- New work in zebrafish shows that sorbitol, a sugar alcohol used as a low calorie sweetener, can drive fatty liver disease when it is not cleared by gut bacteria.
- Even without external sorbitol, intestinal cells can convert glucose into sorbitol after meals; if levels overwhelm gut bacteria, sorbitol reaches the liver and is turned into fructose like compounds.
- The findings challenge the idea that sugar alcohols are always harmless and suggest that in high amounts they may still contribute to metabolic associated steatotic liver disease.
Researchers at Washington University in St Louis have spent years studying how fructose harms the liver and fuels cancer cells.
In this latest work, they turned to sorbitol, a sugar alcohol used in many sugar free products and also present in some fruits.
Using adult zebrafish as a model, they examined what happened when gut bacteria were removed.
They looked at how dietary glucose was processed in the intestine and liver and how this related to the development of steatotic liver disease, also known as fatty liver.
What they found
When the gut microbiome was depleted, zebrafish on otherwise standard diets developed fatty liver.
Metabolomics and isotope tracing showed that dietary glucose was being converted into sorbitol by enzymes in the intestinal lining. Sorbitol then travelled to the liver, where it was converted into a fructose derivative that promoted fat accumulation.
Under normal conditions, certain Aeromonas bacteria in the gut can degrade sorbitol into harmless by products.
If these bacteria are missing, or if sorbitol levels are too high, sorbitol escapes this first step and reaches the liver in greater amounts. Experiments also showed that giving sorbitol directly led to its appearance in tissues throughout the body.
The work suggests there are several paths to fructose like metabolites in the liver.
- Mortality risk increased by fatty liver disease
- Fatty liver: cases rise by 131% in the last 30 years
- People who stay up late, snore and nap are at greater risk of fatty liver disease
Excess dietary sorbitol, high glucose intake driving more sorbitol production or an unfavourable gut microbiome could all funnel more sorbitol to the liver.
What this means for sweetener choices
These findings do not mean everyone must now completely avoid sorbitol or all sugar alcohols.
In modest amounts, such as those found naturally in fruit, gut bacteria usually cope well.
Problems are more likely when people consume high amounts from multiple processed sources, particularly if their microbiome is less able to clear sorbitol.
For people with diabetes or fatty liver disease who rely on sugar free products, this research is a reminder that sugar substitutes are not automatically neutral.
There really may be no free lunch when it comes to sweetness.
Reading labels, limiting ultra processed foods and focusing on whole foods with minimal added sugars or sweeteners remains a safer long term strategy.
