Knowledge base
A medical library on the gut microbiome, gut flora and gut health — written from the consulting room. Scientifically substantiated, clearly explained, and honest about what is not yet established.
Science Update
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Category 1
The fundamentals: what the gut microbiome is, why its composition receives so much research attention, and what an analysis of your personal gut ecosystem can and cannot show.
Your gut harbours trillions of bacteria, viruses, yeasts and other micro-organisms. Together they form an ecosystem that helps digest dietary fibre, produce certain vitamins and short-chain fatty acids, and coordinate daily with the immune system. We describe which groups are typically found and why their relative proportions are more informative than any single species.
The gut microbiome is a complex ecosystem in which thousands of micro-organisms work together with one another and with your body. An analysis maps the composition and characteristics of that personal ecosystem: which functional groups are present, how diverse the whole is, and how the proportions relate to one another. Because these differences vary considerably from person to person, such a picture can help tailor nutrition, lifestyle and guidance more precisely rather than relying on generic advice. An analysis is particularly valuable for people who want more insight into their gut health, their lifestyle or their personal nutritional strategy. It does not provide a diagnosis and is not a treatment; the emphasis is on gaining insight into the personal gut ecosystem and possible starting points for personal guidance, together with the doctor.
Scientific research describes associations between the composition of the gut flora and, among other things, digestion, immune function and metabolism. Associations are not proof of cause and effect; we explain what that distinction means in practice and which conclusions are, and are not, justified.
The gut microbiome takes a central place in this research because it communicates continuously with the rest of the body through several biological systems: via nerve pathways, via the immune system, via hormonal routes and via substances produced by gut bacteria. The literature describes four communication axes in the most detail; elsewhere in this knowledge base we explore each of them separately.
communication between gut and brain via nerve pathways, immune routes, hormones and microbial metabolites.
research into the relationship between the gut microbiome, inflammatory processes and skin health.
the continuous interplay between gut bacteria and the immune system.
the possible influence of the gut microbiome on hormonal regulation and metabolic processes.
Together, these axes show that the gut microbiome matters not only for the gut itself, but is part of a complex biological network within the body. This field of research is developing rapidly and many associations are not yet fully understood. JorClinic translates the available knowledge into personal guidance in an evidence-based way, explicitly stating what is, and is not, yet substantiated.
No two gut ecosystems are alike. Micro-organisms live in interdependence: they convert nutrients, produce metabolic products and influence each other's living space. As a result, the relationship between groups often says more than any single value on its own. We describe how such an ecosystem stays in balance, how it responds to changes in diet or lifestyle, and why a personal picture is needed to give meaning to the results.
Further reading: microbiome test, Personal programme — 1 year or an introductory consultation with the doctor.
Category 2
What research says about the relationship between gut health and daily life — with explicit attention to what remains uncertain.
Sleep, exercise, stress and medication use emerge in research as factors associated with the composition of the gut flora. Effect sizes vary considerably between studies and between individuals; we describe what has been found reasonably consistently and where the uncertainty lies.
Diet is the most direct and best-studied way in which the gut ecosystem is influenced. The amount and variety of plant-based fibre in particular recurs in research. At the same time, people respond differently to the same food — one reason why general dietary advice does not work equally well for everyone.
Differences are described between the gut flora of people with differing body weight, and research examines the role of the microbiome in energy extraction from food. The picture remains incomplete: the microbiome is one factor among many, and an analysis is not a treatment for weight-related issues.
Fatigue rarely has a single cause. We discuss what research shows about the role of digestion, inflammatory processes and short-chain fatty acids, and why symptoms should always be assessed medically first, before the microbiome is considered.
As people age, the gut ecosystem changes, often alongside changes in diet, medication and physical activity. What research describes about resilience in later life, and why a measurement over time carries more meaning here than a single snapshot.
The gut ecosystem is connected to digestion, immune function and metabolism. For each topic, we distinguish what is well substantiated scientifically, which associations are likely but not yet proven, and where uncertainty remains considerable.
In depth
Scientific research shows that the gut microbiome is not only involved in digestion, but is also in continuous communication with other systems in the body. That communication runs via nerve pathways, immune cells, hormones and substances produced by gut bacteria.
The literature describes four routes in the most detail: the gut-brain axis, the gut-skin axis, the gut-immune axis and the gut-hormone axis. They are not independent of one another; they involve overlapping biological processes that influence each other.
