Research shows that the types and amounts of bacteria in a baby’s gut during the first few months of life are associated with diarrhea and fever later in infancy. A 2026 study of breastfed Danish infants found that babies with less diverse gut bacteria were more likely to experience diarrhea, and specific bacteria like Granulicatella were present in higher amounts during diarrhea episodes. Certain bacteria present at 3.5 months predicted future diarrhea and fever, suggesting the early microbiome may influence illness risk.
Researchers studied the gut bacteria of healthy breastfed Danish babies to understand why some get diarrhea and fever while others don’t. By analyzing stool samples from infants over 8.5 months, scientists discovered that certain types of bacteria in a baby’s gut are linked to stomach illnesses. Some bacteria increased the risk of future diarrhea and fever, while others seemed protective. This discovery could help doctors predict which babies might get sick and potentially prevent gastroenteritis before it starts.
Key Statistics
A 2026 research article published in mSystems found that alpha diversity (the variety of different bacteria) in infant stool was negatively associated with diarrhea in breastfed Danish infants, meaning babies with less diverse gut bacteria experienced more diarrhea.
According to the 2026 study of Danish infants, Granulicatella abundance was positively associated with diarrhea, and this same bacterium has been linked to diarrhea in children from low-income countries, suggesting a universal pattern.
Research showed that Staphylococcales and Haemophilus abundance measured at 3.5 months of age were positively associated with future diarrhea during ages 3.5 to 8.5 months in breastfed Danish infants.
The 2026 mSystems study found that Bacteroidales abundance between 3.5 and 6 months of age was negatively associated with fever, suggesting this bacterial group may be protective against fever in infants.
The Quick Take
- What they studied: Whether the types and amounts of bacteria in a baby’s poop can predict or explain diarrhea, fever, and vomiting in healthy breastfed infants
- Who participated: Healthy, well-nourished breastfed Danish infants followed from birth through 8.5 months of age, with stool samples collected at three different time points
- Key finding: Certain bacteria in a baby’s gut during the first 3.5 months of life were linked to diarrhea and fever later on. Babies with less diverse gut bacteria were more likely to have diarrhea, and specific bacteria like Granulicatella were found in higher amounts during diarrhea episodes
- What it means for you: Understanding your baby’s gut bacteria composition might one day help doctors predict and prevent stomach illnesses. However, this research is still early-stage and shouldn’t change current feeding or medical practices without consulting your pediatrician
The Research Details
Researchers collected poop samples from Danish babies at three different ages during their first 8.5 months of life. They also asked parents detailed questions about when babies had diarrhea, fever, or vomiting, and about breastfeeding practices. The scientists then analyzed the DNA from the bacteria in these poop samples using special genetic testing to identify which types of bacteria were present and in what amounts.
They used several statistical methods to look for patterns: checking whether certain bacteria were more common when babies were sick, whether the variety of bacteria mattered, and whether the bacteria present early in life could predict illness later. They also used computer learning models to see if early bacterial patterns could forecast future diarrhea and fever.
This research approach is important because it follows babies over time rather than just looking at one moment. This allows researchers to see whether bacteria changes happen before illness (suggesting they might cause it) or after (suggesting they’re just a response to illness). Understanding these patterns in healthy, well-fed babies is crucial because most previous research focused on babies in poorer countries with different nutrition and living conditions
This study was registered in a clinical trials database before it started, which is a sign of good research practices. The researchers used standardized genetic sequencing methods and multiple statistical approaches to verify their findings. However, the exact number of babies studied wasn’t clearly stated in the abstract, and the study only included Danish infants, so results may not apply to all populations
What the Results Show
The research revealed several important patterns about gut bacteria and infant illness. Babies with less diverse gut bacteria—meaning fewer different types of bacteria—were more likely to experience diarrhea. This suggests that a healthy, varied bacterial community might protect against stomach problems.
A specific bacterium called Granulicatella was found in much higher amounts in babies who had diarrhea, suggesting it may play a role in causing or accompanying diarrhea. Interestingly, this same bacterium has been linked to diarrhea in children from low-income countries, suggesting this pattern might be universal across different populations.
When researchers looked ahead at what would happen later in infancy, they found that certain bacteria present at 3.5 months predicted future problems. Babies with higher amounts of Staphylococcales and Haemophilus bacteria at this early age were more likely to develop diarrhea between 3.5 and 8.5 months. In contrast, babies with more Bacteroidales bacteria were less likely to get fever during this same period.
