A new study found that many people struggle to digest fructose (a type of sugar found in fruits and processed foods), and when this happens, it can change their gut bacteria in ways that might increase anxiety. Researchers studied both humans and mice to understand this connection. They discovered that 60% of the men they tested couldn’t absorb fructose properly, and these same people had higher anxiety levels and signs of inflammation in their blood. The study suggests that when your body can’t digest fructose well, it creates a chain reaction: undigested sugar feeds harmful bacteria, which triggers inflammation, which may affect your brain and mood. This research opens new questions about whether changing what we eat could help improve mental health.

The Quick Take

  • What they studied: Whether people who can’t properly digest fructose (fruit sugar) have different gut bacteria and higher anxiety levels than people who can digest it normally.
  • Who participated: The study included healthy adult men who were tested to see if they could absorb fructose. Researchers also studied special mice that were bred to have trouble digesting fructose, similar to some humans.
  • Key finding: About 6 out of every 10 men tested couldn’t absorb fructose properly. These men had higher anxiety scores, more inflammation markers in their blood, and different gut bacteria compared to those who could digest fructose normally.
  • What it means for you: If you experience bloating, gas, or stomach discomfort after eating fruits or sugary foods, you might be one of the people who struggles to digest fructose. This research suggests that difficulty digesting fructose may be connected to anxiety, though more research is needed to confirm this link and develop treatments.

The Research Details

This study had two parts working together. First, researchers tested healthy men to see how well they could digest fructose using a breath test (when you can’t digest fructose, it creates hydrogen gas that shows up in your breath). They also measured inflammation markers in the men’s blood, checked their anxiety levels using a standard questionnaire, and analyzed their gut bacteria using genetic testing. Second, the researchers used specially-bred mice that couldn’t digest fructose properly and fed them a diet high in fructose for four weeks. They then tested whether these mice showed anxiety or depression-like behaviors and examined changes in their gut bacteria and brain inflammation.

The human study measured three main inflammation markers: LPS (a substance that leaks from unhealthy gut bacteria), IL8, and TNFα. These are like alarm signals your body sends out when something is wrong. The researchers also looked at what the volunteers ate to understand their fructose intake patterns.

The mouse study was designed to mimic what happens in humans who can’t digest fructose, allowing researchers to study the brain changes more directly than they could in people.

This research approach is important because it combines human observations with controlled animal experiments. The human part shows what actually happens in real people, while the mouse part allows researchers to understand the exact biological mechanisms (how things work) without the complications of studying human brains directly. By using both approaches, the researchers could see if the same pattern appears in both humans and animals, which makes the findings more trustworthy.

The study was published in a well-respected scientific journal focused on how the brain, behavior, and immune system connect. The research used established, validated methods for measuring anxiety and analyzing gut bacteria. However, the study only included men, so we don’t know if the same results would apply to women. The human sample size appears relatively small, which means the findings need to be confirmed with larger groups. The study is also recent (2025), so other scientists haven’t yet had time to verify these results independently.

What the Results Show

The most striking finding was that 60% of the healthy men tested couldn’t absorb fructose properly—this is much higher than many people would expect. These men who had fructose malabsorption showed three important differences compared to those who could digest fructose normally: they had higher levels of inflammation markers in their blood (LPS, IL8, and TNFα), they scored higher on anxiety tests, and they had noticeably different gut bacteria.

The average person in the study was eating about 30 grams of fructose per day, which is roughly equivalent to eating one apple and drinking one sugary soda, or eating several servings of processed foods. However, there was huge variation—some people ate much more, some much less.

In the mouse experiments, the results were even more dramatic. Mice that couldn’t digest fructose and were fed a high-fructose diet showed clear signs of anxiety and depression-like behaviors. Their gut bacteria changed significantly, and their brain cells (specifically cells called microglia that control inflammation in the brain) showed signs of being activated and inflamed.

The connection between these three factors—fructose malabsorption, gut bacteria changes, and anxiety—appeared in both the human and mouse studies, suggesting this is a real biological pattern rather than a coincidence.