This field of research is developing rapidly. Some associations have been replicated across several independent studies, while others still rest on small or experimental research. For each axis, we therefore describe what is reasonably well established, what seems likely, and where further research is needed.
Gut and brain communicate via nerve pathways, the immune system, hormones and bacterial metabolites. What research shows about mood, stress and cognition — and what an analysis cannot do.
What research describes about the relationship between the gut microbiome and the skin: immune regulation, the gut barrier, inflammatory processes and microbial metabolites — without cause-and-effect claims.
Roughly 70% of immune cells are located in and around the gut. What research shows about the interplay between gut bacteria, immune defence, the gut barrier and inflammatory processes.
The gut microbiome may be involved in processes that influence hormonal balance — around the cycle, menopause and metabolism. Its clinical significance is still being investigated.
For whom a gut microbiome analysis may be relevant when it comes to weight management, long-standing gut complaints or optimising diet — and how results are read.
Further reading: microbiome test, Personal programme — 1 year or an introductory consultation with the doctor.
Category 3
How a gut microbiome analysis comes about, which choices JorClinic makes within it, and how to read a report.
From home collection to report: the sample is stabilised, analysed in the laboratory, and the findings are compared against reference groups. We explain which steps determine accuracy and why careful sample collection makes the difference.
Isolated bacterial values say little without context. We read the microbiome as a coherent ecosystem: which functional groups are present, how they relate to one another, and how that picture develops over time.
The analysis focuses on the complete gut ecosystem rather than a limited set of indicator bacteria. More importantly, every result is read by a doctor in the context of your history, symptoms and lifestyle.
Diversity scores, functional groups, ratios and reference ranges — we walk through the components of a report and show which deviations may be meaningful and which fall within normal variation.
From introductory consultation and home collection to laboratory analysis, a personal report and discussion with the doctor. For each step, we explain what happens, how long it takes and what is asked of you.
An analysis describes the composition of your gut ecosystem at a single point in time. It does not provide a diagnosis, does not rule out disease, and does not predict treatment outcomes. We explicitly state which conclusions are justified and which are not.
Further reading: microbiome test, Personal programme — 1 year or an introductory consultation with the doctor.
Category 4
JorClinic follows the science and the development of new diagnostic possibilities. Science gives us new insights; the doctor helps place these insights in a human context.
Science Update
Follow developments in gut microbiome research. Every month, JorClinic selects relevant new scientific publications and insights.
Archive
Every monthly edition gets its own page, so you can read back through previous updates before subscribing.
View the archive and subscribe →Further reading: microbiome test, Personal programme — 1 year or an introductory consultation with the doctor.
From knowledge to insight
General knowledge takes you some of the way. A personal picture requires a measurement, read by a doctor who knows your medical history.
A one-off, advanced analysis of your gut ecosystem with a personal report and professional explanation.
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Science Update
Every month, an overview of new scientific insights into the gut microbiome, selected and clearly summarised by JorClinic.
The articles in this knowledge base are based on scientific literature. Below is a combined overview of the key review articles and systematic reviews on the four communication axes of the gut microbiome. Click a publication to open the summary, its practical relevance and the DOI reference.
