How Different Diets May Influence the Gut Microbiome: What a New Systematic Review Found
- Josh Williams
- Aug 4
- 6 min read
The relationship between food and the gut microbiome continues to receive significant scientific attention. However, research in this area is often more complex than headlines such as “eat this food for better gut bacteria” might suggest.
A systematic review published in the Journal of Translational Medicine on 7 January 2026 examined how a wide range of dietary interventions were associated with changes in the gut microbiota.
The review included 80 controlled clinical trials involving both healthy adults and people with different health conditions. It explored 13 categories of dietary intervention, including Mediterranean, plant-based, Western, low-FODMAP, gluten-free, ketogenic, high-fibre and calorie-restricted diets.
The findings provide an interesting overview of current microbiome research, but they do not establish one universally ideal diet or prove that changing particular bacteria will produce a specific health outcome.
What did the systematic review investigate?
The researchers searched five major scientific databases and included controlled dietary trials that measured changes in the gut microbiota from the beginning to the end of an intervention.
They examined several types of information, including:
Microbial diversity
Changes in particular bacterial groups
Short-chain fatty acids
Inflammatory markers
Blood, stool and urine biomarkers
Of the 80 included studies, 67 were randomised controlled trials. The remainder were single-arm controlled dietary studies.
The diets examined included:
Mediterranean diets
Korean and Japanese dietary patterns
Nordic diets
Plant-based and animal-based diets
High-fibre and high-polyphenol diets
Low-FODMAP diets
Gluten-free diets
Ketogenic diets
Calorie-restricted diets
Diets with altered fat, protein or carbohydrate content
Rather than investigating one food or supplement, the review attempted to compare broad patterns across many different forms of dietary intervention.
Diet was associated with changes in specific microorganisms
One of the clearest findings was that dietary interventions were frequently associated with changes in the relative abundance of particular bacterial groups.
For example, the researchers reported associations between Mediterranean, Japanese, Korean, plant-based, high-fibre and high-polyphenol dietary interventions and changes in bacteria involved in producing short-chain fatty acids or lactic acid.
Short-chain fatty acids are compounds produced when certain gut microorganisms ferment dietary substrates, particularly some forms of fibre. However, finding more or fewer bacteria associated with these pathways does not automatically demonstrate that a person’s symptoms or health have improved.
The authors also identified associations between some dietary patterns and changes in inflammatory or metabolic biomarkers. These findings varied considerably between studies and should not be interpreted as proof that a particular diet will produce the same result for every person.
Overall microbiome diversity did not consistently change
Microbial diversity is often promoted as one of the main indicators of gut health. The systematic review found that dietary interventions did not consistently change either alpha diversity or beta diversity.
Alpha diversity broadly describes the number and distribution of microorganisms within one sample. Beta diversity compares differences in microbial communities between samples or time points.
Of the 80 studies, 57 reported alpha-diversity measurements. Some Mediterranean, Korean, calorie-restricted and low-fat dietary studies reported increases. However, many plant-based, high-fibre, high-polyphenol and Mediterranean-diet studies reported no change.
Results were also inconsistent for low-FODMAP, ketogenic and lower-carbohydrate diets.
This suggests that a dietary intervention may alter particular organisms or microbial functions without producing a large change in an overall diversity score.
It also means that microbiome diversity should not necessarily be viewed as a simple score where higher is always better. The clinical relevance of diversity may depend on the person, microbial ecosystem, health context and way the sample was analysed.
What did the review find about low-FODMAP diets?
The review included 13 trials investigating low-FODMAP diets, ten of which were randomised controlled trials.
Across these studies, low-FODMAP interventions were associated with several microbial changes, including reduced abundance of some short-chain-fatty-acid and lactic-acid-producing bacteria. However, findings relating to overall microbial diversity were inconsistent.
The review also reported associations between low-FODMAP interventions and changes in certain biological markers.
These results do not mean that a low-FODMAP diet is inherently good or bad. They show that restricting fermentable carbohydrates may influence more than digestive symptoms and that the microbial effects require further consideration.
Low-FODMAP diets also differ between studies in their duration, level of restriction and reintroduction process. The findings therefore cannot be applied automatically to every person who follows a version of the diet.
A naturopath or clinical nutritionist may consider a person’s usual food intake, dietary restrictions, nutritional adequacy and individual circumstances when discussing any restrictive dietary approach. This does not replace medical assessment or mean that a particular diet is appropriate for everyone.
