A practising UK FDN-P's honest account of what Functional Diagnostic Nutrition is, what it isn't, and why it remains the most rigorous training available for health practitioners who want to work with lab data.
FDN is taught globally by Reed Davis and his team in the United States. But if you're based in the UK and asking whether FDN is right for you — which labs to use, how to navigate the NHS context, and what a UK practice actually looks like — there isn't much written from that vantage point.
This page is my attempt to fill that gap. I completed FDN certification after more than two decades of clinical practice, having already trained under Bill Wolcott (Metabolic Typing), Paul Chek, Bryan Walsh, and others. FDN gave me the systematic, lab-anchored framework I had been building towards.
I have no reason to oversell it. But I also have 37 years of clinical results that tell me this approach — test, don't guess — works consistently when other approaches have failed the patient.
"FDN isn't a protocol. It's a way of thinking about the body. Once you see that the same hidden stressors — metabolic, digestive, immune, hormonal — show up underneath almost every chronic complaint, you can't unsee it."
— Stephen Duncan FDN-P BSc Hons MSc, Detective HealthFDN is a health and wellness practice methodology created by Reed Davis, built on two decades of working with chronic health clients at the Functional Medicine Rejuvenation Centre in California. The core premise is straightforward: most chronic health problems — fatigue, weight dysregulation, poor sleep, hormonal symptoms, digestive dysfunction, cognitive issues — share the same underlying drivers, even when their surface presentations differ.
FDN calls these H.I.D.D.E.N. stressors: Hormonal, Immune, Digestive, Detoxification, Energy, and Nervous system imbalances. The training teaches practitioners to investigate these systematically using functional lab testing, then correct them through the D.R.E.S.S. protocol rather than chasing symptoms.
Metabolically appropriate nutrition — not one-size-fits-all eating
Sleep quality, circadian rhythm, recovery between demands
Appropriate movement — not too much, not too little, matched to current capacity
Total load reduction — psychological, physiological, and environmental
Targeted therapeutic supplementation informed by actual lab findings
What distinguishes FDN from most functional health training is that it insists on testing before recommending. The D.R.E.S.S. protocol is only meaningful once you have the lab data to personalise it. This is the discipline most practitioners — however well-intentioned — skip.
FDN teaches practitioners to run tests across five functional systems. In the US, FDN recommends specific labs (including Fluids IQ for OAT panels). In the UK, the landscape is different — and knowing which UK-available equivalents to use is essential before you start practising here.
Dried urine testing of sex and adrenal hormones, metabolites, cortisol curve, and organic acid markers. The gold standard for HPA axis and hormonal pattern analysis.
Comprehensive stool testing for pathogens, parasites, bacteria, fungi, and intestinal markers. Identifies gut dysfunction that drives systemic symptoms.
Urine-based test measuring over 70 metabolic markers: mitochondrial function, B vitamin status, neurotransmitter metabolites, yeast/bacterial overgrowth, and detoxification capacity.
Identifies delayed IgG-mediated food reactions driving inflammation, digestive symptoms, brain fog, and skin conditions. Distinct from IgE allergy testing.
Functional interpretation of standard blood markers — not just flagging out-of-range values but identifying patterns within the "normal" range that signal emerging dysfunction.
FDN's US curriculum references Fluids IQ for Organic Acids Testing, which is not available in the UK. Most UK FDN-Ps default to the Mosaic Diagnostics OAT (formerly Great Plains Laboratory), available via Nordic Labs — a solid, well-validated panel that covers the core organic acids terrain FDN requires.
At Detective Health, we use Metabolomix+ from Genova Diagnostics instead. It covers everything the standard OAT does, and adds amino acid profiling and essential fatty acid analysis in a single urine and blood spot collection. That's a meaningfully more complete metabolic picture — particularly useful for complex fatigue, neurological presentations, and clients where protein metabolism or inflammatory fatty acid balance is part of the clinical picture.
The same logic applies to food sensitivity testing. Most practitioners use a 190-marker IgG panel. We use the HealthBeings 286-marker IgG — 50% more foods tested at a comparable client price point. When your philosophy is to look under every rock, that extra breadth matters. We add the 176-marker IgE panel when allergy mechanisms need ruling in or out alongside sensitivity patterns.
These aren't criticisms of what other FDN-Ps use — the standard panels are good. They're the choices we've made in building a practice where no clinically relevant stone is left unturned.
FDN has trained practitioners across 50+ countries. But in the UK, FDN-P certification remains relatively uncommon — and UK-specific content, lab guidance, and practitioner community is thin. Most UK health practitioners with a testing-based approach have either trained in naturopathy, nutritional therapy, or functional medicine (IFM), none of which centres lab data interpretation the way FDN does.
UK clients are increasingly lab-savvy — they're ordering private blood tests, asking about gut microbiome panels, and coming to practitioners with NHS results they want interpreted properly. The demand for a practitioner who knows what to test and how to read it is significant and growing.
FDN is one of the few training programmes that genuinely builds that skill from the ground up — not as an add-on, but as the foundation.
One of the reasons I built Detective Health's clinical platform was to have resources I wished had existed when I completed my FDN training. UK lab landscape, NHS context, client communication materials, clinical tools — none of that was readily available. If you're a UK FDN-P or in training, you're welcome to use what's here.
In-depth guides on interpreting DUTCH, GI-MAP, OAT, and blood chemistry — written for practitioners, not patients.
Which labs to use, how to order, home draw options, and what each panel covers. The UK-specific guide FDN doesn't publish.
42 clinical tools including lab analysers, pattern recognition engines, and client-facing assessment tools. Built for working practitioners.
Resources, diagrams, and reference materials to support your practice. Including the methylation cycle, HPA axis maps, and more.
AI-powered clinical knowledge assistant trained on DH protocols, lab reference ranges, and functional health principles. For client support between sessions.
The Test, Don't Guess series — practitioner-grade clinical guides covering the TDG methodology and all five functional test categories.
If you're a qualified UK FDN-P looking for clinical support, UK lab guidance, or access to the DH clinical tool suite and practitioner resources, the Detective Health practitioner portal is worth exploring — it was built partly to fill the gap that UK-specific FDN resources have historically left.
We are also open to referral arrangements with qualified UK FDN-Ps — if a client finds Detective Health but is better served by a practitioner closer to them geographically or with a different clinical specialisation, we would rather connect them with the right person than lose them to a generic search result. If that kind of working relationship interests you, get in touch directly.
FDN certification is a significant investment — of time, money, and commitment to a different way of practising. It's worth it. I say that as someone who had already trained for twenty years before completing it. If you're ready to explore, the link below is where to start.
Explore FDN Certification → See the Full Curriculum