In seasonal environments, turning temporary abundance into stored fuel can be a survival advantage. Photo by Pete Nuij / Unsplash.

One of the strangest contradictions in metabolic health is that the body can store more energy than it needs while its cells behave as though fuel is running low.

You can recognize the contradiction long before it becomes a diagnosis. Weight becomes easier to gain. Cravings grow louder. Energy becomes less reliable. The usual explanation is arithmetic: more calories entered than were burned. The arithmetic matters—but it does not explain why the body can become so determined to conserve fuel, or why hunger and fatigue can rise together.

Fructose offers a missing piece.

Glucose and fructose carry the same calories, but they do not deliver the same metabolic instructions. Glucose is widely used as fuel. Fructose enters a pathway built to help living creatures turn temporary abundance into survival: spend energy now, store fuel, reduce unnecessary energy use, and keep searching for more. In a seasonal world, that is elegant biology. In a modern environment, the same program may be activated far more often than the conditions that once made it useful.¹

At the center is ketohexokinase—also called fructokinase or KHK—the enzyme that pulls fructose into this pathway. Its first steps rapidly use ATP, temporarily capture phosphate, and increase uric-acid production. From there, the effects can reach mitochondrial function, fat production, and the way cells respond to incoming fuel.^1–4

Fructose is not the only cause of metabolic dysfunction. It may be something more interesting: a central coordinator that helps explain how a body can have energy everywhere, yet too little usable energy where cells need it. That is the case this article will make.

Why can the body store energy while cells feel short on energy?

Body fat is an energy reserve. ATP is the spendable energy each cell uses to do work. The two can move in different directions.

Think of a city with full oil tanks but an unstable electrical grid. The city is not short of energy in the abstract. It is short of energy in the form, place, and moment where work must happen.

Metabolism has a similar distinction. Calories describe the energy entering the body. They do not fully describe the signals that determine whether fuel is burned, stored, released, or sought again. The cell does not count calories. It responds to chemistry.

This is why a model of metabolism needs both arithmetic and biology. Energy balance tells us that stored energy came from somewhere. Hormones, enzymes, organelles, and nervous-system signals help explain why the body handled it as it did.

What makes fructose metabolically different from glucose?

Fructose and glucose have the same caloric value, but their first metabolic steps are different. Glucose is regulated and distributed broadly; fructose is pulled rapidly into a pathway whose opening reaction consumes ATP and can favor conservation and storage.

Context changes the response. Whole fruit delivers fructose with water, fiber, and intact cellular structure; a sweetened drink can deliver a large sugar load quickly. Those are meaningfully different exposures, but not every acute response differs: in one randomized trial, matched fructose loads from whole apples, apple juice, and a fructose drink raised uric acid similarly, while whole apples produced greater satiety.^5,6

For a deeper look, see LIV3's fructose metabolism master guide, or begin with what fructose is and where it comes from.

What happens when fructokinase processes fructose?

KHK uses ATP to attach a phosphate to fructose, producing fructose-1-phosphate. When fructose arrives quickly enough, phosphate can become temporarily sequestered, ATP can fall, and AMP breakdown can increase uric-acid production.

ATP is often called the cell's energy currency. That metaphor is useful, but incomplete: ATP is also part of the cell's energy-sensing system. A rapid change in ATP and its related phosphate compounds can make abundance look, locally and temporarily, like scarcity.

The opening sequence is well established:

1. Fructose enters a cell and meets KHK.
2. KHK spends ATP to create fructose-1-phosphate.
3. Phosphate is temporarily captured in that molecule, limiting the pool available to regenerate ATP.
4. AMP is broken down through the purine-degradation pathway.
5. Uric acid rises as one product of that breakdown.^1–4

Human studies have observed this energy shift. In one magnetic-resonance study, healthy men given 75 grams of oral fructose showed a mean liver-ATP nadir of roughly 80% of baseline.² Other human experiments have documented changes in liver phosphate metabolites and an acute uric-acid response after fructose.^3,4

This is sometimes described as fructose “stealing” phosphate. Nothing disappears. Phosphate is temporarily trapped in a metabolic intermediate, while ATP is used faster than it can immediately be restored.

The metabolic paradox becomes a loop

The part of this model people often recognize first is not the biochemistry. It is the experience: tired enough to want energy, driven to eat, and frustrated that the body seems determined to store it.