The study found that fever and diarrhea didn’t usually happen together in these babies, suggesting they may have different causes. Early life bacteria called Actinobacteriota were linked to later fever, though this group includes both harmful and helpful bacteria, so the relationship is complex. The research also found that Pielou’s evenness—a measure of how evenly distributed different bacteria are—was protective against fever when measured early in life
According to Gram Research analysis, this study confirms findings from research in low-income countries by showing that Granulicatella bacteria are associated with diarrhea even in well-nourished, breastfed Danish infants. This suggests the relationship between this bacterium and diarrhea may be consistent across different populations and living conditions. However, the specific bacteria that predict future illness appear somewhat different from what’s been found in other populations, indicating that local factors may influence which bacteria matter most
The study only included Danish infants, so the findings may not apply to babies in other countries with different diets, living conditions, or healthcare systems. The exact sample size wasn’t clearly reported, making it hard to assess statistical power. The research is observational, meaning it shows associations but cannot prove that specific bacteria actually cause illness. Additionally, the study couldn’t identify what caused many cases of diarrhea, suggesting other factors beyond bacteria may be important
The Bottom Line
Current evidence suggests monitoring your baby’s gut health through normal pediatric care. Continuing breastfeeding appears beneficial based on this research. Do not make changes to feeding practices based on this study alone—consult your pediatrician before any dietary modifications. This research is preliminary and should not be used for self-diagnosis or treatment decisions (moderate confidence level)
Parents of breastfed infants, pediatricians, and researchers studying infant health should find this relevant. This is particularly important for families concerned about recurrent diarrhea or fever in their babies. Healthcare providers may eventually use this information to identify at-risk infants, though that application is not yet ready for clinical practice
This research represents early-stage science. It may take 5-10 years of additional research before findings translate into practical tools for predicting or preventing infant gastroenteritis. Parents should not expect immediate changes to medical practice based on this single study
Frequently Asked Questions
Can gut bacteria tests predict if my baby will get diarrhea?
Early research suggests certain bacteria patterns may predict future diarrhea, but this technology isn’t yet available for clinical use. Current testing methods are research tools, not diagnostic tests. Talk to your pediatrician about monitoring your baby’s health through standard checkups
What can I do to help my baby develop healthy gut bacteria?
Continuing breastfeeding appears beneficial based on this research, as the study focused on breastfed infants. Avoid unnecessary antibiotics when possible, as they disrupt healthy bacteria. Consult your pediatrician about any dietary changes or probiotic supplements
Does this research apply to formula-fed babies?
This study only examined breastfed Danish infants, so the findings may not directly apply to formula-fed babies. Formula-fed infants have different gut bacteria patterns, and separate research would be needed to understand their microbiome and illness risk
Should I be worried if my baby has diarrhea?
Occasional diarrhea is common in infants and usually resolves on its own. However, contact your pediatrician if diarrhea lasts more than a few days, is severe, or your baby shows signs of dehydration like fewer wet diapers or unusual lethargy
How does this research change infant care recommendations?
This research is preliminary and doesn’t change current infant care guidelines. Continue following your pediatrician’s recommendations for feeding, hygiene, and medical care. This study provides insights for future research but isn’t ready for clinical application
Want to Apply This Research?
- Log daily stool consistency (normal, loose, or watery) and any fever episodes with temperature readings. Track breastfeeding duration and any dietary changes. Record these observations weekly to identify patterns over time
- Use the app to maintain detailed records of your baby’s health symptoms and feeding patterns. Share this data with your pediatrician during checkups to help identify any concerning trends early. Set reminders for scheduled pediatric visits to discuss any patterns you’ve noticed
- Create a long-term health timeline in the app tracking diarrhea and fever episodes from birth through 12 months. Compare patterns across seasons and developmental stages. Use this information to discuss preventive strategies with your pediatrician at each visit
This article summarizes research findings and is not medical advice. Gut microbiome research in infants is an emerging field, and findings from this study should not be used for self-diagnosis or to make changes to your baby’s care without consulting your pediatrician. If your baby experiences persistent diarrhea, fever, vomiting, or signs of dehydration, contact your healthcare provider immediately. Always follow your pediatrician’s recommendations for feeding, hygiene, and medical care. This research applies specifically to breastfed infants and may not generalize to all populations or feeding methods.
This research translation is published by Gram Research, the science division of Gram, an AI-powered nutrition tracking app.