What is Ketohexokinase?

Ketohexokinase is the enzyme that performs the first committed step in the body’s principal fructose-metabolism pathway. It transfers a phosphate from ATP to fructose, producing fructose-1-phosphate:

Fructose + ATP → fructose-1-phosphate + ADP

The International Union of Biochemistry and Molecular Biology assigns this mammalian enzyme the accepted name ketohexokinase and the identifier EC 2.7.1.3. “Fructokinase” remains a common synonym in medicine and nutrition, although the formal enzyme catalogue also uses that word for a different enzyme, EC 2.7.1.4, that produces fructose-6-phosphate. On this page, fructokinase means human ketohexokinase, or KHK. IUBMB Enzyme Nomenclature

KHK is not simply another sugar-processing enzyme. By converting fructose to fructose-1-phosphate, it traps fructose inside the cell and commits it to fructolysis. That makes KHK the gateway through which both dietary fructose and fructose made inside the body enter the same metabolic pathway.

Why KHK Makes Fructose Different From Glucose

Glucose is a broadly available fuel. Its early metabolism is governed by feedback controls that respond to the cell’s energy state.

Fructose enters through a different door. KHK does not share one of glycolysis’s most important feedback brakes. When a sufficiently large amount of fructose arrives quickly, KHK can continue phosphorylating it while ATP and free phosphate fall. This does not mean that every bite of fruit “drains” a cell. Dose, speed, tissue and metabolic context matter. But the reaction gives fructose a cellular-energy signature that glucose does not share.

The sequence is worth remembering:

1. KHK uses ATP to convert fructose into fructose-1-phosphate.
2. Phosphate is temporarily trapped in fructose-1-phosphate.
3. ATP can fall when fructose is processed rapidly.
4. AMP can then be broken down through the purine pathway.
5. Uric acid rises as one downstream product.
6. Fructose carbon and signaling also promote pathways involved in fat synthesis and energy storage.

The Phosphate Event Is Not Merely Theoretical

This first reaction happens quickly. ATP supplies the phosphate attached to fructose, and that phosphate remains temporarily sequestered in fructose-1-phosphate until the pathway catches up. If fructose enters faster than downstream metabolism can clear it, available phosphate and ATP can fall while AMP breakdown accelerates.

Researchers have watched this happen in the human liver. In one study, healthy volunteers drank 75 grams of fructose while phosphorus magnetic-resonance spectroscopy tracked their liver-energy reserves. Average hepatic ATP reached a low point about 50 minutes later, at roughly 80% of baseline. Earlier intravenous challenge studies also recorded rapid, dose-dependent falls in liver ATP and inorganic phosphate as sugar phosphates accumulated. These were deliberately large experimental loads, not ordinary servings of whole fruit, but they confirm the central chemistry in people: rapid fructose metabolism can create a measurable hepatic energy event. Bawden and colleagues, 2016

This is the part of fructose biology that a calorie count cannot describe. The same molecule provides energy carbon while its first committed step can transiently signal low energy inside the cell.

The result is a paradox: the body can be surrounded by calories while cells experience a short-term low-energy signal. The Fructose Model asks whether repeated activation of this conservation program helps explain why cravings, fatigue, fat storage and metabolic dysfunction so often travel together.

The biochemical pathway is established. Its quantitative importance varies by dose, tissue and condition, and the broader disease implications remain an active research program. A major 2026 review in Nature Metabolism nevertheless describes fructose not only as a calorie, but as a regulator of metabolic health and disease. Johnson and colleagues, 2026

From Uric Acid to Redox and Mitochondrial Pressure

Uric acid is not just a passive end product in this model. The same purine-degradation sequence that produces it is linked experimentally to oxidative stress and altered energy handling. In cultured human kidney proximal-tubule cells, post-meal concentrations of fructose increased intracellular uric acid, reactive oxygen species and an inflammatory mediator. Knocking down KHK prevented those effects, tying the response to fructose metabolism rather than sweetness or calories alone. Cirillo and colleagues, 2009

Experimental research connects this redox pressure with impaired mitochondrial function and a shift away from readily spending energy. That offers a plausible biological bridge between a brief ATP-consuming reaction and the longer conservation response emphasized in the Fructose Model. The exact size and persistence of that effect in each human tissue are still being mapped. The logic, however, is coherent: repeated signals of cellular energy stress can favor making and preserving fuel even when fuel is abundant outside the cell.

