What does eating too much sugar do to your gut health?

Eating a lot of sugar, particularly refined carbohydrates and fructose from soft drinks and added sugars, is linked to a disrupted gut microbiome and a higher risk of metabolic diseases such as metabolic syndrome. High fructose intake in particular appears to make the gut wall more permeable. This allows bacterial toxins to leak into the bloodstream, which in turn contributes to fatty liver disease. Researchers stress that larger studies are needed to confirm this definitively.

An important indirect effect of a high-sugar diet is that it crowds out fibre. Fibre is precisely the fuel for beneficial gut bacteria: they convert fibre into short-chain fatty acids, substances that support metabolism. Eating little fibre is associated with a higher risk of inflammatory bowel diseases, obesity and type 2 diabetes. Seen in that light, the pattern of a high-sugar, low-fibre diet is at least as harmful as sugar itself.

One finding is surprising: in a randomised study in healthy adults, twelve weeks of eating fewer free sugars barely changed the composition of the gut microbiome, although LDL cholesterol did fall. This suggests that restricting sugar alone does not automatically lead to a measurably better microbiome, at least not in the short term in healthy people.

The gut bacterium Akkermansia muciniphila is associated with better blood sugar regulation and less abdominal fat in people who are overweight. A high-sugar diet can reduce the amount of this bacterium, but whether that is directly the cause of metabolic problems has not yet been proven. Regarding artificial sweeteners such as aspartame and sucralose, the picture remains neutral for now: a small two-week study saw no effect on the microbiome, but that study was too small and too short to offer genuine reassurance. Moreover, one of the researchers had a commercial relationship with a soft drink manufacturer.