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Fructolysis

Fructolysis is the biochemical process by which dietary fructose, a monosaccharide abundant in fruits, honey, and high-fructose corn syrup, is metabolized primarily in the liver to generate energy through conversion into glycolytic intermediates.^[1] This pathway begins with the phosphorylation of fructose to fructose-1-phosphate by the enzyme ketohexokinase (also known as fructokinase), followed by cleavage by aldolase B into dihydroxyacetone phosphate and glyceraldehyde, which are then further processed to enter glycolysis or other metabolic routes such as gluconeogenesis and lipogenesis.^[1] Unlike glycolysis, fructolysis is insulin-independent and bypasses the rate-limiting phosphofructokinase-1 step, allowing for rapid but less regulated metabolism that can deplete ATP and promote triglyceride synthesis under high fructose loads.^[2] Fructose absorption occurs passively in the small intestine via the GLUT5 transporter (SLC2A5), with subsequent uptake into hepatocytes via GLUT2, enabling efficient hepatic processing where approximately 70% of ingested fructose is metabolized.^[2] In extrahepatic tissues like adipose and muscle, fructose is alternatively phosphorylated by hexokinase to fructose-6-phosphate, directly feeding into glycolysis, though this route is less efficient due to hexokinase's preference for glucose.^[1] The process yields two ATP and two NADH per fructose molecule when fully oxidized to pyruvate, similar to glucose, but its unregulated nature links excessive fructolysis to metabolic disorders including obesity, non-alcoholic fatty liver disease, and hyperuricemia through enhanced de novo lipogenesis and uric acid production.^[2] Genetic disruptions, such as aldolase B deficiency in hereditary fructose intolerance, underscore the pathway's critical role, causing toxic fructose-1-phosphate accumulation and severe hypoglycemia upon fructose exposure.^[1] Overall, fructolysis exemplifies how dietary sugars influence systemic metabolism, with implications for nutrition and disease prevention.^[2]

Overview

Definition and Biological Role

Fructolysis is the metabolic process by which fructose, a monosaccharide derived from dietary sources, is catabolized primarily in the liver, with lesser contributions from the small intestine and kidney, to generate triose phosphates that integrate into downstream pathways.^[1] Unlike glucose metabolism via glycolysis, which is subject to regulatory controls, fructolysis proceeds through a distinct entry point that bypasses key rate-limiting steps, enabling unregulated flux into hepatic intermediary metabolism.^[1] This pathway was first delineated in the mid-20th century, with the term "fructolysis" emerging to describe the fructose-specific enzymatic cascade identified in mammalian liver tissues during studies led by Henri-Géry Hers in the 1950s.^[3] The biological role of fructolysis centers on its contribution to rapid ATP production in the liver, independent of phosphofructokinase-1 (PFK-1) regulation, which allows for efficient energy provision during periods of high fructose availability without the feedback inhibition typical of glucose handling.^[1] This unregulated metabolism supports hepatic energy homeostasis by channeling fructose carbons directly into triose intermediates, thereby meeting local ATP demands in hepatocytes.^[4] Furthermore, fructolysis preferentially directs metabolic flux toward de novo lipogenesis rather than glycogen synthesis, promoting triglyceride production and lipid export from the liver, a process that underscores its role in adapting to dietary sugar loads.^[5] The initial step of fructolysis involves the phosphorylation of fructose to fructose-1-phosphate, catalyzed by the enzyme fructokinase (also known as ketohexokinase), as represented by the reaction:

\text{Fructose} + \text{ATP} \rightarrow \text{Fructose-1-phosphate} + \text{ADP}

This ATP-consuming step commits fructose to hepatic catabolism and sets the stage for subsequent cleavage into glycolytic intermediates.^[1]

Dietary Sources and Intake

Fructose occurs naturally in various foods, primarily as free fructose or as a component of sucrose, a disaccharide composed of equal parts glucose and fructose. Key natural sources include fruits such as apples, pears, grapes, and bananas; vegetables like onions, artichokes, and asparagus; and honey, which contains approximately 40% fructose. Sucrose, found in table sugar and many plant-based foods, contributes additional fructose upon digestion. These sources provide fructose in relatively low concentrations compared to processed forms, with whole fruits offering fiber that moderates intake.^[6]^[7]^[8] In modern diets, a significant portion of fructose comes from processed sources, particularly high-fructose corn syrup (HFCS), which is widely used as a sweetener in beverages, snacks, and baked goods due to its cost-effectiveness and stability. HFCS, typically containing 42-55% fructose, is a primary ingredient in soft drinks, fruit-flavored beverages, cereals, and condiments, accounting for about 30% of total fructose intake in the United States. Other processed contributors include sweetened fruit juices, candies, and desserts, where HFCS or sucrose replaces natural sugars.^[9]^[10] Average daily fructose intake in Western diets ranges from 49 to 55 grams, representing approximately 10% of total caloric energy, with added sugars comprising 10-20% of calories and fructose accounting for roughly half of that due to its presence in sucrose and HFCS; however, HFCS consumption peaked in the 1990s-2000s and has since declined, with per capita deliveries at about 39.5 pounds (18 kg) as of 2022.^[11]^[12]^[13] This intake has increased since the 1970s, coinciding with the widespread adoption of HFCS in food production, which rose from negligible levels to over 40% of the sweetener market by the 1990s, driving higher overall sugar consumption.^[14] The World Health Organization recommends limiting free sugars—including those providing fructose—to less than 10% of total energy intake, with a further reduction to below 5% for additional health benefits. Excessive intake exceeding 50 grams per day has been associated with potential metabolic strain, though moderate levels from natural sources are generally well-tolerated.^[15]^[16]