The Fructose Model proposes that repeated activation of the pathway can push cells already struggling to recover their energy balance toward conservation. Fat storage, reduced metabolic flexibility, cravings, and fatigue may then become reinforcing parts of one loop rather than unrelated failures.

The mechanism does not stop at one brief ATP dip. Fructose metabolism supplies building blocks for fat synthesis, and controlled human studies show that fructose-containing drinks can increase hepatic de novo lipogenesis—the creation of new fat in the liver—under the conditions tested.^7,8

Preclinical research also connects fructose metabolism and uric-acid signaling to mitochondrial oxidative stress.⁹ Mitochondria are the structures that turn fuel into usable cellular energy. If their performance becomes less flexible, a cell may be surrounded by fuel while becoming worse at converting that fuel into ATP.

Now the paradox begins to make biological sense. A system experiencing cellular energy strain may reduce optional energy use, preserve stored fuel, and intensify signals to find more. In the short term, that is a survival program. Repeated without a period of recovery, it could become a self-reinforcing metabolic trap.

In this model, insulin resistance is not simply a broken lock on the cell door. It may sometimes be partly protective: a way for an already stressed cell to resist additional fuel pressure. That does not make insulin resistance harmless. It reframes it as a response with a history. Changing incoming fuel can help, but it may be only one layer if the cellular stress encouraging conservation remains.

This is the Fructose Model's central, testable prediction.

Many roads lead to fructose in the body

The body can also make fructose from glucose through the polyol pathway. Endogenous fructose is real in humans, although its quantitative contribution to common metabolic disease remains an open question.

The route has two steps: glucose is converted to sorbitol, and sorbitol is converted to fructose. In a small human study using a hyperglycemic clamp, brain fructose rose alongside brain glucose, providing direct evidence that the human brain can produce fructose from glucose.¹⁰

This does not mean every gram of excess glucose becomes fructose. It means dietary fructose is not the only possible entrance to the pathway. Experimental research suggests that high glucose, high salt, dehydration, ischemia, and oxidative stress can activate endogenous fructose production in particular contexts.¹¹

That may help explain why simply removing obvious sources of fructose does not settle every question. Many roads can lead to fructose in the body. What matters is the total pathway activity: where fructose appears, how quickly it is metabolized, which tissues are exposed, and whether their energy systems recover.

What has fructose research demonstrated in people?

Human research answers three important questions. Fructose exposure can measurably change liver ATP, uric acid, and fat synthesis. The human brain can produce fructose internally. And pharmaceutical KHK inhibition can engage the pathway and alter liver-fat outcomes in defined study populations.^{1–4,7,8,10,12–14}

PF-06835919 has produced dose-dependent target-engagement signals in people. In two phase 2a studies, it modestly reduced MRI-measured liver fat in defined groups, although it did not normalize every metabolic marker.^12–14 That matters because it moves KHK from an interesting mechanism to a human intervention point with measurable consequences.

What remains to be tested is the larger prediction: whether repeated pathway activation is a major causal bridge connecting cravings, fatigue, insulin resistance, and persistent fuel conservation in everyday human life.

The Fructose Model adds another layer

These models operate at different levels. Energy balance describes what must be true when body stores change. Insulin helps coordinate fuel storage and availability. GLP-1 medicines act through a different therapeutic layer, including appetite and energy-intake regulation.¹⁶ The Fructose Model asks what signals may push cells toward conservation in the first place.

A treatment can work at one layer even while another layer remains active. The useful question is not, “Which model wins?” It is, “How many layers must change for a person to regain metabolic flexibility?”

What does this reframing change?

It shifts attention from fructose content alone to pathway activation, delivery rate, cellular recovery, and KHK as a measurable control point. It also turns familiar experiences—cravings, energy instability, and easy fat storage—into biological questions rather than character judgments.

The first practical implication is not fear of fruit. It is reducing large, rapidly delivered sugar loads—especially sweetened drinks.^5–8 The model also asks how sleep, hydration, muscle activity, and the overall diet shape tissue exposure and recovery context. Dose, delivery rate, tissue, and recovery all shape what the pathway does.

At a research level, KHK creates a specific intervention point. Fructokinase (KHK) sits at the pathway's entrance, so a credible intervention should be able to show target engagement, downstream biomarker changes, and eventually meaningful outcomes in people.

