Fructose, allulose and your liver
Fructose and allulose are almost the same molecule: the only difference is one flipped carbon. The body treats them completely differently. Table sugar, which is half fructose, pushes the liver to make new fat. Allulose mostly passes through the body unchanged.
At a glance
One overflow, two ways to lose
Your small intestine can handle only so much fructose at once. In animal studies it clears most of a small dose on its own. Past that limit, the fructose overflows, and it overflows in two directions.
Some reaches the liver, where it pushes the liver to make new fat. Some reaches the colon, where gut bacteria ferment it and make gas, which is why large amounts of fructose can mean bloating and discomfort. One overflow, two ways to lose. Table sugar is the biggest source of that fructose in most diets, because it is half fructose and it is everywhere.
What table sugar does to the liver
The clearest human evidence comes from a controlled trial, and table sugar came out worst.
In a seven-week trial, 94 healthy men drank 80 g a day of sugar in sweetened drinks. Table sugar roughly doubled the rate at which their livers made new fat: from a median 9.1% a day with no added sugar to 20.8% a day, the highest of any group. Fructose on its own did nearly the same (19.7%). The same amount of glucose did not change it (11.0%).
Pooled randomized trials point the same way. A 2023 BMJ review of sugar and health found that eating added sugar, compared with none, increased fat stored in the liver, a finding the review rated moderate certainty.
Sugar sold as “real cane sugar” is sucrose, the same table sugar in this trial. In acidic drinks it splits into the same fructose and glucose that high-fructose corn syrup supplies. More on table sugar →
Allulose takes a different path
One carbon apart from fructose, and handled completely differently.
Allulose is absorbed, but the body does not use it for fuel. In a human study, about 70% of a dose was recovered unchanged in urine, and it supplied almost no usable energy, which is why the FDA puts it at 0.4 calories per gram against about 4 for sugar.
The liver leaves it alone, too. When researchers gave liver cells from people and rats fructose or allulose, the cells had used up about half the fructose after four hours (43–53% left) but left 95–97% of the allulose untouched.
These studies show how the body handles allulose. No human trial has yet measured the liver’s fat-making rate on allulose the way the sugar trial above did.
Same test, different gas
Only bacteria make hydrogen gas. So breath hydrogen shows how much of a food your gut bacteria ate.
In a human study, people drank allulose at three doses, and on other days the same amounts of a food gut bacteria readily ferment. Allulose barely registered; the comparison food produced 6 to 11 times more gas. After eight weeks of daily allulose, hydrogen output had still not risen, so the gut did not learn to ferment it.
Why so little gas? Bacteria need a specific enzyme to ferment allulose. A 2025 genetic survey searched 85,202 bacterial genomes and found it in only 116 species, and in about 16% of gut samples from healthy adults.
Three questions, one clean answer
Does it push the liver to make fat? Does it make gas? Does it bake and brown like sugar?
| Sweetener | Pushes the liver to make fat? | Makes gas? | Bakes and browns like sugar? |
|---|---|---|---|
| Table sugar | Yes | In excess | Yes |
| Honey | Yes | Yes | Yes |
| Agave nectar | Yes | Yes | Yes |
| High-fructose corn syrup | Yes | Yes | Yes |
| Xylitol, sorbitol, maltitol | No | Yes | No |
| Erythritol | No | Little | No |
| Inulin and chicory root fiber | No | Yes | No |
| Stevia, monk fruit | No | No | No |
| Sucralose, aspartame | No | No | No |
| Allulose | No | No | Yes |
The sugars and syrups work in the kitchen but push the liver to make fat. Most sugar alcohols and fibers spare the liver but feed gas-making bacteria. The high-intensity sweeteners avoid both but cannot bake or brown. Allulose is the only one that avoids both and still behaves like sugar. More on allulose →
Common questions
Does fructose go straight to the liver?
Not at small doses. The small intestine handles most of a small amount. It is the overflow from larger amounts, like a sweetened drink, that reaches the liver and pushes it to make fat.
Is cane sugar better for your liver than high-fructose corn syrup?
No. Both deliver fructose and glucose in similar amounts. In the seven-week trial above, table sugar raised the liver’s fat-making rate the most of any group.
Does allulose turn into fat?
The body does not use allulose for fuel: about 70% leaves unchanged, and liver cells leave it almost untouched. That is why it carries only 0.4 calories per gram.
Selected sources
- Geidl-Flueck et al., 2021, Journal of Hepatology — randomized trial in 94 men: fructose, sucrose and glucose drinks and the liver’s rate of making new fat.
- Huang et al., 2023, BMJ — umbrella review of sugar and health; added sugar versus none and liver fat in randomized trials (moderate certainty).
- Jang et al., 2018, Cell Metabolism — mouse study: the small intestine clears most fructose at low doses; larger doses spill to the liver and gut bacteria.
- Maeng et al., 2019, Foods — human and rat liver cells: allulose versus fructose over four hours.
- Iida et al., 2010, Metabolism — urinary excretion, energy value and breath hydrogen after allulose, including an 8-week adaptation study.
- Minabou Ndjite et al., 2025, Communications Biology — how few gut bacteria carry the enzyme to ferment allulose.
- US Food & Drug Administration — guidance on declaring allulose and its calories (0.4 kcal/g) on Nutrition Facts labels.
The sugar that behaves differently
Everything on allulose in one place: how it bakes, what the trials show about blood sugar, and how it compares with every other sweetener.
Read the allulose profile →