Absorption and Tissue Distribution

Intestinal Absorption Mechanisms

Dietary fructose is primarily absorbed in the jejunum and ileum of the small intestine through enterocytes, entering the bloodstream via the portal vein for subsequent delivery to the liver. The process relies on facilitative diffusion mediated by specific glucose transporter family members, distinct from the sodium-dependent uptake of glucose via SGLT1.^[17] The apical membrane of enterocytes expresses GLUT5 (SLC2A5), a fructose-specific transporter that facilitates Na+-independent uptake of fructose from the intestinal lumen into the cell cytosol. This transporter exhibits high specificity for D-fructose with low affinity for glucose, and its absence in knockout models results in near-complete elimination of transepithelial fructose transport while leaving glucose absorption intact. Once inside the enterocyte, fructose exits across the basolateral membrane primarily via GLUT2 (SLC2A2), a low-affinity, high-capacity facilitative transporter that also handles glucose and galactose, delivering fructose into the portal circulation.^[18]^[17] Fructose absorption is enhanced when co-ingested with glucose, as luminal glucose promotes the recruitment and stabilization of GLUT2 to the apical membrane, increasing overall sugar flux and preventing back-diffusion. This synergistic effect is evident in dietary scenarios where mixed sugars like sucrose (glucose-fructose disaccharide) are consumed, upregulating both GLUT5 and GLUT2 expression. However, the system has limited capacity; in healthy adults, absorption is efficient up to approximately 25 g of fructose per dose, but intakes of 30–50 g often saturate transporters, leading to malabsorption in 60–80% of individuals. Unabsorbed fructose exerts an osmotic effect in the gut, drawing water and causing diarrhea, while the remainder reaches the colon for bacterial fermentation, producing short-chain fatty acids and gases.^[19]^[17] Developmentally, GLUT5 expression is minimal during the suckling phase in mammals, contributing to higher rates of fructose malabsorption in infants (up to 90%). Post-weaning, with the introduction of solid foods containing fructose, GLUT5 mRNA and protein levels surge dramatically—often within hours of exposure—enabling efficient absorption in juveniles and adults; this induction requires luminal fructose and is modulated by hormones like glucocorticoids.^[20]

Tissue-Specific Metabolism

Fructose metabolism is predominantly hepatic, with the liver accounting for approximately 50-70% of the total dietary fructose load due to its high expression of fructokinase (ketohexokinase, KHK) and aldolase B, which facilitate efficient fructolysis.^[21] This prioritization stems from the delivery of absorbed fructose via the portal vein, where hepatic extraction can reach up to 70% of an oral fructose load, far exceeding the 15-30% extraction rate for glucose.^[2] The liver's GLUT2 and GLUT8 transporters further support this rapid uptake, ensuring minimal fructose spillover into systemic circulation under normal conditions.^[21] In the small intestine, fructolysis handles a significant portion of incoming fructose, metabolizing about 90% of low doses (e.g., <0.5 g/kg body weight) and roughly 20% (or 3-5 g) of higher dietary loads through the same key enzymes, KHK-C and aldolase B, expressed in enterocytes.^[22]^[21] This intestinal clearance, mediated by apical GLUT5 uptake, acts as a protective barrier, converting much of the fructose to glucose, lactate, and other metabolites before it reaches the portal vein, thereby limiting the hepatic and systemic fructose burden.^[22] The kidney contributes to minor fructolysis, processing around 10-20% of circulating fructose via GLUT5 and SGLT5 transporters and comparable enzyme expression to the liver and intestine.^[21] This renal metabolism helps maintain fructose homeostasis, with clearance rates averaging about 19% per pass through the kidney.^[23] Metabolism in other tissues is limited; skeletal muscle and adipose tissue exhibit low fructokinase activity and minimal GLUT5 expression, restricting their role to less than 5% under baseline conditions, though inducible pathways may activate during high fructose exposure.^[21] In the brain, fructose is primarily generated endogenously via the polyol pathway—converting glucose to sorbitol and then to fructose—particularly in regions like the hypothalamus under metabolic stress, with specific areas expressing KHK and supporting localized fructolysis.^[24]^[25] Extrahepatic sites such as sperm also utilize GLUT5 for fructose uptake, contributing negligibly to overall clearance.^[1]

Core Biochemical Pathway

Initial Phosphorylation by Fructokinase

The initial committed step in fructolysis is the phosphorylation of fructose to fructose 1-phosphate (F1P), catalyzed by the enzyme ketohexokinase, also known as fructokinase (KHK).^[26] KHK exists in two main isoforms: KHK-A, which is ubiquitously expressed with low affinity for fructose (Km approximately 5-7 mM), and KHK-C, which is predominantly expressed in the liver with high affinity (Km ~0.5 mM), enabling efficient fructose processing at physiological concentrations.^[27]^[28] In the liver, KHK-C predominates and drives the majority of fructose metabolism under typical dietary conditions.^[29] The reaction proceeds as fructose + ATP → F1P + ADP, which is irreversible due to the high energy barrier for dephosphorylation and occurs rapidly without allosteric regulation.^[26] This phosphorylation takes place in the cytosol of hepatocytes, where KHK is localized, facilitating immediate sequestration of incoming fructose.^[30] The accumulation of F1P effectively traps fructose intracellularly, as the charged phosphate group prevents its efflux across the plasma membrane, committing it to further metabolism. F1P subsequently serves as the substrate for cleavage in the downstream pathway.^[26] KHK exhibits a high maximum velocity (Vmax ~10 μmol/min per gram of liver tissue), allowing rapid fructose clearance but risking transient ATP depletion in hepatocytes when fructose intake overwhelms downstream glycolytic capacity.^[27] This unregulated kinetics underscores the potential for metabolic imbalance during excessive fructose consumption, as ATP hydrolysis outpaces replenishment.^[29]

