Researchers led by Gary Patti at Washington University in St. Louis report that the sugar alcohol sorbitol can be converted into fructose in the liver and may contribute to liver dysfunction. The work, published in Science Signaling, used zebrafish to trace how sorbitol moves from the gut into tissues. Patti’s earlier studies showed that fructose processed in the liver can promote cancer cells and contribute to steatotic liver disease, which affects 30% of adults worldwide.
Sorbitol appears in the gut in two main ways: it can be eaten directly (it is used in many low-calorie candies, gums and some stone fruits), and it can be produced from glucose by an intestinal enzyme. That enzyme has a low affinity for glucose, so it usually makes sorbitol when glucose levels are high. This explains why sorbitol production is linked to diabetes, although the zebrafish work shows it can also occur in healthy animals after feeding.
Whether sorbitol becomes harmful depends in part on gut bacteria. Some microbes, including Aeromonas strains, can degrade sorbitol and turn it into a harmless byproduct, so much sorbitol is cleared in the gut. If those bacteria are absent or overwhelmed, sorbitol can reach the liver and be converted into a derivative of fructose with possible damaging effects. Gut microbes clear sorbitol well at modest levels such as those normally found in fruit, but problems arise when dietary sorbitol is high or when excessive glucose in the diet raises glucose-derived sorbitol.
The findings raise questions about the safety of sugar substitutes such as aspartame (Equal), sucralose (Splenda) and sugar alcohols. Patti noted that polyols like sorbitol may not always be harmlessly expelled and that sorbitol given to animals appears in tissues throughout the body; he also found a large amount of sorbitol in his own protein bar. Further research is needed to define the specific bacterial mechanisms that clear sorbitol and to understand the health implications. The study was supported by the National Institutes of Health. The authors write that "there is no free lunch" when seeking sugar alternatives.
Difficult words
- sorbitol — a sugar alcohol used as a sweetener
- fructose — a simple sugar found in fruit and honey
- enzyme — a protein that speeds up chemical reactions
- affinity — natural tendency to bind or react with
- degrade — to break into smaller parts or molecules
- microbe — a tiny living organism like bacteriamicrobes
- polyol — a class of sugar alcohol compoundspolyols
- steatotic — relating to abnormal fat accumulation
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Discussion questions
- How could differences in a person's gut microbes change the health effects of sugar alcohols for them?
- If sorbitol can form from glucose when glucose is high, what dietary changes might reduce that production?
- What further experiments or information would help decide whether sugar alcohols are safe for widespread use?
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