline, producing clinical manifestations that include nausea, vomiting, abdominal pain, and deep, labored respirations (Kussmaul breathing).[8]

Accumulation of acetone, a volatile ketone body generated through spontaneous decarboxylation of acetoacetate, produces a characteristic fruity odor on the breath. Severe DKA can result in a combination of metabolic acidosis, dehydration, and electrolyte disturbances that compromise cerebral function, leading to delirium, unconsciousness, or coma. Rapid recognition and timely treatment of DKA are critical to preventing life-threatening complications.[9]

Molecular Level

Ketogenesis is a critical metabolic pathway that preserves energy homeostasis during carbohydrate and protein scarcity, including prolonged fasting or starvation. In these states, hepatic metabolism prioritizes glucose production via gluconeogenesis to supply glucose-dependent tissues such as the brain and red blood cells. Oxaloacetate, an essential intermediate of the citric acid cycle, is diverted toward gluconeogenesis, thereby reducing cycle flux and limiting hepatic oxidation of acetyl-CoA generated through β-oxidation of fatty acids. The resulting accumulation of acetyl-CoA in liver mitochondria necessitates an alternative metabolic fate.

Ketogenesis provides this outlet by converting excess acetyl-CoA into the ketone bodies acetoacetate and D-β-hydroxybutyrate, which are subsequently released into the circulation. Peripheral tissues, including cardiac and skeletal muscle and, during sustained fasting, the brain, take up ketone bodies and reconvert them to acetyl-CoA for adenosine triphosphate (ATP) generation. This metabolic adaptation is essential for survival under conditions of limited glucose availability.

In clinical practice, particularly in the emergency setting, patients presenting with severe ketoacidosis require careful evaluation to determine the underlying cause. Although DKA is frequently associated with hyperglycemia resulting from insulin deficiency, a comparable metabolic crisis may arise from insulin overdose or extreme carbohydrate deprivation, both of which can produce hypoglycemia. Despite differing etiologies, these states share impaired cellular glucose uptake and increased ketone production.

Measurement of blood glucose concentration is the critical first step in differentiating the cause of ketoacidosis. Markedly elevated glucose levels strongly suggest DKA, in which case, insulin therapy is the treatment of choice. Conversely, low glucose levels point to ketoacidosis secondary to insulin overdose or carbohydrate deficiency, where management requires prompt administration of intravenous glucose. Administering insulin in this context would further depress blood glucose concentration and could precipitate seizures, coma, or death.[10][11]

Function

Unlike long-chain fatty acids, ketone bodies readily cross the blood–brain barrier and serve as an energy source for the brain. This capacity makes them essential fuels during prolonged fasting, starvation, or low-carbohydrate intake, when glucose availability is limited. Although most tissues can oxidize fatty acids for energy, the brain is primarily dependent on glucose. During extended fasting, ketone bodies become the principal alternative substrate for cerebral ATP production.[12]

Despite being the site of ketone body synthesis, the liver cannot utilize them for its own energy needs. Hepatocytes lack thiophorase (succinyl-CoA:acetoacetate CoA transferase), the key enzyme required to convert ketone bodies back to acetyl-CoA for entry into the citric acid cycle. Consequently, ketone bodies are produced exclusively for export to peripheral tissues, including the brain, heart, skeletal muscle, and kidneys, where they are reconverted to acetyl-CoA and fully oxidized to generate ATP.[13]

Mechanism

Ketogenesis is a highly regulated metabolic process that adjusts to changing energy demands, particularly during fasting, starvation, or uncontrolled diabetes. Regulation is predominantly hormonal. Insulin functions as the principal inhibitor, whereas several counterregulatory hormones promote ketone body production.

Ketogenesis is stimulated by hormones that enhance lipolysis, including glucagon, cortisol, catecholamines (epinephrine and norepinephrine), and thyroid hormones. These hormones activate hormone-sensitive lipase in adipose tissue, increasing the release of FFAs into the circulation. The elevated supply of FFAs augments hepatic β-oxidation, producing an excess of acetyl-CoA that is subsequently directed into ketone body synthesis.

Despite contributions from other hormones, insulin is the principal hormonal regulator of ketogenesis. In the fed state, elevated insulin concentrations suppress ketone body production by inhibiting lipolysis in adipose tissue, promoting fatty acid synthesis, and stimulating glycolysis and glucose oxidation. In contrast, low insulin levels—characteristic of fasting, starvation, or type 1 diabetes—activate ketogenesis through several coordinated mechanisms.