Cleavage by Aldolase B and Triose Formation

In the fructolysis pathway, the second major step following phosphorylation involves the cleavage of fructose-1-phosphate (F1P) by aldolase B, a liver-predominant isoform of fructose-1,6-bisphosphate aldolase (ALDOB). This enzyme is highly expressed in the liver, kidney, and small intestine, where it facilitates the breakdown of F1P, distinguishing fructolysis from glycolysis by operating in these specific tissues.^[2] Aldolase B catalyzes the reversible aldol cleavage of F1P into dihydroxyacetone phosphate (DHAP) and glyceraldehyde, without requiring additional ATP investment—a key contrast to the glycolytic aldolase reaction on fructose-1,6-bisphosphate, which follows two prior ATP-consuming phosphorylations. The reaction can be represented as:

\text{Fructose-1-phosphate} \rightleftharpoons \text{[Dihydroxyacetone phosphate](/page/Dihydroxyacetone_phosphate)} + \text{[Glyceraldehyde](/page/Glyceraldehyde)}

This step generates equal proportions of the triose intermediates.^[1]^[2] The non-phosphorylated glyceraldehyde produced is subsequently phosphorylated at the expense of ATP by triokinase (also known as triose kinase or TKFC) to form glyceraldehyde-3-phosphate (G3P), enabling its integration into downstream metabolism. This phosphorylation proceeds as:

\text{[Glyceraldehyde](/page/Glyceraldehyde)} + \text{ATP} \rightarrow \text{Glyceraldehyde-3-phosphate} + \text{[ADP](/page/ADP)}

Thus, both trioses—DHAP and G3P—become available as glycolytic intermediates.^[2] The significance of this cleavage lies in its role as a branch point that bypasses the regulated phosphofructokinase-1 (PFK-1) step of glycolysis, allowing unregulated flux of fructose-derived carbons into central metabolism and potentially favoring lipogenesis under high fructose loads. By producing 50% DHAP and 50% G3P directly, it ensures balanced entry into gluconeogenic or glycolytic pathways without the allosteric controls that limit glucose utilization.^[2]

Relation to Glycolysis

Bypass of Regulatory Steps

Fructolysis in the liver bypasses several key regulatory steps of glycolysis, including the phosphorylation by glucokinase, the isomerization by phosphoglucose isomerase, and the committed step catalyzed by phosphofructokinase-1 (PFK-1). Instead of entering the glycolytic pathway at glucose-6-phosphate, fructose is rapidly phosphorylated by fructokinase to form fructose-1-phosphate (F1P), which is then cleaved by aldolase B into dihydroxyacetone phosphate (DHAP) and glyceraldehyde. These triose intermediates directly join the lower glycolytic pathway, evading the upper regulatory gates.^[1]^[31] PFK-1, the primary rate-limiting enzyme in glycolysis, is subject to allosteric regulation by activators such as AMP and fructose-2,6-bisphosphate, as well as inhibitors like citrate and ATP, which fine-tune flux based on cellular energy status; this control is entirely circumvented in fructolysis.^[2] The absence of these regulatory checkpoints allows fructolysis to proceed with minimal feedback inhibition, enabling a high, unregulated flux of carbon into downstream metabolism. Unlike glucose, whose breakdown is modulated to match energy demands, fructose metabolism lacks inhibition by end products such as pyruvate, acetyl-CoA, ATP, or citrate, potentially leading to excessive substrate accumulation. Rapid F1P formation sequesters inorganic phosphate and depletes ATP, as fructokinase operates without the product inhibition seen in hexokinases, which can indirectly limit the process through substrate availability once ATP levels drop. This contrasts sharply with glucose metabolism, where PFK-1 gating prevents overload by aligning glycolytic rate with energetic needs, thereby maintaining homeostasis.^[1]^[31]^[2] Experimental evidence from isotopic tracing studies supports this bypass mechanism. Using 13C-labeled fructose in human subjects, researchers observed that fructose carbons preferentially appear in lower glycolytic intermediates like lactate and pyruvate, with minimal incorporation into upper pathway markers, indicating direct entry distal to PFK-1 without activating regulatory controls. For instance, during oral fructose administration, approximately 28% of the label was recovered in lactate, reflecting unregulated flux into the triose pool, while studies combining fructose with exercise showed 55-60% conversion to circulating glucose via lower pathway integration. These findings highlight how fructolysis floods the distal glycolytic segment, bypassing the energy-responsive upper controls inherent to glucose processing.^[32]^[33]

Integration with Glycolytic Intermediates

The products of fructolysis, dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (G3P), integrate directly into the glycolytic pathway at the triose phosphate level, allowing fructose-derived carbons to converge with those from glucose metabolism. DHAP enters glycolysis immediately and is rapidly isomerized to G3P by triose phosphate isomerase (TPI), an enzyme that catalyzes the reversible interconversion without energy expenditure. G3P, produced from the phosphorylation of glyceraldehyde by triokinase (which consumes one ATP), joins the pathway at the same point, proceeding through subsequent steps including oxidation by glyceraldehyde-3-phosphate dehydrogenase (GAPDH) to 1,3-bisphosphoglycerate, followed by substrate-level phosphorylation at phosphoglycerate kinase and pyruvate kinase.^[2]^[3] This convergence enables both triose phosphates to advance to pyruvate via enolase and the lower glycolytic reactions, yielding a net energy production comparable to that of glucose. The initial phosphorylation steps in fructolysis—fructokinase and triokinase—consume 2 ATP to generate DHAP and G3P from one fructose molecule, while the lower glycolytic phase from two triose phosphates produces 4 ATP through substrate-level phosphorylation, resulting in a net yield of 2 ATP per fructose, alongside 2 NADH. The fructose carbons thus distribute equivalently to pyruvate, which can enter the tricarboxylic acid (TCA) cycle as acetyl-CoA for oxidative phosphorylation or be directed toward gluconeogenesis from phosphoenolpyruvate or oxaloacetate intermediates.^[34]^[2] The overall process can be summarized as follows:

