What is an FDN-P?
An FDN-P — Functional Diagnostic Nutrition Practitioner — is a health practitioner trained to identify hidden stressors in the body using functional laboratory testing. Rather than treating symptoms, an FDN-P uses test data to uncover the metabolic, hormonal, digestive, and immune dysfunctions driving poor health, then builds personalised protocols to restore function.
AD — the two FDN training links above are affiliate links. If you enrol on the full FDN programme through them I receive a commission. I trained with FDN and hold the credential; that is why the page exists, and the disclosure is here so you can weigh it.
Functional Diagnostic Nutrition Practitioner. A credentialled health practitioner trained by Functional Diagnostic Nutrition® to use functional laboratory testing — including hormone panels, stool analysis, organic acids, blood chemistry, and food sensitivity — to identify hidden metabolic, hormonal, digestive, and immune dysfunction. Protocols are derived from test data, not symptom patterns alone. Core principle: Test, Don't Guess.
What the FDN-P Certification Covers
The FDN-P designation is awarded by Functional Diagnostic Nutrition®, a professional training programme founded by Reed Davis. It represents completion of a rigorous curriculum covering functional lab interpretation, clinical reasoning, and the D.R.E.S.S. protocol — Diet, Rest, Exercise, Stress Reduction, and Supplementation.
Unlike conventional nutrition qualifications, the FDN-P credential is built around laboratory data. Practitioners learn to order and interpret functional tests — including hormone panels, stool analysis, organic acids, and blood chemistry — and use those results to guide clinical decisions rather than relying on symptom patterns alone. The emphasis is on identifying why dysfunction is occurring, not just naming what symptoms are present.
The programme was designed for practitioners who found that symptom-based practice had limits — that two people with identical presentations could have completely different underlying drivers, and that without testing, protocol design is always, at some level, guesswork.
What Does an FDN-P Actually Do?
An FDN-P works with clients who haven't found answers through conventional routes — people with persistent fatigue, hormonal imbalance, digestive dysfunction, weight resistance, or complex symptom pictures that don't fit a single diagnosis.
FDN-P vs Other Practitioners
The FDN-P approach differs from conventional nutrition and medical practice primarily in its use of functional lab testing as the starting point — not an add-on.
| Approach | FDN-P | Nutritionist | GP / Doctor |
|---|---|---|---|
| Uses functional lab testing | ✓ Central | Rarely | Standard ranges only |
| Investigates root causes | ✓ Always | Sometimes | Symptom management |
| Hormones, gut, metabolism together | ✓ Integrated | Rarely | Specialist referral |
| Protocol driven by | Test data | Diet + symptoms | Symptoms + standard bloods |
| Optimal vs normal ranges | ✓ Functional ranges | Not typically | Population reference only |
| Cross-system pattern recognition | ✓ Core skill | Limited | Siloed by specialty |
FDN Foundation — and the TDG Advanced Layer
The FDN programme teaches practitioners to test before treating. The Test, Don't Guess Five-Test System extends that foundation further — adding comprehensive blood chemistry as a second clinical layer and integrating all five tests simultaneously, rather than sequentially. The result is a fuller picture of metabolic function than the core FDN panel alone can provide.
Clients on a GLP-1: what the evidence actually says
If you work with people on semaglutide or tirzepatide, three claims come up constantly. I have graded each one with sources — including the muscle-loss claim, which is not what most people think: in the SURMOUNT-1 body-composition substudy, the placebo arm lost lean mass in the same proportion. There is also a section on what US scope-of-practice advice does and does not transfer to the UK.
Read the graded write-up →The TDG Clinical Manual
Most people who land on this page are here to find out what the letters mean, not to book anything. That is a fair reason to be here, and it deserves something better than a sales pitch at the end of a definition.
So: the clinical reasoning behind all of it — why a test gets ordered, what a marker means when it sits inside a functional range but outside an optimal one, how findings in two different systems turn out to be one problem — is written down in thirteen modules. Module 01 is free and complete, no email required to read it. The remaining twelve are £97. The £97 comes off the TDG Five-Test Programme if you enrol as a client within 60 days of buying the manual. It isn’t credited against single tests or combined with the retest discount.
Practitioner? See how the programme runs in practice, from intake to retest: the TDG pathway →
Not an affiliate link — this one is mine
Occasional notes on FDN, written from the UK
Which labs work here and which need substituting, where the US material needs adapting for UK scope, and corrections when I change a position. It is not an automated sequence: I send something when there is something worth sending, and you can leave at any time. Module 01 of the TDG Clinical Manual is on the next page, free.
Who Works Well with an FDN-P Practitioner
FDN-P work is particularly suited to people who have tried conventional routes and found incomplete answers. If your tests come back normal but you don't feel normal, functional testing is designed exactly for that gap.
You've seen multiple practitioners without resolution — no clear diagnosis, or a diagnosis that doesn't fully explain your symptoms.
You suspect hormonal, digestive, or metabolic dysfunction — fatigue, weight resistance, gut symptoms, mood dysregulation, poor sleep.
You want test data driving your protocol — not a generic plan based on what usually works for most people.
You're ready to engage with the findings — functional medicine requires active participation, not passive treatment.
You've been told your blood tests are normal, but the reference ranges used were population averages — not optimal functional thresholds.
You want to understand your own biology, not just receive instructions — the TDG process is educational as well as clinical.
Stephen Duncan FDN-P MSc
Stephen Duncan is an Edinburgh-based Functional Diagnostic Nutrition Practitioner with nearly forty years of experience working with people across nutrition, functional medicine, movement, and coaching. He holds a BSc (Hons) in Developmental Biology, a PG Dip in Health Informatics, and an MSc in Coaching Studies and Applied Physiology.
Trained under Reed Davis (FDN), Bill Wolcott (Healthexcel / Metabolic Typing), Bryan Walsh, and Paul Chek, Stephen built the Test, Don't Guess clinical framework around five integrated functional tests — creating a protocol process that is fully derived from laboratory data, not clinical assumption.
He works with clients across the UK and internationally, with all consultations conducted online.
Ready to Test, Not Guess?
A free 20-minute call is the starting point — no obligation, no upsell. We look at what you've tried, what the data might show, and whether the TDG approach is a good fit for where you are.