Does this prove that the Mediterranean diet is best?
The Mediterranean diet was one of the most extensively studied dietary patterns in the review.
Some included trials reported associations with greater abundance of bacteria involved in short-chain-fatty-acid production, changes in certain microbial species and reductions in selected inflammatory markers.
However, not every Mediterranean-diet study produced the same findings. Several reported no measurable change in overall alpha or beta diversity.
The Mediterranean diet itself also varies between trials. Different studies may emphasise different quantities of vegetables, fruit, legumes, whole grains, nuts, olive oil, fish and other foods.
The review therefore does not prove that one standard Mediterranean diet will create a particular microbiome profile or produce guaranteed outcomes.
Why individual dietary responses may differ
The gut microbiome varies considerably between individuals.
People may begin a dietary intervention with different:
Microbial communities
Eating patterns
Health histories
Medication use
Levels of dietary adherence
Cultural food preferences
Fibre tolerances
Digestive symptoms
The microbiome-testing methods used across the studies also differed. Most studies used 16S ribosomal RNA sequencing, while others used metagenomics, PCR, bacterial culture or other analytical techniques. Different technologies can produce different levels of microbial detail.
This variation helps explain why two people may not show identical microbial changes after following a similar diet.
It also highlights why a naturopath or clinical nutritionist should avoid presenting a single dietary study as a guaranteed blueprint for every patient.
Important limitations of the review
Although systematic reviews sit high within the evidence hierarchy, their conclusions still depend on the quality and consistency of the studies they include.
The authors identified substantial differences across the studies in:
Participant populations
Dietary interventions
Study duration
Microbiome-testing technology
Outcome reporting
Dietary-adherence monitoring
Because the studies were so different, the researchers could not conduct a meta-analysis. This means they could not calculate a pooled effect size showing the overall magnitude or precision of the reported changes.
The review also focused largely on changes occurring within each dietary group rather than consistently comparing changes between intervention and control groups. The authors specifically stated that this limits the ability to make causal conclusions.
Of the 67 randomised controlled trials, 69% were assessed as having either some risk-of-bias concerns or a high risk of bias. Only 31% were rated as having a low risk of bias.
These limitations do not make the review unhelpful. They show why its findings should be viewed as an overview of emerging patterns rather than a final answer about the ideal microbiome diet.
What does this mean for gut health testing?
Gut health testing can provide information about microorganisms and microbial functions detected in a stool sample. However, a result should not be interpreted in isolation or used to predict that one particular diet will produce a guaranteed result.
This review suggests that dietary patterns may be associated with changes in selected bacteria and biomarkers even when overall diversity remains stable.
It also shows why one microbiome marker should not automatically determine an entire food plan.
A broader assessment may consider:
The person’s symptoms
Existing dietary intake
Medical history and investigations
Medication and supplement use
Nutritional adequacy
Food tolerances and preferences
The limitations of the testing method
A naturopath and clinical nutritionist may use microbiome research as one source of information while recognising that the science is continuing to evolve.
The role of a naturopath and clinical nutritionist
A naturopath or clinical nutritionist can help patients understand how scientific findings may relate to general dietary planning without presenting emerging microbiome associations as established medical facts.
This may involve reviewing current food intake, identifying unnecessary dietary restrictions, explaining the purpose of different foods and discussing how changes could be introduced in a practical way.
However, no practitioner can determine from this systematic review alone that a particular diet will correct an individual’s microbiome, resolve symptoms or prevent disease.
At The Gut Guy, current research is considered alongside the individual’s clinical presentation, dietary history, gut health testing where appropriate and relevant medical care.
The intention is not to label foods as universally good or bad. It is to understand the available evidence, acknowledge its limitations and avoid making decisions based on microbiome trends alone.
The main takeaway
The 2026 systematic review suggests that diet is associated with measurable changes in particular gut microorganisms and biological markers.
At the same time:
Overall microbiome diversity did not consistently change.
Results varied substantially between diets and studies.
Many studies had methodological limitations.
The review could not establish causation.
No single dietary pattern was proven to create an ideal microbiome for everyone.
The research supports a more balanced discussion about diet and the gut microbiome—one that considers the entire dietary pattern, the individual and the limitations of current testing.
For people considering dietary changes or gut health testing, speaking with an appropriately qualified naturopath, clinical nutritionist or medical practitioner may help place the available information into context.
This article is for general education only and does not provide medical advice, diagnosis or an individual treatment recommendation.
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