\text{Fructose} + 2 \text{ ATP} \rightarrow \text{DHAP} + \text{G3P} + 2 \text{ ADP} + 2 \text{ P}_i

\text{DHAP (isomerized to G3P)} + \text{G3P} + 2 \text{ NAD}^+ + 4 \text{ ADP} + 2 \text{ P}_i \rightarrow 2 \text{ Pyruvate} + 2 \text{ NADH} + 2 \text{ H}^+ + 4 \text{ ATP}

Net: \text{Fructose} \rightarrow 2 \text{ Pyruvate} + 2 \text{ ATP} + 2 \text{ NADH} + 2 \text{ H}^+. Although the primary flux proceeds to pyruvate, a minor portion of DHAP can be diverted to glycerol-3-phosphate via reduction by cytosolic glycerol-3-phosphate dehydrogenase (GPD1), using NADH as a cofactor, thereby linking fructolysis to lipid precursor synthesis without further glycolytic progression.^[34]^[2]^[35]

Downstream Metabolic Products

Glycogen Synthesis Pathways

In the liver, the triose phosphates dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (G3P), produced from fructose via aldolase B cleavage, serve as key intermediates for glycogen synthesis.^[1] These trioses undergo interconversion via triose phosphate isomerase and are then condensed by aldolase to form fructose-1,6-bisphosphate, which is dephosphorylated by fructose-1,6-bisphosphatase-1 (FBPase-1) to fructose-6-phosphate. This is followed by isomerization to glucose-6-phosphate (G6P), effectively bypassing the regulatory phosphofructokinase-1 step of glycolysis.^[1] The resulting G6P is directly utilized for glycogen synthesis without involvement of glucose-6-phosphatase, as the enzyme is not required for intracellular storage. G6P is converted to UDP-glucose by UDP-glucose pyrophosphorylase, and glycogen synthase then catalyzes the addition of glucose units from UDP-glucose to the growing glycogen chain. Insulin enhances this process by promoting the dephosphorylation and activation of glycogen synthase through signaling pathways involving protein phosphatase-1.^[1]^[36] When hepatic glycogen stores are low, up to 50% of ingested fructose may be directed toward hepatic glycogen synthesis, though under typical fed conditions this is lower (around 10-25%) due to a metabolic preference for lipogenic pathways with fructose. Postprandially, 10-15% of fructose-derived carbons are incorporated into liver glycogen within a few hours, supporting energy homeostasis during the post-meal period.^[36]^[37]

Lipogenesis and Triglyceride Formation

In the liver, a primary site of fructolysis, the triose phosphates generated from fructose metabolism—dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (G3P)—serve as key precursors for lipogenesis and triglyceride (TAG) synthesis. DHAP is reduced to glycerol-3-phosphate by the enzyme glycerol-3-phosphate dehydrogenase, providing the essential glycerol backbone for TAG assembly. This backbone is then esterified with fatty acids through acyltransferases, such as acyl-CoA:diacylglycerol acyltransferase (DGAT), to form triglycerides stored in hepatocytes or secreted as very low-density lipoproteins (VLDL).^[38]^[5] G3P contributes to the carbon pool for fatty acid synthesis by being converted to pyruvate via glycolytic enzymes, followed by decarboxylation to acetyl-CoA by the pyruvate dehydrogenase (PDH) complex. This acetyl-CoA enters the mitochondria, where it is transported to the cytosol as citrate and cleaved to regenerate acetyl-CoA, which is carboxylated to malonyl-CoA by acetyl-CoA carboxylase (ACC). Malonyl-CoA then serves as the building block for fatty acid elongation and desaturation by fatty acid synthase (FAS), ultimately producing palmitate and longer-chain fatty acids that are incorporated into TAGs. The overall pathway thus diverts fructolytic intermediates from fructose through triose phosphates, pyruvate, and PDH to acetyl-CoA, malonyl-CoA, palmitate, and TAG assembly, distinguishing fructose metabolism from glucose by its unregulated flux into these lipogenic routes.^[38]^[5]^[39] Studies indicate that up to 30% of ingested fructose can be converted to lipids via de novo lipogenesis (DNL) in the liver, particularly under conditions of high intake, with the remainder partitioned to other fates like oxidation or export as lactate. For instance, the synthesis of one palmitate molecule (16 carbons) theoretically requires carbons derived from approximately four fructose molecules (each providing 4 carbons to acetyl-CoA via trioses, after decarboxylation), highlighting the multi-substrate nature of this process. High fructose consumption promotes DNL over fatty acid oxidation by generating excess malonyl-CoA, which allosterically inhibits carnitine palmitoyltransferase 1 (CPT1), thereby limiting mitochondrial beta-oxidation and favoring lipid accumulation.^[5]^[40]^[41]

