Early in my clinical career, I kept encountering a puzzle that conventional nutrition training couldn't explain. Two clients would follow the exact same dietary recommendations — same macronutrient ratios, same meal timing, same food quality — and get completely opposite results. One would thrive: energy improving, weight normalising, mood stabilising. The other would struggle: fatigue worsening, weight unchanged, cravings intensifying.
The conventional explanation was compliance. "They must not be following it properly." But I knew they were. I could see the food diaries, hear the frustration in their voices, observe the genuine effort. Something else was happening — something that generic nutrition had no framework to explain.
The answer is Metabolic Nature. Not a diet. Not a food philosophy. A framework for understanding why the same fuel produces such dramatically different outcomes in different people — and how to identify which fuel your specific biochemistry requires.
The Science of Biochemical Individuality
In 1956, biochemist Roger Williams published his groundbreaking work documenting remarkable variation in human anatomy, physiology, and biochemistry. His research demonstrated that nutritional needs can vary by 700% or more between individuals for certain nutrients. Enzyme activity differs dramatically between healthy people — some produce ten times more of certain digestive enzymes than others. Organ sizes and shapes vary considerably — stomachs can differ by 600% in capacity between individuals. Metabolic rates differ by 50% or more between people of similar size and activity levels. Neurotransmitter levels show enormous individual variation affecting mood, cognition, and behaviour.
His conclusion: generic nutritional recommendations would be optimal for virtually no one and potentially harmful for some. Nearly seventy years later, mainstream nutrition still largely ignores this, continuing to issue one-size-fits-all guidelines that fail millions of people. The TDG Metabolic Nature framework is, in part, the clinical application of what Williams understood in 1956.
Consider the diet wars: low-fat versus low-carb, vegan versus carnivore, Mediterranean versus paleo. Each has passionate advocates who've experienced genuine transformations. Each has equally passionate critics for whom it failed miserably.
What if everyone is right — for themselves? What if the vegan who thrives on plants and the carnivore who thrives on meat are both accurately reporting their experience? The reason they need such different approaches isn't discipline, belief, or willpower — it's fundamental differences in how their cells convert food to energy. Discovering your Metabolic Nature is how you end the confusion about what you should eat.
The Two Underlying Systems — What Determines Your Nature
Metabolic Nature rests on two interconnected physiological systems that determine how your body converts food to energy. These systems interact — which is why six distinct Metabolic Nature profiles emerge from their combination rather than a simple three-way split.
System 1 — Autonomic Dominance
Your autonomic nervous system has two branches: sympathetic (fight-or-flight) and parasympathetic (rest-and-digest). While these systems respond to circumstances, people tend toward constitutional dominance in one branch, and this affects everything from digestion speed to stress response to ideal macronutrient ratios.
Sympathetic-dominant individuals tend to be more anxious and wired. Their nervous systems run "hot." They have faster metabolisms and often burn through nutrients quickly. Heavy, grounding foods — proteins and fats — help calm their system and provide sustained energy. These are often the people who can't skip meals, who get "hangry" easily, who feel anxious when they haven't eaten.
Parasympathetic-dominant individuals tend to be calmer and more laid-back. Their nervous systems run "cooler." They have slower metabolisms and don't need as much fuel. Heavy meals can overwhelm their slower digestive processes. These are the people who can forget to eat, who feel better with lighter meals, who feel sluggish after a heavy Sunday roast.
System 2 — Cellular Oxidation Rate
Your cells convert nutrients to energy through oxidation — controlled burning of fuel. The rate at which this happens varies significantly between individuals and is the most important single metabolic variable in nutrition.
Fast oxidisers burn through fuel quickly. Simple carbohydrates combust too fast, causing blood sugar spikes followed by crashes. They need the sustained-release energy from proteins and fats — fuels that burn slowly and provide steady energy over hours rather than minutes. They crave rich, savoury foods. They feel unsatisfied by "light" meals.
Slow oxidisers burn fuel more gradually. They can extract sufficient energy from carbohydrates without the crashes fast oxidisers experience. Heavy proteins and fats can bog down their system, requiring more metabolic effort than the energy they return. They have smaller appetites. They prefer chicken to beef, fish to red meat.
These two systems interact — a sympathetic-dominant fast oxidiser has different needs than a parasympathetic-dominant fast oxidiser, even though both are fast oxidisers. The combination produces six distinct Metabolic Nature profiles.
The Grounded Nature — Why I Felt Terrible as a Vegetarian
For years, I followed the prevailing wisdom. Eat less meat. More vegetables. Reduce fat. I became largely vegetarian, convinced I was doing the healthiest thing possible. All the books said so. All the health authorities agreed. Plant-based was optimal.
