Microbiome Studies: Association vs Causation
Why finding a microbe linked to a disease does not show it causes the disease, with the main pitfalls in microbiome studies and a checklist for reading headlines.
Content last updated . This page has not been medically reviewed.
The core problem
Most microbiome studies compare the microbes of people with a condition against people without it and report what differs. That shows an association. It does not show whether the microbes caused the condition, were changed by the condition, or were changed by something else, such as a medicine, a diet or a bowel habit. A 2020 review of the field put it plainly: whether altered gut microbes are causal, consequential or bystanders to disease is, for the most part, unresolved. [1]Evidence labels used on this page: association only, preclinical (cells/animals only) and insufficient evidence. The wider idea is covered in Observational Studies and Confounding.
Four common pitfalls
1. Confounding by host factors. In a 2020 analysis, alcohol intake and bowel movement quality were strong sources of gut microbiota variation that differed between healthy people and people with disease. Matching cases and controls on such variables reduced observed differences and spurious associations for several common diseases. [2] Stool consistency had the largest effect size and medication explained the largest share of variance in a 2016 population analysis of two cohorts of about 1,100 people each. [3]2. Mouse studies that look more certain than they are. Researchers often move gut microbes from people into rodents to test cause. A systematic review found that 95% of such studies (36 of 38) reported a transfer of disease features to the recipient animals. The authors called that rate implausible and said it overstates the microbiome's role in human disease. [1] Label: preclinical. Do not carry animal results over to people. See Cell Studies vs Animal Studies vs Human Trials.
3. One snapshot of a changing community. A 2019 expert workshop noted it is not established whether dysbiosis is a cause, a consequence or both, and said future studies should use repeated measures within individuals because a single sample misses change over time. [4]
4. Lab and analysis differences. In a blinded study across 15 laboratories, variation depended most on the sample type and origin, then DNA extraction, sample handling and analysis methods. [5] Researchers have also published a 17-item reporting checklist, called STORMS, because common reporting guidelines lacked microbiome-specific features. [6]
How the pieces fit together
| Type of finding | What it can show | What it cannot show |
|---|---|---|
| People with vs without a disease | A difference in microbes | Which came first |
| Microbes moved into mice | A possible mechanism in animals | A cause of human disease |
| Randomized trial of a product or medicine in people | Effect on the outcome measured | That the microbiome was the reason |
| Change in a microbe count or marker | That the community moved | That health improved |
A checklist for headlines
- Was this in people, mice or a dish? Mice and cells give preclinical evidence.
- Did the study account for medicines, diet, alcohol and bowel habits?
- Was the outcome a symptom or disease, or only a change in microbes?
- How many people took part, and was there a comparison group?
- Does the report say "linked" or "associated" while the headline says "causes"?
More tools are in How to Read Health Product Marketing Claims and Why a Microbial Association Is Not a Treatment Target. For the idea behind "dysbiosis" see Dysbiosis in Research and Marketing.
Frequently asked questions
If a microbe is higher in people with a disease, does it cause it?
Not by that finding alone. It may be a cause, a result, or a side effect of a medicine or lifestyle difference. [1][2]
Why do microbiome studies disagree?
Low agreement between studies is a known problem, and differences in participants and lifestyle contribute. [2] Laboratory methods add variation too. [5]
Do mouse transplant studies prove a microbe causes disease?
No. They give animal evidence only, and a systematic review questioned how often such studies report transfer of disease. [1]
Is there a standard way for researchers to report microbiome studies?
A 17-item checklist called STORMS was proposed for this purpose. [6]
More details: evidence, limits and cautions
The details behind this page: cautions, limits of the evidence and where sources disagree. Open any note to read it.
Safety note
References
- Walter J, Armet AM, Finlay BB, Shanahan F. Establishing or exaggerating causality for the gut microbiome: lessons from human microbiota-associated rodents. Cell 2020. PubMed 31978342. Read 2026-10-22.
- Vujkovic-Cvijin I et al. Host variables confound gut microbiota studies of human disease. Nature 2020. PubMed 33149306. Read 2026-10-22.
- Falony G et al. Population-level analysis of gut microbiome variation. Science 2016. PubMed 27126039. Read 2026-10-22.
- McBurney MI et al. Establishing what constitutes a healthy human gut microbiome: state of the science, regulatory considerations, and future directions. J Nutr 2019. PubMed 31373365. Read 2026-10-22.
- Sinha R et al. Assessment of variation in microbial community amplicon sequencing by the Microbiome Quality Control (MBQC) project consortium. Nat Biotechnol 2017. PubMed 28967885. Read 2026-10-22.
- Mirzayi C et al. Reporting guidelines for human microbiome research: the STORMS checklist. Nat Med 2021. PubMed 34789871. Read 2026-10-22.