Regulation and Physiological Control

Enzymatic and Hormonal Regulation

Fructokinase, the enzyme catalyzing the initial phosphorylation of fructose to fructose 1-phosphate (F1P) in the liver, kidney, and intestine, operates without strong allosteric regulation but is constrained by cellular ATP/ADP ratios, as the reaction consumes ATP to drive fructose trapping and metabolism. High fructose loads rapidly deplete ATP due to this kinase activity, generating AMP and limiting further phosphorylation until ATP is replenished. This ATP depletion also activates AMP-activated protein kinase (AMPK), which inhibits lipogenic pathways as a feedback mechanism. The liver-predominant isoform C (KHK-C) exhibits high affinity for fructose and minimal feedback sensitivity, promoting swift metabolism and ATP depletion upon fructose exposure, whereas the ubiquitous isoform A (KHK-A) metabolizes fructose more slowly in extrahepatic tissues, effectively sequestering substrate and mitigating hepatic overload from isoform C activity. Aldolase B, responsible for cleaving F1P into dihydroxyacetone phosphate and glyceraldehyde, displays substrate-specific kinetics suited to fructose metabolism, with F1P serving as an efficient activator at physiological concentrations to facilitate triose production. Additionally, aldolase B exhibits pH sensitivity, maintaining optimal activity within the physiological range of 5.5 to 7.8, beyond which fructose cleavage efficiency declines, linking its function to intracellular acidification during metabolic stress. Hormonal influences on fructolysis primarily involve insulin and glucagon, which modulate substrate availability and competing pathways. Insulin does not directly regulate fructose uptake via GLUT2, which is constitutively expressed in hepatocytes, but enhances downstream lipogenic pathways. In contrast, glucagon inhibits fructolytic flux via cAMP signaling, which promotes gluconeogenesis from triose intermediates while suppressing glycolytic conversion of fructose-derived products to lactate, effectively competing for shared downstream metabolites like glyceraldehyde 3-phosphate. Triokinase (also known as glyceraldehyde kinase), which phosphorylates glyceraldehyde to glyceraldehyde 3-phosphate, controls the flux of this triose into lipogenic or glycolytic routes. The low Michaelis constant (Km ≈ 0.5 mM) of fructokinase for fructose ensures its metabolic priority over glucose in mixed-sugar environments, as hepatic hexokinase exhibits a much higher Km for fructose (exceeding 10 mM), rendering it inefficient for fructose phosphorylation at physiological concentrations. This kinetic advantage allows rapid clearance of fructose from circulation, maintaining homeostasis during dietary loads containing both sugars.

Fructose-Induced Lipogenic Enzyme Expression

Fructose metabolism in the liver initiates a transcriptional program that upregulates lipogenic enzymes through the activation of carbohydrate response element-binding protein (ChREBP). Upon ingestion, fructose is rapidly phosphorylated by fructokinase to form fructose-1-phosphate (F1P), which serves as a key allosteric activator of ChREBP, promoting its nuclear translocation and binding to carbohydrate response elements in the promoters of target genes.^[42] This activation leads to the transcription of key lipogenic enzymes, including acetyl-CoA carboxylase (ACC), fatty acid synthase (FAS), and stearoyl-CoA desaturase-1 (SCD-1), thereby enhancing de novo lipogenesis.^[42] The process is particularly pronounced in the liver, where fructokinase expression is high, contrasting with glucose, which activates ChREBP more modestly through intermediates like glucose-6-phosphate without the potent F1P-mediated effect.^[2] In parallel, fructose metabolites such as dihydroxyacetone phosphate (DHAP) contribute to the upregulation of sterol regulatory element-binding protein-1c (SREBP-1c), another major transcription factor for lipogenic genes. DHAP, generated from F1P cleavage by aldolase B, amplifies insulin signaling pathways in hepatocytes, facilitating SREBP-1c processing and nuclear entry to co-activate genes like those encoding ACC and FAS.^[2] This synergistic interaction with insulin is evident even in models with impaired insulin receptor function, indicating a partial insulin-independent component driven by fructose flux.^[2] The liver-specific nature of this regulation underscores fructose's role in hepatic lipid homeostasis, differing from glucose's broader systemic effects. The temporal dynamics of this induction show rapid mRNA upregulation for lipogenic enzymes within 3-6 hours following an acute fructose load, with sustained elevation observed during chronic consumption.^[42] In rodent models fed a 20% fructose diet, hepatic expression of ChREBP target enzymes such as FAS and ACC increases 2- to 5-fold, correlating with enhanced lipid synthesis capacity.^[2] These adaptations help mitigate fructose-induced metabolic stress in the short term but contribute to long-term lipid accumulation when intake is excessive.

Pathological and Clinical Aspects

Inborn Errors of Fructolysis

Inborn errors of fructolysis are rare genetic disorders resulting from deficiencies in enzymes critical to fructose metabolism, primarily affecting the liver, kidney, and intestine. These conditions disrupt the sequential phosphorylation and cleavage of fructose, leading to metabolite accumulation or excretion, with varying clinical severity from benign to life-threatening. The most well-characterized defects involve fructokinase, aldolase B, and triokinase, each altering distinct steps in the pathway.^[43]^[44]^[45] Essential fructosuria, also known as fructokinase deficiency, is an autosomal recessive disorder caused by mutations in the KHK gene encoding ketohexokinase (fructokinase). This enzyme catalyzes the initial phosphorylation of fructose to fructose-1-phosphate (F1P) in the liver. In its absence, F1P is not formed, resulting in benign urinary excretion of unmetabolized fructose following fructose ingestion, without accumulation of toxic intermediates. The condition is asymptomatic and harmless, with no dietary restrictions required, as it does not impair overall energy metabolism or cause long-term complications. Its estimated incidence is 1 in 130,000 individuals worldwide.^[43]^[46]^[31] Hereditary fructose intolerance (HFI) arises from biallelic mutations in the ALDOB gene, which encodes aldolase B, the enzyme responsible for cleaving F1P into dihydroxyacetone phosphate and glyceraldehyde in the liver, kidney, and small intestine. Over 50 pathogenic variants have been identified, leading to reduced or absent enzyme activity and accumulation of F1P. This sequesters inorganic phosphate and depletes ATP, causing severe metabolic disturbances including hypoglycemia, lactic acidosis, hypophosphatemia, and hyperuricemia upon fructose exposure. Clinical manifestations typically emerge in infancy during weaning, with symptoms such as vomiting, abdominal pain, failure to thrive, and aversion to sweet-tasting foods; chronic exposure can result in liver and kidney damage, potentially progressing to failure or death if untreated. HFI has an incidence of approximately 1 in 20,000 to 100,000 births, varying by population, with higher rates in some European groups due to founder effects.^[44]^[47]^[48]^[49]^[50] Triokinase deficiency, resulting from biallelic variants in the TKFC gene encoding triokinase/FMN cyclase, is an extremely rare autosomal recessive disorder that impairs the phosphorylation of glyceraldehyde (a product of F1P cleavage) to glyceraldehyde-3-phosphate, hindering the integration of fructose-derived carbons into glycolysis and gluconeogenesis. First described in 2020, it manifests as a multisystem syndrome with congenital cataracts, developmental delay, failure to thrive, liver dysfunction, and variable neurological features, reflecting disrupted endogenous fructose metabolism and FMN homeostasis. The condition's rarity limits precise incidence data, but it underscores the pathway's role beyond dietary fructose processing.^[45]^[51] Diagnosis of these disorders relies on clinical suspicion triggered by symptoms following fructose exposure, supported by biochemical tests showing elevated plasma or urinary fructose (in essential fructosuria) or metabolic derangements like hypoglycemia and hyperuricemia (in HFI). Confirmatory methods include enzyme activity assays on liver biopsy tissue, which demonstrate reduced aldolase B function in HFI, and molecular genetic testing to identify pathogenic variants, particularly in the ALDOB gene for HFI or KHK for essential fructosuria; sequencing of TKFC is used for suspected triokinase deficiency. Genetic testing is preferred for its non-invasiveness and ability to guide family counseling.^[47]^[49]^[44]