Reduced insulin signaling decreases inhibition of hormone-sensitive lipase in adipose tissue, enhancing triglyceride breakdown and increasing the release of FFAs into the circulation for hepatic uptake. Within hepatocytes, diminished insulin activity lowers acetyl-CoA carboxylase activation, resulting in decreased malonyl-CoA synthesis. Since malonyl-CoA normally inhibits carnitine palmitoyltransferase 1, its reduction permits greater mitochondrial import of FFAs, thereby accelerating β-oxidation. The resulting rise in mitochondrial acetyl-CoA exceeds the capacity of the tricarboxylic acid cycle, in part because oxaloacetate is diverted toward gluconeogenesis. Excess acetyl-CoA is therefore channeled into ketogenesis, with increased activity of mitochondrial HMG-CoA synthase driving the synthesis of acetoacetate and D-β-hydroxybutyrate.

Testing

The presence and concentration of ketone bodies, the products of ketogenesis, may be assessed using urine or blood testing. These measurements are particularly valuable for diagnosing and monitoring conditions such as DKA and starvation ketosis.

Urinary ketone detection is commonly performed using standard urinalysis test strips, which primarily measure acetoacetate. Results are reported semiquantitatively, ranging from 0 through +4, with higher values indicating progressively greater ketonuria. Since urine ketone measurements reflect past rather than real-time concentrations, they may underestimate the severity of acute ketoacidosis and fail to capture D-β-hydroxybutyrate, which predominates during severe DKA.

Blood-based assays provide a more accurate and timely assessment of ketone status by directly quantifying D-β-hydroxybutyrate and acetone in serum or plasma. Acetone, generated through spontaneous decarboxylation of acetoacetate, is normally present at concentrations below 0.6 mmol/L. Elevated levels may reflect increased ketogenesis due to fasting, carbohydrate restriction, or uncontrolled diabetes. Measurement of D-β-hydroxybutyrate is considered the most reliable indicator in acute settings such as DKA, where it accounts for up to 75% of circulating ketones.

Monitoring ketone levels is critical for diagnosing and managing DKA, as well as differentiating the causes of metabolic acidosis. Measurement is also useful for assessing patients for dietary adherence to ketogenic diets or evidence of starvation, alcoholism, or prolonged vomiting.

Clinical Significance

Excessive ketogenesis can lead to clinically significant complications because of the acidic nature of ketone bodies. The most recognized example is DKA, a potentially life-threatening state in which ketone body production exceeds peripheral utilization.

DKA occurs most commonly in individuals with type 1 diabetes, where insulin deficiency is absolute. The condition may also develop in advanced type 2 diabetes when insulin levels fall to critically low concentrations, although most patients with type 2 diabetes retain sufficient insulin secretion to suppress unchecked ketone production and thereby avoid DKA.

Cellular glucose uptake is severely impaired in the absence of adequate insulin, creating a state of perceived starvation despite marked hyperglycemia. This starvation signal activates gluconeogenesis, which further elevates blood glucose levels. Glucose cannot enter insulin-dependent tissues. Consequently, serum glucose concentrations typically rise well above 250 mg/dL, the diagnostic threshold for DKA, and may reach substantially higher levels.

Reliance on ketogenesis intensifies as carbohydrate stores are depleted, and gluconeogenesis cannot meet metabolic demands. This metabolic shift leads to marked overproduction of ketone bodies, primarily D-β-hydroxybutyrate and acetoacetate, both of which are acidic and drive the development of an anion gap metabolic acidosis.[14][15][16][17]

Patients with DKA typically present with severe dehydration resulting from osmotic diuresis. Hyperglycemia increases the osmotic pressure within the renal tubules, impairs water reabsorption, and produces substantial fluid losses. Neurologic and gastrointestinal manifestations, including confusion, nausea, vomiting, and abdominal pain, are common and may mimic an acute abdomen.

Respiratory compensation manifests as Kussmaul respirations—deep, labored, and rapid breathing that facilitates carbon dioxide clearance and partially corrects the acidosis. Prolonged hyperventilation may ultimately result in respiratory fatigue and distress. A characteristic fruity or nail polish-like odor on the breath reflects acetone accumulation, a volatile byproduct of ketogenesis. Cerebral edema may develop in severe cases, particularly during treatment, as a consequence of rapid shifts in serum osmolality.

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Disclosure: Nader Rahimi declares no relevant financial relationships with ineligible companies.

Disclosure: Sonu Gupta declares no relevant financial relationships with ineligible companies.