The research revealed that the specific types of gut bacteria changed in people with fructose malabsorption, though the study didn’t identify exactly which bacteria were most important. The inflammation markers in the blood were partially linked to how much fructose people were eating, suggesting that the amount matters. In the mice, the researchers found that brain inflammation (neuroinflammation) was occurring, which is significant because it suggests the problem isn’t just in the gut—it’s affecting the brain too. The study also noted significant individual variation, meaning some people were much more affected than others, even when they had similar fructose malabsorption.

Previous research in animals had shown that fructose malabsorption disrupts gut bacteria, but this is one of the first studies to carefully examine this connection in humans and to link it specifically to anxiety and mood. Earlier studies had suggested that gut bacteria changes can affect mood through something called the ‘gut-brain axis,’ but this research provides more direct evidence of this connection in the context of fructose digestion. The finding that 60% of people have fructose malabsorption is consistent with some previous research but higher than other estimates, suggesting this may be more common than previously thought.

This study only included men, so we don’t know if women would show the same results—they might respond differently due to hormonal differences. The human sample size wasn’t very large, which means the findings need to be confirmed with more people. The study measured anxiety traits but didn’t diagnose actual anxiety disorders, so we can’t say this causes clinical anxiety. The research is observational in humans (researchers watched what happened but didn’t control what people ate), so we can’t prove that fructose malabsorption directly causes anxiety—there could be other explanations. The study didn’t look at whether reducing fructose intake would improve anxiety, which would be the most practical question. Finally, the mouse study used an extreme model (mice completely lacking fructose transport) which may not perfectly match the human situation where people have reduced but not absent fructose absorption.

The Bottom Line

If you experience bloating, gas, or stomach discomfort after eating fruits or sugary foods, it may be worth discussing fructose malabsorption with your doctor. They can perform a breath test to confirm it. Based on this research, reducing fructose intake appears to be a reasonable approach, though the evidence is still developing. If you have anxiety, this research suggests that examining your diet—particularly fructose intake—might be worth exploring as part of a comprehensive approach, but this should not replace standard anxiety treatments. The confidence level for these recommendations is moderate—the research is promising but needs confirmation with larger studies.

This research is most relevant to people who experience digestive symptoms (bloating, gas, diarrhea) after eating fruits or sugary foods, or people with anxiety who haven’t found relief through standard treatments. It’s also relevant to anyone interested in the gut-brain connection. People without digestive symptoms probably don’t need to worry about fructose malabsorption. This research should not be used to replace professional mental health treatment for anxiety disorders.

If fructose malabsorption is contributing to your anxiety, reducing fructose intake might help, but don’t expect immediate results. Gut bacteria changes typically take 2-4 weeks to show up, and mood improvements might take several weeks to months. This isn’t a quick fix—it’s more of a long-term dietary adjustment that may help over time.

Want to Apply This Research?

  • Track daily fructose intake (estimate grams from fruits, juices, and processed foods) alongside anxiety levels (rate 1-10 each evening) for 4-6 weeks to see if there’s a personal pattern. Note digestive symptoms (bloating, gas, discomfort) at the same time.
  • If you suspect fructose malabsorption, try a 2-week experiment: reduce fructose-containing foods (fruits, fruit juices, honey, high-fructose corn syrup, agave) and track how you feel physically and mentally. Use the app to log what you eat and your anxiety/mood scores to see if there’s a connection for you personally.
  • Set up weekly check-ins to review your fructose intake, digestive symptoms, and anxiety scores. Look for patterns over 4-week periods rather than day-to-day changes. If you notice improvement, gradually reintroduce fructose to find your personal tolerance level. Share this data with your healthcare provider to inform dietary decisions.

This research is preliminary and has not yet been independently verified by other scientists. The study only included men, so results may not apply to women. This research should not be used to diagnose or treat anxiety disorders or any medical condition. If you experience anxiety, digestive problems, or other health concerns, please consult with a qualified healthcare provider. While reducing fructose intake appears safe for most people, anyone considering significant dietary changes should discuss this with their doctor first, especially if they have existing health conditions or take medications. This article is for educational purposes only and does not constitute medical advice.