Associations with Metabolic Disorders

Dysregulated fructolysis promotes unregulated de novo lipogenesis in the liver, leading to hepatic steatosis and insulin resistance through the rapid phosphorylation of fructose to fructose-1-phosphate, which bypasses phosphofructokinase-1 regulation and floods glycolytic intermediates into lipogenic pathways.^[40] This process activates transcription factors such as carbohydrate-responsive element-binding protein (ChREBP), enhancing expression of lipogenic enzymes like acetyl-CoA carboxylase and fatty acid synthase.^[40] Studies from 2021 to 2025, including those on ChREBP-β activation by fructose-1-phosphate, demonstrate hyperactivity of ChREBP in non-alcoholic fatty liver disease (NAFLD), correlating with increased hepatic triglyceride accumulation and impaired insulin signaling.^[40]^[52] High-fructose diets exacerbate obesity and metabolic syndrome by elevating very low-density lipoprotein-triglyceride (VLDL-TAG) export from the liver and promoting visceral fat deposition, as fructose-derived lipids are preferentially stored in abdominal adipose tissue.^[53] High-fructose diets have been associated with an increased risk of metabolic syndrome components, including dyslipidemia and central obesity, independent of total calorie consumption.^[53] These effects stem from fructose-induced hepatic lipid overproduction, which drives systemic inflammation and further insulin resistance in adipose and muscle tissues.^[53] Fructose metabolism during fructolysis accelerates purine degradation by depleting ATP via fructokinase activity, resulting in increased AMP deaminase activation and subsequent uric acid production, which contributes to hyperuricemia.^[54] This hyperuricemia impairs endothelial function by reducing nitric oxide bioavailability and promoting oxidative stress, thereby linking dysregulated fructolysis to vascular complications.^[54] Beyond these, fructose involvement in the polyol pathway generates endogenous fructose that exacerbates gout through sustained hyperuricemia and contributes to cardiovascular disease via endothelial inflammation and atherosclerosis progression.^[55] In the brain, fructose metabolism disrupts hippocampal insulin signaling and synaptic plasticity, leading to cognitive impairments such as memory deficits, as highlighted in 2025 reviews on chronic high-fructose effects.^[56] Clinical interventions restricting dietary fructose have shown efficacy in mitigating these associations, with a trial demonstrating approximately 10% reduction in liver fat content in adolescent boys with NAFLD after 8 weeks of isocaloric dietary sugar restriction, alongside decreased de novo lipogenesis and improved insulin kinetics.^[57] Emerging pharmacological approaches, such as ketohexokinase (KHK) inhibitors, have demonstrated reductions in liver fat and inflammatory markers in adults with NAFLD in phase 2 trials as of 2024.^[58]