Luteolin is especially intriguing because it offers a specific way to test the model. A 2017 study found that luteolin inhibited fructokinase in an in-vitro enzyme assay and in lysates from immortalized human proximal-tubule cells; the researchers also administered luteolin intravenously in a mouse model of acute kidney injury.¹⁵ That finding begins a clear human research program: can an oral formulation achieve useful exposure, engage KHK in relevant tissues, change downstream biomarkers, and ultimately improve meaningful outcomes?

SugarShield contains luteolin and was developed around this research direction. Its relevance here is as an application of the hypothesis—not as proof that the finished product inhibits KHK in humans or treats metabolic disease.

The clue hidden inside the contradiction

The body can store too much energy and still send signals associated with scarcity. Once those two facts are allowed to coexist, cravings, fatigue, fat storage, and insulin resistance no longer look like a random collection of failures.

They begin to look like parts of a conservation program.

Fructose may be central to that program because it is both fuel and signal. Through KHK, it can spend cellular energy, capture phosphate, generate uric acid, promote fat production, and influence whether fuel is burned or conserved. The pathway's components are measurable. The unifying model is testable.

The most useful question may not be only how much energy entered the body, but what instructions that energy gave the cells—and whether those instructions can be changed.

Commercial-interest disclosure: LIV3 Health publishes this article and sells SugarShield, a formulation containing luteolin. The ingredient and pathway research discussed here did not test SugarShield.

This article explains research and presents an evidence-led hypothesis. It is not medical advice and is not intended to diagnose, treat, cure, or prevent disease. If you have symptoms, a medical condition, take medication, or are considering a supplement, discuss your circumstances with a qualified healthcare professional.

References

1. Johnson RJ, et al. Fructose: Metabolic Signal and Modern Hazard. Nature Metabolism. 2026.
2. Bawden SJ, et al. Investigating the effects of an oral fructose challenge on hepatic ATP reserves in healthy volunteers: A 31P MRS study. Clinical Nutrition. 2016.
3. Masson S, et al. Hepatic metabolism during constant infusion of fructose; comparative studies with 31P-magnetic resonance spectroscopy in man and rats. Biochimica et Biophysica Acta. 1994.
4. Carran EL, et al. Acute effect of fructose intake from sugar-sweetened beverages on plasma uric acid: a randomised controlled trial. European Journal of Clinical Nutrition. 2016.
5. Semnani-Azad Z, et al. Association of Major Food Sources of Fructose-Containing Sugars With Incident Metabolic Syndrome: A Systematic Review and Meta-analysis. JAMA Network Open. 2020.
6. White SJ, et al. The effects of apples and apple juice on acute plasma uric acid concentration: a randomized controlled trial. American Journal of Clinical Nutrition. 2018.
7. Geidl-Flueck B, et al. Fructose- and sucrose- but not glucose-sweetened beverages promote hepatic de novo lipogenesis: A randomized controlled trial. Journal of Hepatology. 2021.
8. Schwarz JM, et al. Effect of a High-Fructose Weight-Maintaining Diet on Lipogenesis and Liver Fat. Journal of Clinical Endocrinology & Metabolism. 2015.
9. Lanaspa MA, et al. Uric acid induces hepatic steatosis by generation of mitochondrial oxidative stress: potential role in fructose-dependent and -independent fatty liver. Journal of Biological Chemistry. 2012.
10. Hwang JJ, et al. The human brain produces fructose from glucose. JCI Insight. 2017.
11. Andres-Hernando A, et al. Endogenous fructose production: what do we know and how relevant is it?. Current Opinion in Clinical Nutrition and Metabolic Care. 2019.
12. Gutierrez JA, et al. Pharmacologic inhibition of ketohexokinase prevents fructose-induced metabolic dysfunction. Molecular Metabolism. 2021.
13. Kazierad DJ, et al. Inhibition of ketohexokinase in adults with NAFLD reduces liver fat and inflammatory markers: A randomized phase 2 trial. Med. 2021.
14. Saxena AR, et al. A phase 2a, randomized, double-blind, placebo-controlled, three-arm, parallel-group study to assess the efficacy, safety, tolerability and pharmacodynamics of PF-06835919 in patients with non-alcoholic fatty liver disease and type 2 diabetes. Diabetes, Obesity and Metabolism. 2023.
15. Andres-Hernando A, et al. Protective role of fructokinase blockade in the pathogenesis of acute kidney injury in mice. Nature Communications. 2017.
16. U.S. Food and Drug Administration. Wegovy prescribing information. 2026.