Many people report that a gut flora home test or microbiome test has helped them. Some consultants also say:

“I have been working with these tests for years and regularly see improvements.”

The first part of this series dealt with how microbiome tests work technically and why their informative value can be limited. This second part deals with a different question: Why are gut flora tests often so convincing – even if their diagnostic value is limited?

The explanation often lies in a combination of statistical effects, psychological mechanisms and general lifestyle changes.

Many variables – something always fits.

A typical gut flora report often contains:

• Dozens or even hundreds of bacteria groups
• Various scores or rankings
• Comparisons with reference populations

Statistically, it is very likely that some of these values will appear conspicuous or seem to match individual complaints. For example, if a report assesses 80 microorganisms, it is hardly surprising that some of them fall outside an average range. In scientific studies, this issue is addressed through large samples and statistical models (Knight et al., 2018). This is much more difficult with individual home tests.

Wide scope for interpretation

Many microbiome reports use formulations such as:

• “could be associated with inflammation”
• “is associated with digestive problems in some studies”
• “can be influenced by diet”

Such statements are scientifically correct, but often formulated in very general terms. As a result, they can apply to many situations and easily create the impression of a precise diagnosis.

Standard ranges from average data

Many providers compare their own microbiome with a reference population. The problem: even among healthy people, microbiomes differ greatly. The Human Microbiome Project has already shown that the composition of the gut microbiome can vary considerably between healthy people (Human Microbiome Project Consortium, 2012). A value outside an average range therefore does not automatically mean that something is pathological.

Regression to the center

Another significant effect is the so-called regression to the center. Many people have tests carried out when their symptoms are particularly severe. Statistically, it is likely that the symptoms will improve again afterwards – regardless of any intervention. If a measure is subsequently recommended, the natural improvement can easily be attributed to this intervention.

Confirmation bias

Another psychological effect is the confirmation bias. People tend to pay more attention to information that confirms their expectations. Contradictory information, on the other hand, is often given less attention. If a microbiome report provides a plausible explanation (“This bacterium could be related to your symptoms”), this information is particularly memorable. Especially when consultants – experts to whom you pay money, after all – explain and confirm this.

Lifestyle changes after the microbiome test

After a gut flora test, many people change several aspects of their lifestyle at the same time:

• more fiber
• less ultra-processed food
• more movement
• More conscious eating behavior

These changes have been well studied and can actually improve digestion. Diet can change the gut microbiome within just a few days (David et al., 2014). However, if symptoms improve as a result, the effect is easily attributed to the test. Incidentally, very few people stick to such changes for long and revert to their old diet after 3-6 months. Then they do another microbiome test and the game starts all over again.

Why consultants can honestly believe that gut flora tests work

Many therapists report positive experiences with microbiome tests. This assessment is not necessarily dishonest. Several factors can contribute to this.

Selective perception

In practice, particularly successful cases are remembered. Cases without a visible effect, on the other hand, are remembered less strongly. This effect is described in psychology as selective perception.

Complex interventions

After a test, more than one measure is usually implemented. Several recommendations often follow at the same time:

• Change of diet
• Probiotics
• Prebiotics
• Stress reduction

If complaints improve, it is difficult to identify which measure was actually responsible.

Why microbiome reports are so convincing

In addition to psychological effects, the design of many reports also plays a role.

Lots of data generates trust

Microbiome reports often contain numerous diagrams and rankings. These visualizations give the impression of a very precise analysis. In fact, many of these visualizations are based on relative frequencies and statistical comparisons, not clinical thresholds.

Personalized language

Many reports use formulations such as:

• “Your microbiome shows …”
• “Your gut type is …”
• “The following diet could be useful for you …”

This language easily gives the impression of an individualized medical assessment.

Scientific references

Many providers refer to scientific studies. These studies are often real, but usually refer to populations or statistical correlations, not individual diagnoses. The difference between association and individual diagnosis is not always easy for readers to recognize. Actually never.

Why intestinal flora tests are convincing

Intestinal flora home tests can provide fascinating insights into the world of intestinal bacteria. However, many of the compelling elements do not necessarily come from precise medical diagnostics.

A combination of

• statistical effects
• broad interpretations
• Lifestyle changes
• psychological perception mechanisms

can make tests appear very plausible – and sometimes even contribute to improvements. However, this does not automatically mean that the microbiome analysis itself was the cause of these improvements.

FAQ – Frequently asked questions about intestinal flora home tests

Literature

David, L. A., Maurice, C. F., Carmody, R. N., et al. (2014). Diet rapidly and reproducibly alters the human gut microbiome. Nature, 505, 559-563. https://doi.org/10.1038/nature12820

Human Microbiome Project Consortium. (2012). Structure, function and diversity of the healthy human microbiome. Nature, 486, 207-214. https://doi.org/10.1038/nature11234

Knight, R., Vrbanac, A., Taylor, B. C., et al. (2018). Best practices for analyzing microbiomes. Nature Reviews Microbiology, 16(7), 410-422. https://doi.org/10.1038/s41579-018-0029-9