References

[1]
Biochemistry, Fructose Metabolism - StatPearls - NCBI Bookshelf - NIH
Oct 17, 2022 · Fructose is an abundant monosaccharide in the human diet that the body needs to metabolize. It is present in honey, fruits, vegetables, and high-fructose corn ...
[2]
Fructose metabolism and metabolic disease - PMC - PubMed Central
Here we will review the biochemistry and molecular genetics of fructose metabolism as well as potential mechanisms by which excessive fructose consumption ...
[3]
Molecular Aspects of Fructose Metabolism and Metabolic Disease
We review the biochemistry, genetics and physiology of fructose metabolism and consider mechanisms by which excessive fructose consumption may contribute to ...
[4]
Molecular aspects of fructose metabolism and metabolic disease
Dec 7, 2021 · Upon transport into the cell, fructose is metabolized by a cascade of three “fructolytic” enzymes, originally delineated by Henri Hers during ...
[5]
Fructose drives de novo lipogenesis affecting metabolic health - PMC
There is evidence from both human and animal studies that fructose is a more potent inducer of hepatic lipogenesis than glucose.
[6]
Fructose intolerance: Which foods to avoid? - Mayo Clinic
Fructose is a sugar found naturally in fruits, fruit juices, some vegetables and honey. Table sugar, called sucrose, also has fructose. High-fructose corn ...
[7]
Top 10 Foods Highest in Fructose
Nov 10, 2024 · Top foods high in fructose include fruit juices (grape), sugary drinks (cola), dried fruit (sweetened cranberries), and honey.List of High Fructose Foods · More Fructose Rich Foods
[8]
Is fructose bad for you? Benefits, risks, and other sugars
Fructose is a very sweet, naturally occurring caloric sweetener. It can come from fruits, fruit juices, honey, and even some vegetables. Pure fructose is also ...
[9]
Dietary Fructose Consumption Among US Children and Adults
Overall, the most important contributors of fructose were sugar-sweetened beverages, 30.1%; grains (which includes processed foods, such as cakes, pies, and ...
[10]
What are some foods high in fructose?
Mar 31, 2023 · High fructose foods include honey, agave syrup, fruit juices, dried fruit, grapes, figs, apples, pears, and some imported sweet drinks.
[11]
The Role of Fructose as a Cardiovascular Risk Factor: An Update
Jan 12, 2022 · The general intake in a Western diet is of 49 g fructose/day, only 8 g being from natural sources [7]. The metabolization process mainly ...
[12]
Dietary fructose consumption among US children and adults
Aug 6, 2025 · The mean consumption of fructose was estimated to be 54.7g/day (range, 38.4-72.8) and accounted for 10.2% of total caloric intake. Consumption ...<|separator|>
[13]
National Estimates of Dietary Fructose Intake Increased from 1977 to ...
The high-fructose corn syrup percentage of sweeteners increased from 16% in 1978 to 42% in 1998 and then stabilized. Since 1978, mean daily intakes of added and ...
[14]
Reducing free sugars intake in adults to reduce the risk of ...
Aug 9, 2023 · In both adults and children, WHO recommends reducing the intake of free sugars to less than 10% of total energy intake. WHO suggests a further ...
[15]
Health implications of fructose consumption: A review of recent data
It has been considered that moderate fructose consumption of ≤50g/day or ~10% of energy has no deleterious effect on lipid and glucose control and of ≤100g/day ...
[16]
Intestinal Absorption of Fructose - PMC - PubMed Central
New insights in our knowledge of intestinal fructose absorption mediated by the facilitative glucose transporter GLUT5 in the apical membrane and by GLUT2 in ...
[17]
Slc2a5 (Glut5) Is Essential for the Absorption of Fructose in the ... - NIH
We conclude that Glut5 is essential for the absorption of fructose in the intestine and plays a fundamental role in the generation of fructose-induced ...
[18]
Simple-sugar meals target GLUT2 at enterocyte apical membranes ...
Dietary glucose or fructose manipulation affects facilitative-transporter mRNA expression. Sugar-rich diets efficiently upregulate intestinal GLUT5 and GLUT2 ...
[19]
Regulation of the fructose transporter GLUT5 in health and disease
GLUT2 in a bidirectional manner is involved mainly in fructose uptake across the hepatic plasma membrane into the liver (154) and in the basolateral membrane of ...
[20]
Tissue-Specific Fructose Metabolism in Obesity and Diabetes - PMC
The primary sites of fructose metabolism are the liver, intestine, and kidney. Skeletal muscle and adipose tissue can also metabolize a large portion of ...
[21]
The Small Intestine Converts Dietary Fructose into Glucose and ...
We find that small intestine plays a major role in dietary fructose metabolism, converting fructose to glucose and other circulating metabolites. In this manner ...<|control11|><|separator|>
[22]
Fructose Production and Metabolism in the Kidney - PubMed Central
Apr 6, 2020 · They found that decreases in fructose concentration on the passage of blood through the kidney—about 19% on average—were accompanied by ...Missing: percentage | Show results with:percentage
[23]
The human brain produces fructose from glucose - PMC
Feb 23, 2017 · Our findings suggest that the polyol pathway contributes to endogenous CNS production of fructose and that the effects of fructose in the CNS ...
[24]
Specific regions of the brain are capable of fructose metabolism - PMC
Liver and kidneys are responsible for metabolism of 40–60% of ingested fructose, while the physiological fate of the remaining fructose remains poorly ...
[25]
Ketohexokinase-Dependent Metabolism of Fructose Induces ... - NIH
In the liver, as well as in the kidney, fructose is phosphorylated by fructokinase (ketohexokinase [KHK]) to yield fructose-1-phosphate followed by cleavage by ...
[26]
INBORN ERRORS OF FRUCTOSE METABOLISM - Annual Reviews
Fructokinase has a high affinity for fructose (Km is - 0.5 roM) and a high Vmax (- 10 JlmoVmin per gram of liver at 37°C). Recently, a rat liver fructokinase ...
[27]
Both isoforms of ketohexokinase are dispensable for normal growth ...
The isoforms have comparable maximum catalytic activities, but only Khk-C appears adapted for dietary fructose clearance (Km = 0.8 mM), while Khk-A has a 10- ...
