Eating a lot of fructose might mess up the gut bacteria in mice and rats, leading to inflammation and a leaky gut — especially when they get 20–60% of their food as fructose for over two months.

What we've found so far is that high fructose intake may affect gut bacteria and promote inflammation in rodents. Our current analysis is based on limited evidence, but what we’ve reviewed suggests this possibility. We analyzed one assertion from the research, and it indicates that when mice or rats consume 20–60% of their daily calories as fructose for more than two months, their gut bacteria may become disrupted [1]. This shift in gut bacteria has been linked to inflammation and increased gut permeability—sometimes called “leaky gut” [1]. Leaky gut means the lining of the intestines may allow substances to pass into the bloodstream that normally wouldn’t, which the body can respond to with inflammation. So far, all the evidence we’ve reviewed supports this idea [1]. No studies we’ve seen so far contradict it. However, we’ve only looked at one assertion, which means our understanding is still very limited. We can’t yet say how strong this effect is, or under what conditions it might happen more often. We also don’t know if the same changes would occur in humans, or if shorter periods of fructose intake would have the same impact. Our analysis is ongoing, and as we review more studies, our understanding may change. Right now, the evidence we’ve reviewed leans toward a link between long-term, high-dose fructose consumption and negative changes in gut health in rodents. Practical takeaway: In rodents, very high fructose diets over long periods might harm gut health. While this doesn’t prove the same happens in people, it suggests that consistently eating large amounts of fructose could be worth watching if you’re aiming to support gut health.