How KHK Turns Fructose Into a Storage Signal

The pathway does not stop at low ATP or uric acid. Aldolase B splits fructose-1-phosphate into three-carbon molecules that can enter glycolysis, glucose production, lactate production or lipid synthesis. Fructose therefore supplies building material for fat—but its influence is larger than the percentage of fructose carbon converted directly into fat.

Fructose-1-phosphate also changes how the liver handles glucose, and fructose-derived metabolites help activate carbohydrate-responsive element-binding protein, or ChREBP. ChREBP coordinates genes involved in fructose processing, glycolysis, de novo lipogenesis and lipid export. In effect, KHK opens both a substrate route and a signaling route toward energy storage. Herman and Birnbaum, 2021

This helps explain why KHK inhibition is so informative. In experimental work with PF-06835919, blocking KHK reduced de novo lipogenesis and inactivated ChREBP alongside improvements in liver fat and other metabolic measures. The strongest causal data in that study came from cells and rats, while a small human Phase 1 component confirmed target engagement by showing a dose-dependent rise in plasma fructose. Gutierrez and colleagues, 2021

KHK does not merely dispose of fructose. It helps decide what the cell is instructed to do with incoming energy.

KHK-C and KHK-A Are Not the Same Enzyme in Practice

The human KHK gene produces two principal isoforms through alternative splicing: KHK-C and KHK-A. They are closely related proteins, but they differ in where they are expressed and how efficiently they process fructose.

• Isoform: KHK-C; Where it is most relevant: Especially liver, kidney and small intestine; What distinguishes it: High affinity for fructose and responsible for most rapid fructose catabolism
• Isoform: KHK-A; Where it is most relevant: More widely distributed across tissues; What distinguishes it: Lower affinity for fructose; emerging noncanonical roles remain under investigation

KHK-C is the isoform most directly connected to the familiar fructose pathway. It is abundant in the organs positioned to receive or clear fructose: the small intestine, liver and kidney. KHK-A is more broadly expressed and has attracted growing interest for functions that may extend beyond ordinary fructose catabolism. Eberhart and colleagues, 2020

This distinction matters. “Blocking KHK” can mean inhibiting KHK-C, KHK-A or both. Drugs and experimental compounds are not automatically interchangeable, and findings from one tissue or isoform should not be applied to every other context.

Where Does Fructose Metabolism Happen?

KHK-C is prominently expressed in the small intestine, liver and kidney—three tissues that encounter fructose for different reasons.

The small intestine can metabolize part of an oral fructose load before it reaches the liver. How much it clears depends on dose and context, and much of the most detailed mapping comes from animal experiments. Fructose that escapes intestinal first-pass handling travels through the portal circulation to the liver, where KHK-C rapidly converts it to fructose-1-phosphate. The kidney also expresses KHK-C, particularly in proximal tubular cells, and can metabolize both circulating and locally produced fructose.

The old picture of fructose as a liver-only story is therefore too simple. KHK connects the intestine, liver and kidney in a pathway whose distribution changes with the size and timing of the fructose load.

That distribution also affects interpretation. Intestinal clearance may protect the liver from smaller oral loads, while larger or faster loads can exceed first-pass capacity. The liver is positioned to receive what escapes the intestine, and the kidney can encounter both circulating fructose and fructose produced locally. A result in one organ cannot simply be pasted onto another, but the same KHK reaction gives these tissues a shared biochemical starting point.

Fructose Can Come From Food—or From Glucose

KHK does not know where a fructose molecule came from.

Dietary fructose is found in fruit, honey, sucrose and sweeteners such as high-fructose corn syrup. Whole fruit normally delivers fructose with water, fiber, micronutrients and a relatively modest dose. Concentrated beverages and refined foods can deliver much larger amounts far more quickly.

The body can also make fructose from glucose through the polyol pathway:

Glucose → sorbitol → fructose → KHK

Human evidence confirms that endogenous fructose production is real. In a small Yale experiment, eight healthy adults underwent a controlled hyperglycemic clamp while magnetic resonance spectroscopy tracked metabolites in the brain. Intracerebral fructose rose with intracerebral glucose even though the change was not explained by circulating fructose. Hwang and colleagues, 2017

Experimental studies show that high glucose, high osmolarity, dehydration, salt and low-oxygen states can activate components of this pathway. Mouse kidney research has also demonstrated KHK-dependent effects of locally generated fructose in models of diabetic and ischemic injury. Those condition-specific results should not be mistaken for proof that endogenous fructose drives every human disease. They do establish the central point: reducing dietary fructose does not necessarily eliminate fructose production or KHK activity inside the body.