Except I felt terrible. My energy was erratic. My mood was unstable. I was hungry constantly, yet nothing satisfied me. I developed cravings for red meat that I tried to suppress through willpower. I told myself the cravings meant my body was "detoxing" from years of heavy meat consumption. I just needed to push through.
Then I assessed my Metabolic Nature properly. And there it was, clear as day: I was a strongly Grounded type — a slow oxidiser with parasympathetic dominance. My body literally needed the very foods I'd been denying it for years.
The shift was dramatic. I started eating according to my nature — substantial protein at every meal, quality fats, fewer grains and starches. Within weeks, I felt like a different person. Energy stabilised. Mood evened out. Hunger normalised. The cravings disappeared because they'd never been "detox" — they'd been my body screaming for what it needed.
A Grounded type as a vegetarian is like running a diesel engine on petrol. Neither the food nor the diet was wrong in absolute terms. The diet was wrong for me. Understanding this distinction — and applying it — is what the Metabolic Nature framework delivers.
"Nothing was wrong with them. They were just using the wrong fuel."
Stephen Duncan FDN-P MScThe Six Metabolic Natures — Complete Profiles
From the interaction of autonomic dominance and cellular oxidation rate, six distinct Metabolic Nature profiles emerge. These are not personality types or dietary philosophies — they are biochemical templates that determine how your body converts different macronutrients to energy, how quickly it processes fuel, and what combinations of food make you feel your best.
Sympathetic Dominant
The High-Performance Engine
The most metabolically demanding profile. Kinetic types burn through fuel rapidly and run "hot" — their nervous systems are primed for action and their cellular metabolism is fast. They need the most substantial fuel mix of all six natures: rich, dense proteins and fats at every meal to maintain blood sugar stability and sustain energy.
Without adequate protein and fat, Kinetic types crash — hard. Simple carbohydrates combust too quickly, producing a brief energy spike followed by a dramatic drop that triggers anxiety, irritability, and intense food cravings. They are typically the "hangry" person. They cannot skip meals. They feel anxious when not properly fuelled.
Recognise this nature if: Strong appetite even after meals; crave steak, cheese, eggs, nuts; feel unsatisfied by salads or light meals; energy crashes noticeably between meals; coffee can cause jitteriness; vegetarian diets made you feel significantly worse.
Optimal: Protein 35–40% · Fat 35–40% · Carbs 20–30%Prioritise
Beef, lamb, pork, dark poultry meat, salmon, sardines, whole eggs, organ meats, butter, ghee, avocado, full-fat dairy (if tolerated), nuts. Non-starchy vegetables and berries for carbohydrate.
Parasympathetic Dominant
The Steady, Efficient Engine
The Grounded nature has a cooler, more efficient metabolism. Slow oxidation means fuel burns gradually — carbohydrates are extracted efficiently without the blood sugar swings that fast oxidisers experience. The parasympathetic dominance means the digestive system operates well with proper pacing. Substantial meals are welcomed; excessive grazing is not needed.
Heavy, fat-rich diets can bog down a Grounded type — the liver and digestive system face a metabolic burden that the slow-burning oxidation rate can't efficiently process. This was the vegetarian disaster described above: removing the foods that grounded a Grounded type. Ironically, the Grounded nature often thrives on significant protein and fat — but at its own pace, in substantial meals rather than constant snacking.
Recognise this nature if: Can comfortably go 4–5 hours between meals; feel sated with substantial, well-composed meals; feel sluggish after very heavy fatty meals but energised after protein-rich meals; prefer darker meat; do well with moderate carbohydrate and high fat; feel best with 2–3 meals rather than constant eating.
Optimal: Protein 30–35% · Fat 30–35% · Carbs 30–40%Prioritise
Quality red meat and dark poultry, whole eggs, oily fish, full-fat dairy, moderate nuts. Plenty of non-starchy vegetables. Smaller, well-spaced carbohydrate portions from whole food sources.
Variable Autonomic
The Hybrid Engine
Catalyst types experience characteristics of both fast and slow oxidation — they need balance across all three macronutrients and are the most sensitive to dietary extremes. Going too heavy (keto, carnivore) or too light (high-carb vegan) throws them off. They thrive on variety and moderation. Extreme approaches consistently disappoint them even when they work spectacularly for someone else.
The Catalyst nature is also the most challenging to identify precisely because of its variability. Appetite can shift between strong and light depending on stress, sleep, exercise, and season. This adaptability is a metabolic strength — but it can produce confusion when trying to identify a stable dietary approach. The answer is balance, not extremes.
