I came to Functional Diagnostic Nutrition with a science degree, two decades of clinical practice, an MSc, and what I thought was a comprehensive understanding of human health. I was wrong. Not about the basics — the physiology was solid. But about the approach. About how to actually find out what was going wrong in a specific person’s body, rather than making educated guesses based on symptoms that could mean a hundred different things.
That’s what FDN gave me. A methodology. A systematic approach to identifying the actual root causes of dysfunction rather than chasing symptoms from pillar to post with generic protocols that worked for some people and not for others — and nobody could explain why.
If you’ve come across the phrase “test, don’t guess” anywhere on this site, FDN is where it comes from. This post explains what FDN actually is, what the training covers, and why I think it’s the right foundation for anyone serious about practising functional medicine — or understanding their own health at a level that goes beyond what conventional medicine offers.
What Is Functional Diagnostic Nutrition?
Functional Diagnostic Nutrition — FDN — is a clinical training programme developed by Reed Davis, a Board Certified Holistic Health Practitioner who spent years running one of the most productive wellness clinics in California. Davis recognised that the most common reason health interventions fail is that practitioners are working with incomplete information. They’re making recommendations based on symptoms and assumptions rather than objective data about what’s actually happening inside a specific person’s body.
FDN addresses this by training practitioners to use functional laboratory testing — the same tests I use in the TDG five-test programme — to identify what Davis calls H.I.D.D.E.N. stressors: the upstream root causes that drive most of the chronic symptoms people present with.
Hormones — HPA axis, thyroid, sex hormones, adrenal function
Immune — gut-associated immunity, inflammation, autoimmune patterns
Digestion — digestive capacity, microbial balance, barrier integrity
Detoxification — liver phase I and II, methylation, oxidative stress
Energy — mitochondrial function, Krebs cycle, ATP production
Nervous system — autonomic balance, neurotransmitter status, vagal tone
The key distinction from conventional nutritional therapy or health coaching is this: FDN practitioners don’t guess. They test. They read results using functional reference ranges — ranges that reflect optimal physiology rather than population averages — and design interventions based on what the testing actually reveals, not on what the symptoms suggest.
The most common reason health interventions fail is that practitioners are working with incomplete information. FDN addresses this at the foundation — not by treating symptoms, but by identifying their actual cause.
How FDN Is Different From Other Training
I’ve had a lot of training across my career. BSc in Developmental Biology from Glasgow. MSc in Coaching Studies with Applied Sports Science. Advanced Metabolic Typing under Bill Wolcott, C.H.E.K. certifications, MTHFR and methylation training, Nordic Laboratories dnalife diploma. Each added something. FDN was categorically different in one specific way: it taught me to integrate.
Most training teaches you a system in isolation. Blood chemistry is taught separately from hormone testing, which is taught separately from stool analysis, which is taught separately from organic acids. You end up with knowledge in silos. FDN teaches you how tests work together — how a cortisol pattern on the DUTCH panel relates to the gut immune markers on the GI-MAP, how both relate to the neurotransmitter metabolites on the OAT, how all three feed into the blood chemistry picture. It’s the cross-referencing of findings across multiple tests that reveals patterns invisible to any single investigation.
This is what the 900+ data points I reference in the TDG programme actually means. Not 900 isolated numbers — 900 interrelated data points that together produce a clinical picture far more detailed and actionable than anything a standard blood panel can show.
The D.R.E.S.S. Protocol
FDN training is built around what Davis calls the D.R.E.S.S. protocol — Diet, Rest, Exercise, Stress reduction, and Supplementation. This isn’t a rigid formula; it’s a framework for addressing the lifestyle foundations that determine whether any clinical intervention will actually hold. You can prescribe the most targeted supplement protocol in the world. If the person’s diet is undermining it, their sleep is inadequate, and they have no stress management practice, the protocol will produce temporary results at best.
FDN teaches practitioners to address all five domains simultaneously rather than sequentially. It also teaches the intervention hierarchy — the principle that supplementation is the last layer, not the first, and that removing the cause of dysfunction always precedes any attempt to support the affected system. This is the framework that Module 1 of the TDG Resource Manual is built on.
If you’re a health professional who wants to integrate functional laboratory testing into your practice — or someone who wants the most rigorous training available for understanding and improving your own health — the FDN programme is where I’d point you. This is the training behind the TDG methodology. Comprehensive, clinically grounded, and unlike most health certifications, it teaches you to work with objective data rather than educated guesswork.
