Why Fructose Behaves Less Like a Calorie and More Like a Hormone

Drop a gram of fructose into a human liver cell and something strange happens in the first few seconds. An enzyme called ketohexokinase grabs it, slaps a phosphate onto the first carbon, and sets off a small metabolic fire. ATP, the cell’s energy currency, drains faster than the cell can make more of it. Uric acid spikes. A lipogenic signal fans out. All this, from a sugar that, calorie for calorie, is identical to the glucose sitting in your bloodstream right now.

That paradox, that fructose is chemically a near-twin of glucose but behaves as if it belongs to a different category of molecule entirely, is the central argument of a sweeping review published today in Nature Metabolism. Its authors, a nine-strong group led by Richard Johnson at the University of Colorado Anschutz, want us to stop thinking of sugar as a lump of energy and start thinking of one half of it as something closer to a hormone.

A Sugar That Sends Signals

Table sugar and high-fructose corn syrup both dissolve in the mouth as roughly equal parts glucose and fructose. The glucose half is old territory. Insulin rises, cells take it up, the liver stores what isn’t needed as glycogen, and any excess gets converted to fat in a tightly regulated fashion. Fructose runs a different route entirely. It bypasses the most important regulatory gate in the glycolytic pathway, the enzyme PFK1, which normally slams the brakes when energy is plentiful. Fructose just walks around it.

The consequence, as Johnson and colleagues lay out, is that fructose metabolism has almost no off-switch. “Fructose is not just another calorie,” Johnson said in a statement accompanying the paper. “It acts as a metabolic signal that promotes fat production and storage in ways that differ fundamentally from glucose.”

Signals, in biology, tend to be potent at small doses. That is roughly what the data show. Ingest 75 grams of fructose and, within minutes, liver ATP measurably dips, a phenomenon confirmed in humans by magnetic resonance spectroscopy. The cell responds as though something important has happened. Transcription factors called ChREBP and SREBP1c switch on. Fat synthesis accelerates. Fat burning, oddly, slows. The body starts behaving as though winter is coming.

That last part is not a metaphor the authors are shy about. The fructose pathway, they argue, is an evolutionary relic, a survival circuit honed when ripe fruit was a rare and seasonal bonanza. A bear gorging on autumn berries needs to convert a brief glut into fat fast, because the lean months are coming. The same circuit, sparked by an apple or a fig or a drink of honey-sweetened water, seems to have worked beautifully for most of mammalian history.

What has changed, of course, is the seasonality. Or rather, the absence of it.

The mechanism, teased out over decades, is now fairly well charted. Fructose enters intestinal cells through a transporter called GLUT5. The small intestine handles modest doses on its own, shielding the liver, but a fizzy drink glugged in a minute overwhelms that gut-level filter and sends a bolus straight to the hepatocytes. There, ketohexokinase goes to work. Fructose-1-phosphate accumulates. It acts as a chemical whistle, triggering glucokinase to import still more glucose, driving fat synthesis (lipogenesis), and nudging the liver toward insulin resistance. Isotope tracing in humans suggests roughly a quarter of ingested fructose ends up as lactate; much of the rest becomes fat, or the raw material for it. And about 10 to 20 per cent escapes the liver altogether, circulating to the kidneys, muscle, heart, adipose tissue and brain.

The Body Makes Its Own

Here is where the story gets stranger. Dietary sugar is not the only source of fructose. The body, it turns out, can make fructose from glucose via the polyol pathway, an obscure backroad of carbohydrate metabolism that runs through an enzyme called aldose reductase. Salt intake stimulates it. So does alcohol. So does hyperglycaemia, dehydration, heat stress, kidney ischaemia and, disconcertingly, sustained consumption of high-glycaemic starches. One study found that mice fed only plain glucose developed fatty liver and metabolic syndrome, and the culprit appeared to be fructose they had manufactured themselves. Mice bred without the ability to metabolise fructose were protected from the damage.