Recognise this nature if: Moderate appetite that varies day to day; enjoy a wide range of foods without strong cravings in either direction; feel off-balance on exclusive approaches; can identify with both Kinetic and Grounded descriptions; do poorly on any extreme diet; need the most flexibility.
Optimal: Protein 30% · Fat 30% · Carbs 40%Prioritise
Both light and dark meats, full range of fish, whole eggs, moderate dairy, legumes, whole grains, root vegetables, fruits, rotating fat sources. Variety is the key principle.
Slow-to-Moderate Oxidiser
The Balanced, Resilient Engine
Endurance types have the most metabolically balanced autonomic nervous system — neither sympathetic nor parasympathetic dominant — combined with a moderate oxidation rate. This produces a naturally resilient metabolism that handles a wide range of foods well and maintains stable energy across varied nutritional inputs. It is the profile most associated with good metabolic flexibility.
Endurance types rarely have dramatic reactions to food — they don't crash hard on carbohydrates, nor do they feel dramatically better on high protein. This can make identification harder: they often feel "fine" on many different approaches. The goal for this nature is optimisation rather than correction — finding the specific composition that produces their personal best rather than simply adequate function.
Recognise this nature if: Steady, reliable energy across the day; tolerate most foods without dramatic responses; moderate appetite with good satiety signals; function well with or without breakfast; neither extreme diet type produces strong positive or negative response; good exercise recovery even with varied nutrition.
Optimal: Protein 25–30% · Fat 25–30% · Carbs 40–50%Prioritise
Lean proteins alongside moderate fatty proteins, whole grains, root vegetables, abundant non-starchy vegetables, moderate fruit, moderate healthy fats. Quality and variety over restriction.
Mixed Oxidiser
The Stress-Primed Engine
Adaptive types have sympathetic nervous system dominance combined with a variable oxidation rate — they are constitutionally primed for stress response and can perform well under pressure, but they pay a higher metabolic cost for chronic stress than other natures. Their dietary needs shift significantly with stress load: under high demand, they need more substantial, grounding nutrition; during recovery periods, lighter foods serve them well.
The Adaptive nature often looks like a Kinetic type during stressful periods and a Catalyst type during calmer ones. This variability is not inconsistency — it is the Adaptive nature's characteristic responsiveness to autonomic state. Nutrition must track the stress cycle for this type to work consistently.
Recognise this nature if: Nutritional needs feel notably different in busy versus quiet periods; feel anxious or irritable when meals are missed during high-stress times but can skip meals easily when relaxed; benefit from grounding foods under pressure; experience more energy crashes during stressful periods than similar-effort workloads would seem to justify.
Optimal: Context-dependent — Protein 30–40% · Fat 25–35% · Carbs 25–40%During high-demand periods
Increase protein and fat substantially. Regular meals, no skipping. Under lighter load: moderate all macros, higher carbohydrate tolerance. Track stress cycle to match nutrition accordingly.
Variable Oxidiser
The Cool, Light-Running Engine
Calibrated types have a parasympathetic-dominant nervous system combined with a variable oxidation rate that tends toward slow. They have the lightest dietary requirements of the six natures — heavy, rich foods genuinely feel burdensome rather than satisfying. They thrive on higher carbohydrate from whole food sources, lean proteins, and moderate fat. A large, protein-and-fat-heavy meal leaves them tired and sluggish for hours.
Calibrated types often do very well with plant-forward eating, provided protein remains adequate. They may have been misled by the prevailing high-fat, high-protein dietary trend into eating in a way that exhausts rather than energises them. They run cooler metabolically, prefer warm environments and warm foods, and are more likely to run cold and need warmth than other natures.
Recognise this nature if: Prefer chicken breast over steak, white fish over salmon; feel genuinely heavy after rich meals; naturally gravitate toward salads, grains, vegetables; can skip breakfast comfortably; run cold and prefer warm environments; have done well on Mediterranean or plant-forward approaches; coffee rarely causes jitteriness.
Optimal: Carbs 50–60% · Protein 20–25% · Fat 15–25%Prioritise
Chicken breast, turkey, white fish (cod, halibut, sole), egg whites, low-fat dairy, legumes, whole grains, root vegetables, fruits, moderate plant-based fats (olive oil, small amounts of avocado and nuts).
The Self-Assessment — A Starting Point, Not a Diagnosis
This 20-question assessment provides a preliminary indication of your Metabolic Nature. Answer based on your natural tendencies — what feels true when you're not following any particular diet or health programme. Choose whichever response is most typical for you. Where both seem equally true, consider that you may lean toward Catalyst or Endurance — the balanced profiles.