Learn More About FDN Training → Affiliate disclosure: I recommend FDN because it is the training that shaped my clinical approach. I receive a commission if you enrol through this link, at no additional cost to you.Why I Still Use This Framework After 37 Years
I qualified as an FDN-P after more than two decades of clinical practice. By that point I had already developed strong views about what worked and what didn’t. I was not easy to impress. But FDN gave me a structured language for things I had been observing clinically for years without being able to systematically explain. It gave me a reproducible framework for identifying why a protocol that worked brilliantly for one client produced no result in another with identical symptoms.
The answer, in almost every case, came down to the same thing: the underlying causes were different. Same symptoms, completely different root causes. And without testing — without actual data — there is no reliable way to tell the difference.
This insight seems obvious once you’ve internalised it. But it runs directly against the way most of the health industry operates. Most practitioners — good, well-intentioned, trained practitioners — are still working from symptom patterns and population-level research to design interventions for individual people. FDN is one of the few training programmes that systematically addresses this problem rather than working around it.
What FDN Training Gave Me That Nothing Else Did
Three things stand out after many years of clinical integration:
Functional reference ranges. The difference between a ferritin of 18 being “normal” and being inadequate for thyroid conversion, dopamine production, and sustained energy is not taught in conventional nutritional therapy programmes. FDN trains practitioners to interpret markers against optimal physiological benchmarks rather than population statistical thresholds. This single shift changes clinical interpretation fundamentally.
Cross-test integration. Reading a DUTCH hormone panel in isolation is useful. Reading it in the context of GI-MAP immune markers, OAT neurotransmitter metabolites, and blood chemistry inflammatory status is transformative. FDN teaches the clinical logic for how these tests interact — how the HPA axis pattern on the DUTCH predicts what you’ll find on the GI-MAP, how both inform what the blood chemistry is showing. This is the foundation of the Detective Health platform’s cross-test analysis architecture.
The practitioner as investigator. The FDN model positions the practitioner as a detective — following the evidence, not fitting symptoms to preferred diagnoses. The clue is in the name. Test, don’t guess. Find the actual cause, then address it. This philosophy is so embedded in my practice that it became the title of my book and the brand name of this platform.
FDN and the Future of Personalised Health
Mainstream medicine is moving, slowly, toward what functional medicine has been doing for decades. Precision medicine, nutrigenomics, pharmacogenetics — these are all expressions of the same underlying insight: that individual biochemical variation matters enormously, that population averages tell you very little about any specific person, and that interventions need to be designed around data rather than demographics.
FDN is ahead of that curve. It has been applying precision thinking to clinical practice since Reed Davis developed the framework in the early 2000s. The laboratory technology has improved dramatically — the DUTCH hormone test, DNA-based GI-MAP stool analysis, and advanced organic acids panels all postdate the early FDN curriculum but slot naturally into its framework. The underlying logic hasn’t changed because it didn’t need to.
If you’re a practitioner looking to add genuine clinical depth to your work — particularly if you’re already using functional testing and want a more rigorous interpretive framework — FDN is the most comprehensive training available. If you’re not a practitioner but are deeply interested in your own health and have been frustrated by the limitations of standard medical investigation, the FDN health coach programme is worth knowing about.
For practitioners thinking about how AI is changing clinical practice — and how to use it effectively without losing the clinical reasoning that makes functional medicine work — Mark Playne’s book is one of the most practically useful things I’ve come across. He writes about AI integration in health practice with the kind of nuance and clinical grounding that most AI-for-health content completely lacks.
Get AI & I by Mark Playne → Affiliate disclosure: I receive a commission if you purchase through this link, at no additional cost to you.The TDG Platform — FDN in Practice
Everything on the Detective Health platform is an expression of FDN methodology. The five-test programme uses the same testing framework FDN trains practitioners to use. The clinical tools — the blood chemistry interpreter, the DUTCH analyser, the GI-MAP interpretation engine, the OAT pattern layer — are all built on the cross-test integration logic that FDN teaches. The clinical self-assessment tools in the TDG platform encode the same HIDDEN stressor framework that Davis developed and that I’ve been applying and refining for nearly two decades.
The resource manual — a 13-module clinical reference library — draws heavily on the FDN interpretive framework alongside the wider functional medicine evidence base: Metabolic Nature Profiling, Bryan Walsh’s blood chemistry interpretation, the HPA axis clinical hierarchy. FDN is the methodological spine. Everything else attaches to it.
If you want to understand the thinking behind the TDG approach in depth, the FDN training is the most direct route. If you want to experience it as a client, the TDG five-test programme is where that methodology is applied to your specific biochemistry.