This is why KHK is such a strategic control point. Dietary and endogenous fructose converge at the same enzyme.

What Happens When KHK Is Missing?

Rare loss-of-function variants in the KHK gene cause essential fructosuria. People with this condition process much less fructose through the usual hepatic pathway, so more fructose remains in circulation and is excreted in urine. The condition is generally described as benign and is often discovered incidentally. Asipu and colleagues, 2003

Essential fructosuria is an important natural experiment. It shows that markedly reduced hepatic KHK activity can be compatible with ordinary life. It does not prove that every drug, dose or pattern of partial inhibition is automatically safe. It does, however, help explain why KHK became such an attractive pharmaceutical target: the enzyme is upstream, measurable and apparently not required for normal glucose metabolism.

How Researchers Know KHK Has Been Engaged

KHK research is unusually testable because investigators can follow the pathway at several levels:

1. Direct enzyme activity: purified KHK or cell systems show whether a compound reduces conversion of fructose to fructose-1-phosphate.
2. Immediate metabolites: fructose-1-phosphate, ATP, phosphate, AMP breakdown products and uric acid reveal what happens just downstream of the enzyme.
3. Fructose handling: if KHK is inhibited, more fructose remains unmetabolized and appears in blood or urine after a controlled challenge.
4. Pathway signaling: changes in ChREBP activity and de novo lipogenesis show whether the broader storage program has shifted.
5. Organ outcomes: liver fat, insulin sensitivity, triglycerides or other endpoints ask whether pathway engagement produces a meaningful physiological result.

These layers should not be confused. A compound can inhibit purified KHK without reaching the enzyme in a person. A rise in plasma fructose can prove human target engagement without proving a clinical benefit. And a change in liver fat can be clinically interesting even if it does not improve every metabolic marker. The strongest case moves in sequence from biochemical inhibition to human target engagement and then to a relevant outcome.

Can Ketohexokinase Be Inhibited in People?

Yes. Human drug studies have now demonstrated both target engagement and metabolic effects.

In a randomized Phase 2a trial, 53 adults with fatty liver disease received placebo or the Pfizer KHK inhibitor PF-06835919 for six weeks. The higher 300-mg dose significantly reduced MRI-measured liver fat relative to placebo and improved several inflammatory markers; the 75-mg dose did not significantly reduce liver fat. The study was sponsored by Pfizer. Kazierad and colleagues, 2021

A later 16-week Pfizer trial in 164 adults with fatty liver disease and type 2 diabetes again found a significant liver-fat reduction at the higher tested dose, although HbA1c did not improve significantly. Saxena and colleagues, 2023

Eli Lilly tested a different inhibitor, LY3522348, in 65 healthy adults. After a fructose drink, plasma fructose rose in a dose-dependent pattern—the expected result when less fructose is being processed through KHK. That was direct evidence of human target engagement. Fukuda and colleagues, 2025

These studies do not show that KHK inhibition solves every feature of metabolic dysfunction. They establish the more fundamental point: KHK is druggable in people, its inhibition can be measured, and changing the pathway can change a human metabolic outcome.

Our 2026 ketohexokinase evidence review examines the pharmaceutical programs and their implications in greater depth.

Does Luteolin Inhibit KHK?

In laboratory research, yes.

A 2017 Nature Communications study tested luteolin directly against fructokinase. Luteolin inhibited the enzyme in a biochemical assay with a reported IC50 of 11.2 micromolar and inhibited fructokinase activity in a human proximal-tubule cell system. Intravenous luteolin also changed fructose handling and reduced injury in a mouse model in which KHK had independently been shown to contribute. Andres-Hernando and colleagues, 2017

That is unusually specific evidence for a natural compound. It is not yet evidence that an oral luteolin supplement reaches and inhibits KHK to the same degree in people. The decisive next experiment is therefore clear: test oral luteolin or a defined formulation against a controlled fructose challenge and measure plasma or urinary fructose alongside uric acid, glucose, insulin and luteolin metabolites.

Our article Is Luteolin a Fructokinase Inhibitor? walks through the experiment, its strengths and the remaining human question.

Why LIV3 Built SugarShield Around This Pathway

SugarShield is a dietary supplement built around luteolin and the hypothesis that fructose metabolism is a controllable layer of metabolic health.