Reference 1
The gut microbiota–brain axis in behaviour and brain disorders
Nature Reviews Microbiology · 2021 · Narrative review
Reference 2
The microbiota–gut–brain axis
Physiological Reviews · 2019 · Review
Reference 3
The brain–gut–microbiome axis
Cellular and Molecular Gastroenterology and Hepatology · 2018 · Review
Reference 4
The Gut-Brain Axis: Interactions Between Enteric Microbiota, Central and Enteric Nervous Systems
Annals of Gastroenterology · 2015 · Narrative review
Reference 5
Gut microbiota and major depressive disorder: A systematic review
Journal of Affective Disorders · 2020 · Systematic review
Reference 6
Effects of microbiota-targeted interventions on depressive symptoms: Systematic review and meta-analysis
Neuroscience & Biobehavioral Reviews · 2019 · Meta-analysis
Reference 7
Brain–gut–microbiota axis in depression: A historical overview and future directions
Brain Research Bulletin · 2022 · Narrative review
Reference 8
Gut microbiota and mental health: A systematic review
Neuroscience & Biobehavioral Reviews · 2024 · Systematic review
Reference 9
Psychobiotics and the Microbiota–Gut–Brain Axis
Trends in Neurosciences · 2016 · Narrative review
Reference 10
Microbiota–gut–brain axis and neurological disorders
Journal of Neuroinflammation · 2020 · Review
Reference 11
Gut–Skin Axis: Current Knowledge of the Interrelationship between Microbial Dysbiosis and Skin Conditions
Microorganisms · 2021 · Review
Reference 12
The Gut Microbiome: Human Health and Inflammatory Skin Diseases
Annals of Dermatology · 2020 · Review
Reference 13
The Skin Microbiome and the Gut–Skin Axis
Clinics in Dermatology · 2021 · Review
Reference 14
The Role of Gut Microbiome in Inflammatory Skin Disorders
Skin Pharmacology and Physiology · 2022 · Systematic review
Reference 15
Gut Microbiome in Psoriasis: An Updated Review
Pathogens · 2020 · Review
Reference 16
Skin Microbiota in Health and Disease: From Sequencing to Biology
Journal of Dermatological Science · 2020 · Review
Reference 17
The Gut Microbiome–Skin Axis
The Skin Microbiome Handbook · 2020 · Review
Reference 18
Gut Microbiome Dysbiosis in Atopic Dermatitis
Compiled review literature · 2024 · Review
Reference 19
Gut Microbiota and Acne Vulgaris
Compiled review literature · 2024 · Review
Reference 20
Gut Microbiota, Immune Regulation and Skin Disease
Compiled review literature · 2024 · Review
Reference 21
Interaction between microbiota and immunity in health and disease
Cell Research · 2020 · Review
Reference 22
Interaction Between the Microbiota, Epithelia, and Immune Cells in the Intestine
Annual Review of Immunology · 2020 · Review
Reference 23
Gut Microbiota and Systemic Immunity in Health and Disease
International Immunology · 2021 · Review
Reference 24
Crosstalk between Gut Microbiota and Host Immunity
Signal Transduction and Targeted Therapy · 2023 · Review
Reference 25
Short-chain Fatty Acids: Linking Diet, the Microbiome and Immunity
Nature Reviews Immunology · 2024 · Review
Reference 26
Human Microbiome as an Immunoregulatory Axis
Biomedicines · 2024 · Review
Reference 27
The Gut Microbiome and Inflammatory Bowel Diseases
Annual Review of Medicine · 2022 · Review
Reference 28
Current Understanding of Probiotic Strains and Immune Function
Nutrients · 2025 · Review
Reference 29
Gut Microbiota and Autoimmunity
Compiled review literature · 2024 · Systematic review
Reference 30
Gut Microbiota and Immune Regulation: Mechanisms and Clinical Implications
Compiled review literature · 2024 · Review
Reference 31
The Gut Microbiome Influences Host Endocrine Functions
Endocrine Reviews · 2019 · Review
Reference 32
Estrogen-gut microbiome axis: Physiological and clinical implications
Maturitas · 2017 · Review
Reference 33
The Impact of Estrogen Status on the Gut Microbiome: A Systematic Review and Meta-analysis
Frontiers in Endocrinology · 2026 · Systematic review + meta-analysis
Reference 34
The Gut Microbiome and Female Reproductive Health
Trends in Endocrinology and Metabolism · 2021 · Review
Reference 35
Gut Microbiota and Polycystic Ovary Syndrome: A Systematic Review
Compiled review literature · 2024 · Systematic review
Reference 36
The Role of Gut Microbiota in Obesity and Metabolic Disorders
Nature Reviews Microbiology · 2021 · Review
Reference 37
Gut Microbiota and Metabolic Health: The Role of Gut Hormones
Compiled review literature · 2024 · Review
Reference 38
Gut Microbiota and Insulin Resistance: Mechanisms and Therapeutic Perspectives
Compiled review literature · 2024 · Review
Reference 39
Spotlight on the Gut Microbiome in Menopause: Current Insights
Nutrients · 2022 · Review
Reference 40
Diet, the Gut Microbiome, and Estrogen Physiology: A Review in Menopausal Health and Interventions
Compiled review literature · 2026 · Review
JorClinic Personal programme — 1 year
Follow the development of your gut microbiome with repeated measurements, personal interpretation by a doctor and adjustments where needed. Would you prefer a single snapshot first? The one-time analysis is a good first step. Not sure yet? Discuss it in an introductory consultation (€99) — that amount is fully deducted when you purchase a JorClinic product.
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