Which raises a provocative question: how much of what we call diet-related metabolic disease is actually fructose-driven, even on diets that contain almost no fructose? The paper stops short of a firm answer, noting that human evidence remains thin. But cerebrospinal fluid from people with Alzheimer’s disease has been reported to contain fructose and sorbitol at five to six times the levels found in healthy controls.

Not everyone is convinced the case is closed. Two pharmaceutical companies have already abandoned ketohexokinase-inhibitor programmes after middling phase II results, with Pfizer’s compound PF-06835919 managing only around a 20 per cent reduction in liver fat and no meaningful change in insulin, uric acid or weight. Short trial, suboptimal dosing, wrong patient population, the authors suggest. Possibly. Or possibly the biology is simply messier than the mouse models predicted.

What Happens If the Hypothesis Holds

There is, though, a curious epidemiological footnote worth dwelling on. Sugary drink consumption has been falling in many wealthy countries for two decades, yet obesity kept rising until roughly 2020, when prevalence plateaued, and then began easing in 2023. The GLP1 drugs (Ozempic and its cousins) get most of the credit, and they deserve a lot of it. But diabetes incidence, not prevalence, started falling well before those drugs became widespread, about a decade after the sugar declines began. Cause and effect is murky. The lag, if it is a lag, is roughly what you would expect if fructose was a slow-acting poison rather than a fast calorie.

If the survival-circuit framing turns out to be right, the implications ripple outward. It would mean that hydration (which suppresses vasopressin, another fructose-triggered hormone) might blunt metabolic disease. It would mean that hereditary fructose intolerance, the rare genetic inability to break down fructose safely, could become a model for therapies in the general population. It would mean that the 10 per cent guideline on free sugars from the WHO is less a recommendation about calories and more a warning about a signalling molecule we have been dosing ourselves with, several times a day, for most of a century. A molecule telling the body to prepare for a famine that never arrives.

Source: Johnson RJ et al., Fructose: metabolic signal and modern hazard. Nature Metabolism (2026). DOI: 10.1038/s42255-026-01506-y

Frequently Asked Questions

Is fructose worse for you than regular sugar?

Table sugar is roughly half fructose and half glucose, and high-fructose corn syrup is only modestly more fructose-heavy, so the practical difference is small. What the new review argues is that the fructose half of any sugar behaves uniquely in the body, bypassing normal metabolic regulation in ways glucose does not. The problem is less about which sweetener you choose and more about how much fructose you consume overall, especially in drink form.

Does fruit cause the same metabolic damage as soda?

No, and the review is careful on this point. Whole fruit contains fructose, but it also contains fibre, flavanols, vitamin C and potassium, all of which slow fructose absorption or blunt its downstream effects. The dose is also lower and the delivery slower. Fizzy drinks, by contrast, deliver a concentrated fructose bolus fast enough to overwhelm the small intestine’s protective filtering.

How does fructose trigger fat storage if it contains no more energy than glucose?

Fructose metabolism in the liver produces a molecule called fructose-1-phosphate, which acts as a chemical signal rather than just an energy source. It activates transcription factors that switch on fat synthesis, while simultaneously suppressing fat burning. The effect is a net push toward storage, which would have been useful for surviving seasonal famine but is counterproductive in an environment of constant food.

Can your body make fructose on its own?

Yes, through something called the polyol pathway, which converts glucose into fructose via the enzyme aldose reductase. This pathway is switched on by high salt intake, dehydration, alcohol, and persistently elevated blood sugar. That means a diet with very little dietary fructose can still result in fructose accumulating in tissues, particularly the liver, kidneys and brain, a finding that has implications for Alzheimer’s disease and diabetic complications.

Will blocking fructose metabolism become a treatment for obesity?

Possibly, though the first attempts have been underwhelming. Two companies have shelved ketohexokinase inhibitors after phase II trials produced only modest improvements in liver fat. The authors of the new review argue that the trials may have been too short or used the wrong patients, and that such drugs might work better as prevention than treatment. For now, reducing sugary-drink intake and improving hydration remain the more practical levers.

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