[28]
Opposing effects of fructokinase C and A isoforms on fructose ...
Fructokinase C is expressed primarily in liver, intestine, and kidney and has high affinity for fructose, resulting in rapid metabolism and marked ATP depletion ...Missing: Vmax | Show results with:Vmax
[29]
Ketohexokinase: Expression and Localization of the Principal ...
The strong KHK immunostaining in hepatocytes is consistent with the abundance of fructokinase activity in the liver (Weiser and Quill 1975; Raushel and Cleland ...
[30]
Fructose: A Key Factor in the Development of Metabolic Syndrome ...
The consequences of uncontrolled fructose metabolism can be harmful at the cellular level resulting in intracellular ATP depletion, increased uric acid ...<|control11|><|separator|>
[31]
Fructose metabolism in humans – what isotopic tracer studies tell us
Oct 2, 2012 · When fructose was ingested together with glucose, the mean oxidation rate of the mixed sugars increased to 66.0% ± 8.2 in exercising subjects.
[32]
https://pmc.ncbi.nlm.nih.gov/articles/PMC3533803/
[33]
Fructose Metabolism from a Functional Perspective: Implications for ...
In the liver, however, fructolysis consumes 2 ATP and conversion to pyruvate produces 4 ATP, resulting in 2 ATP gained. In contrast, in skeletal muscle, 2 ...Missing: definition | Show results with:definition
[34]
Dihydroxyacetone Phosphate - an overview | ScienceDirect Topics
Dihydroxyacetone phosphate is converted to glycerol 3-phosphate by glycerol-3-phosphate dehydrogenase. This provides a source of glycerol 3-phosphate for ...
[35]
Fructose and metabolic health: governed by hepatic glycogen status?
Fructose interacts with various transcriptional and allosteric (enzymatic) processes along the pathways of glycogen synthesis, glycolysis and DNL within the ...
[36]
Fructose Metabolism from a Functional Perspective: Implications for ...
When fructose is converted into glucose in the liver it consumes 2 ATP. When this newly synthesized glucose is subsequently oxidized in skeletal muscle, the ...
[37]
The sweet path to metabolic demise: fructose and lipid synthesis
Oct 1, 2017 · This review will focus on the biology of fructose. Specifically, we will consider the effects of fructose on metabolic health, how fructose ...
[38]
https://pmc.ncbi.nlm.nih.gov/articles/PMC5035631/
[39]
The Contribution of Dietary Fructose to Non-alcoholic Fatty Liver ...
Recent studies suggest that excessive fructose intake is a driving force in non-alcoholic fatty liver disease (NAFLD).
[40]
Role of Dietary Fructose and Hepatic De Novo Lipogenesis in Fatty ...
Feb 8, 2016 · In summary, fructose metabolism supports DNL more strongly than HFD and hepatic DNL is a central abnormality in NAFLD. Disrupting fructose ...
[41]
Lipogenic transcription factor ChREBP mediates fructose-induced ...
Jun 19, 2017 · We uncovered an intricate pathway through which ChREBP activates de novo lipogenesis but suppresses the UPR and cholesterol biosynthesis in ...
[42]
Entry - #229800 - FRUCTOSURIA, ESSENTIAL - OMIM - (OMIM.ORG)
Essential fructosuria is a benign, asymptomatic defect of intermediary metabolism characterized by the intermittent appearance of fructose in the urine.
[43]
229600 - FRUCTOSE INTOLERANCE, HEREDITARY; HFI - OMIM
Clinical features include recurrent vomiting, abdominal pain, and hypoglycemia that may be fatal. Long-term exposure to fructose can result in liver failure, ...
[44]
Entry - #618805 - TRIOKINASE AND FMN CYCLASE DEFICIENCY ...
Triokinase and FMN cyclase deficiency syndrome (TKFCD) is a multisystem disease with marked clinical variability, even intrafamilially.
[45]
Prevalence and cardiometabolic correlates of ketohexokinase gene ...
Feb 23, 2021 · Its prevalence has been estimated to be 1:130,000 [2]. However, given that EF is asymptomatic, presumed benign, and does not require treatment, ...
[46]
Hereditary Fructose Intolerance - GeneReviews® - NCBI Bookshelf
Dec 17, 2015 · Untreated hereditary fructose intolerance (HFI) is characterized by metabolic disturbances (hypoglycemia, lactic acidemia, hypophosphatemia, hyperuricemia, ...
[47]
Increased prevalence of mutant null alleles that cause hereditary ...
Mutations in the aldolase B gene (ALDOB) impairing enzyme activity toward fructose-1-phosphate cleavage cause hereditary fructose intolerance (HFI).
[48]
Hereditary Fructose Intolerance - StatPearls - NCBI Bookshelf - NIH
The intracellular trapping of phosphate and accumulation of fructose-1-phosphate is responsible for many of the manifestations of the disease. The depletion ...
[49]
Hereditary fructose intolerance: MedlinePlus Genetics
Jun 1, 2011 · After ingesting fructose, individuals with hereditary fructose intolerance may experience nausea, bloating, abdominal pain, diarrhea, vomiting, ...
[50]
Bi-allelic Variants in TKFC Encoding Triokinase/FMN Cyclase Are ...
Feb 6, 2020 · We postulate that deficiency of TKFC causes disruption of endogenous fructose metabolism leading to generation of by-products that can cause cataract.
[51]
https://www.sciencedirect.com/science/article/pii/S0002929720300057
[52]
Fructose metabolism and metabolic disease - JCI
Feb 1, 2018 · FGF21 is a liver-derived hormone that regulates energy, glucose, and lipid homeostasis and may also participate in a feedback mechanism ...
[53]
Fructose and Uric Acid: Major Mediators of Cardiovascular Disease ...
This reaction induces ATP depletion and causes the activation of AMP deaminase, purine degradation and uric acid generation. In addition, fructose generates ...
[54]
Fructose metabolism, cardiometabolic risk, and the epidemic of ...
Sep 7, 2017 · The primary organs capable of metabolizing fructose include liver, small intestine, and kidneys. In these organs, fructose metabolism is ...
[55]
A Deep Insight Into Fructose Metabolism and Its Effects on Appetite ...
Apr 21, 2025 · The review also explores fructose's impact on the brain, with a focus on the hippocampus, a key region for memory and learning. Chronic high ...
[56]
Dietary sugar restriction reduces hepatic de novo lipogenesis ... - JCI
Dec 15, 2021 · Dietary sugar restriction over 8 weeks decreased de novo lipogenesis (DNL) and hepatic fat. The change in DNL correlated with changes in insulin and weight.