The finished product has not yet been tested in a human KHK target-engagement trial, so LIV3 does not claim that SugarShield has clinically demonstrated KHK inhibition. What makes the hypothesis worth pursuing is the convergence of several lines of evidence: KHK is the gateway to fructose metabolism; pharmaceutical KHK inhibition works in people; luteolin inhibits KHK in experimental systems; and customer reports repeatedly cluster around the domains the model predicts—quieter cravings, steadier energy, easier weight management, less subjective puffiness or soreness, and clearer thinking.

Customer experience is not a substitute for controlled evidence. Its consistency is a reason to run the controlled experiment.

That is the broader LIV3 position: fructose is not the sole cause of metabolic dysfunction, and KHK is not the only meaningful target. But the pathway adds an explanatory layer that connects food seeking, cellular energy, fat storage and uric-acid biology with unusual precision.

Frequently Asked Questions

Is ketohexokinase the same as fructokinase?

In human metabolic research, ketohexokinase (KHK) is commonly called fructokinase because it phosphorylates fructose into fructose-1-phosphate. Formally, IUBMB lists ketohexokinase as EC 2.7.1.3 and a different enzyme named fructokinase as EC 2.7.1.4. LIV3 uses “fructokinase” in the common mammalian sense: KHK.

What does ketohexokinase do?

KHK uses ATP to convert fructose into fructose-1-phosphate. This traps fructose inside the cell and commits it to fructose metabolism. With a sufficiently rapid fructose load, the reaction can lower ATP and phosphate, increase AMP breakdown and uric-acid generation, and support pathways involved in fat synthesis.

Where is KHK found?

The high-activity KHK-C isoform is especially important in the small intestine, liver and kidney. The lower-affinity KHK-A isoform is distributed more broadly and may have additional functions that remain under investigation.

What happens when KHK is inhibited?

Less fructose proceeds through the usual fructose-1-phosphate pathway, so circulating or urinary fructose can rise. Human drug trials have measured this target engagement, and higher-dose pharmaceutical KHK inhibition has reduced liver fat in randomized trials. Results depend on the inhibitor, dose, population and outcome being measured.

Does luteolin inhibit ketohexokinase?

Luteolin directly inhibited KHK in an enzyme assay and a human kidney-cell system, and intravenous luteolin produced KHK-consistent effects in mice. Whether an oral luteolin formulation produces meaningful KHK inhibition in humans has not yet been directly established.

Does SugarShield inhibit KHK?

SugarShield was designed around luteolin and the KHK hypothesis, but the finished product has not yet been tested in a human KHK target-engagement trial. LIV3 treats that as a testable research question, not an established clinical claim.

The Enzyme to Remember

The metabolic importance of fructose does not begin with sweetness. It begins with commitment.

Ketohexokinase is the enzyme that makes that commitment. It links dietary and endogenous fructose to fructose-1-phosphate, cellular-energy signaling, uric-acid production and fat synthesis. Human drug development has now shown that the target can be changed and measured.

The remaining questions are no longer whether KHK exists or matters. They are how much it matters in each metabolic context—and which interventions can influence it safely, consistently and meaningfully.

References and Disclosure

1. IUBMB Enzyme Nomenclature: EC 2.7.1.3 ketohexokinase
2. NCBI Gene: Human KHK
3. Johnson et al. Nature Metabolism (2026)
4. Zhang et al. Pharmacological Research (2026)
5. Eberhart et al. Frontiers in Cell and Developmental Biology (2020)
6. Hwang et al. JCI Insight (2017)
7. Asipu et al. Diabetes (2003)
8. Kazierad et al. Med (2021)
9. Saxena et al. Diabetes, Obesity and Metabolism (2023)
10. Fukuda et al. Diabetes Therapy (2025)
11. Andres-Hernando et al. Nature Communications (2017)
12. Bawden et al. Clinical Nutrition (2016)
13. Cirillo et al. Journal of the American Society of Nephrology (2009)
14. Herman and Birnbaum. Cell Metabolism (2021)
15. Gutierrez et al. Molecular Metabolism (2021)

Disclosure

Written by Chris Mearns, founder of LIV3 Health. LIV3 develops and sells SugarShield, a dietary supplement containing luteolin. SugarShield was not used in the KHK drug trials or luteolin experiments cited on this page.

This page is for educational purposes and is not medical advice. SugarShield is not intended to diagnose, treat, cure or prevent disease. Consult a qualified healthcare professional before making changes to your diet, medications or supplements.