What Do Gut Microbiome Tests Actually Measure?
The human gut is home to trillions of microorganisms that play a role in various bodily functions, including the metabolism of certain dietary components and the regulation of the immune system.
Most commercial gut microbiome tests analyse stool samples using genetic sequencing techniques.
These methods allow laboratories to identify a wide range of microorganisms and estimate their relative abundance.
Consumers may therefore receive a report listing different bacteria, measures of microbial diversity and, in some cases, dietary recommendations.
However, identifying a particular bacterium does not establish that it is causing a health problem or that eating certain foods will alter its abundance.
Furthermore, gut microbiome composition varies considerably among healthy individuals and is influenced by factors such as age, diet, medication use, and geographical location. [1], [5]
Can a Gut Microbiome Test Diagnose Dysbiosis?
One of the most common claims made about commercial microbiome tests is that they can determine whether a person’s gut microbiota is balanced.
The term dysbiosis is often used to describe changes in the composition or function of microbial communities.
Although dysbiosis is a recognised scientific concept, there is a fundamental problem.
There is no universally accepted definition of a “normal” gut microbiome that applies to everyone.
In 2025, an international expert consensus statement was published in The Lancet Gastroenterology & Hepatology, bringing together specialists from several scientific disciplines.
The experts concluded that there is insufficient scientific evidence to establish strict reference ranges for the relative abundance of most microorganisms.
They also highlighted that the available evidence does not yet support the use of a general dysbiosis index as a tool for clinical assessment.
The consensus statement also advises against using the Firmicutes/Bacteroidetes ratio as a measure of gut microbiome health. [1]
Therefore, a test result describing someone’s gut microbiome as “imbalanced” does not automatically constitute a diagnosis of a medical condition.
Similarly, finding higher or lower proportions of certain bacteria does not necessarily indicate that treatment is needed.
How Reliable Are Commercial Gut Microbiome Tests?
Another important question is whether different laboratories can produce similar results when analysing the same sample.
In February 2026, a study published in Communications Biology investigated the analytical reliability of seven commercial gut microbiome testing services.
The researchers used a standardised, homogenised stool reference material developed by the US National Institute of Standards and Technology (NIST).
Three testing kits were purchased from each provider to assess both the consistency of repeated measurements and the agreement between different companies.
The results revealed substantial differences, not only between testing services but also between repeated analyses performed by the same provider.
Notably, the variation between companies was comparable to the biological differences observed between individuals.
The researchers attributed these discrepancies primarily to differences in laboratory methods and insufficient standardisation of testing procedures. [2]
If different testing services cannot consistently reproduce the microbial composition of the same sample, this raises further questions about the reliability of the conclusions drawn from their results.
It is important to note, however, that the study examined seven specific services rather than every commercial test available.
It therefore does not establish that all gut microbiome tests are unreliable. Nevertheless, it highlights significant reliability concerns within part of the commercial testing market.
Can Gut Microbiome Test Results Be Used to Personalise Your Diet?
The relationship between diet and the gut microbiome is well established.
Research has shown that different dietary patterns can influence the composition and function of microbial communities. [5]
However, this does not mean we already have sufficient knowledge to determine the ideal diet for each individual based solely on a stool sample.
For such a test to be considered clinically useful, accurately identifying microorganisms is not enough. There must also be evidence that the dietary recommendations based on its results lead to meaningful improvements in health.
The 2025 international consensus statement advises against testing providers offering therapeutic recommendations directly to consumers, as interpreting microbiome results and selecting appropriate interventions requires a broader clinical assessment. [1]
Are There Any Promising Findings?
In 2024, a randomised clinical trial involving 347 participants was published in Nature Medicine. The study evaluated a personalised nutrition programme.
The programme combined data on gut microbiome composition, blood glucose and triglyceride responses, medical history and food characteristics.
After 18 weeks, participants following the personalised nutrition programme experienced a greater reduction in triglyceride levels than those receiving general dietary advice. However, there was no statistically significant difference in LDL cholesterol levels. [3]
Crucially, the intervention was not based on microbiome data alone.
It involved multiple measurements and different forms of nutritional support. The study therefore does not establish that microbiome testing was the essential component responsible for the observed improvement.
It is also worth noting that several authors disclosed financial or professional relationships with the company that developed the programme.
More recently, in August 2026, long-term follow-up findings from a randomised trial involving patients with irritable bowel syndrome (IBS) were published.
The study compared a personalised diet based on gut microbiome analysis with a low-FODMAP diet.
Among participants who completed the follow-up, the personalised dietary intervention was associated with more sustained symptom improvement at 12 months.
However, the long-term analysis included only 69 participants who completed follow-up, and there were limitations relating to how the diets were implemented. The researchers themselves described the findings as preliminary and called for larger trials. [4]
These studies suggest that personalised nutrition incorporating microbiome data is an interesting area of research.
However, they do not demonstrate that commercially available gut microbiome tests can reliably determine the ideal diet for every individual.
Do We Need Gut Microbiome Tests to Improve Our Diet?
Improving dietary quality does not require gut microbiome testing. It can be achieved by following established, evidence-based nutritional recommendations.
For example, the World Health Organization recommends that adults consume at least 25 grams of dietary fibre per day, primarily from foods such as legumes, fruits, vegetables and whole grains. [6]
Dietary fibre also provides an important substrate for various gut microorganisms.
This does not mean that everyone should follow the same diet.
Personalisation is important, particularly for people with specific medical conditions, food allergies or gastrointestinal symptoms.
However, dietary advice can be tailored to an individual’s medical history, clinical presentation and established nutritional guidelines without necessarily requiring a commercial gut microbiome test.