A = tends toward Kinetic/Adaptive/Grounded · B = tends toward Calibrated/Catalyst/Endurance. Score A and B answers separately, then compare to the scoring guide below.
- 1Appetite and meal size: (A) Strong appetite, comfortable with large meals — (B) Smaller appetite, prefer lighter meals
- 2Between-meal hunger: (A) Get hungry between meals, need snacks to maintain energy — (B) Can easily go 4–5 hours between meals
- 3Food preferences: (A) Crave rich, savoury foods — steak, cheese, eggs, nuts — (B) Prefer lighter foods — salads, fish, chicken, fruit, vegetables
- 4After a large meal: (A) Feel satisfied and energised after a hearty meal — (B) Feel tired, sluggish, or uncomfortable after heavy meals
- 5Response to fatty foods: (A) Digest fatty foods well, feel satisfied after eating them — (B) Fatty foods sit heavily; feel better with lower-fat options
- 6Breakfast: (A) Do best with a substantial breakfast — eggs, bacon, meat — (B) Prefer a lighter breakfast or can skip breakfast entirely
- 7Response to carbohydrates: (A) Carb-heavy meals leave me hungry again soon — (B) Carbohydrate-based meals sustain my energy well
- 8Energy patterns: (A) Energy tends to be variable — peaks and crashes — (B) Energy tends to be steady throughout the day
- 9Skipping meals: (A) Irritable, weak, or unable to concentrate if I skip a meal — (B) Can skip meals without much difficulty
- 10Response to coffee: (A) Coffee makes me jittery, anxious, or crashes energy later — (B) Tolerate coffee well; it gives sustained, comfortable energy
- 11Evening eating: (A) Sleep better after a more substantial dinner — (B) Heavy dinners disrupt my sleep; prefer lighter evening meals
- 12Sweet cravings: (A) Sweets trigger more cravings; do better avoiding them — (B) Can enjoy sweets in moderation without triggering cascading cravings
- 13Body temperature: (A) Tend to run warm and rarely feel cold — (B) Tend to feel cold and prefer warm environments
- 14Stress response: (A) Under stress — anxiety, tension, or hyperactivity — (B) Under stress — fatigue, withdrawal, or low mood
- 15Salt preference: (A) Love salt and often add it to food — (B) Not particularly drawn to salty foods
- 16Vegetarian experience: (A) Tried vegetarian eating and felt worse — tired, hungry, foggy — (B) Do well with plant-forward eating or have been vegetarian comfortably
- 17Meat preferences: (A) Prefer dark meat — thighs, legs, fattier cuts — (B) Prefer white meat — chicken breast, turkey, leaner cuts
- 18Snacking: (A) Often need snacks between meals to maintain energy — (B) Rarely need snacks between meals
- 19Digestion: (A) Digest rich, heavy meals without significant problems — (B) Rich, heavy meals cause digestive discomfort or fatigue
- 20Weight patterns: (A) Tend to gain weight easily, especially around the middle — (B) Have difficulty gaining weight or maintain weight easily
14–20 A's: Likely Kinetic or Adaptive nature — prioritise protein and fat, reduce carbohydrates, never skip meals
14–20 B's: Likely Calibrated or Endurance nature — higher carbohydrates from whole foods, lean proteins, moderate fat
8–13 each: Likely Catalyst nature — balance all three macronutrients, avoid extremes in either direction
Variable between A and B depending on stress: Likely Adaptive nature — track stress load and adjust nutrition accordingly
This assessment is a starting point. If none of the profiles clearly fits, consider that functional testing may reveal gut infections, blood sugar dysregulation, or hormonal imbalances masking your true nature.
Post-Meal Awareness — Your Body's Real-Time Feedback
The self-assessment provides a starting point. The most accurate Metabolic Nature refinement comes from systematically paying attention to how you feel 30–120 minutes after eating different meal compositions. Your body provides clear, consistent signals when you've given it the right fuel — and equally clear signals when you haven't.
When you've eaten correctly for your nature
Energy is sustained and stable — neither wired nor tired. You feel genuinely satisfied — not stuffed, not still hungry. Mental clarity is maintained or improved. Mood is even and calm. No cravings for additional food or sweets. Digestion is comfortable.
When you haven't eaten correctly for your nature
Energy drops, crashes, or feels wired then depleted. You feel unsatisfied despite eating an adequate volume. Brain fog or difficulty concentrating. Irritability, anxiety, or low mood. Cravings for carbohydrates or sweets. Digestive discomfort, bloating, or sluggishness.
The five-point post-meal check
30–90 minutes after eating, assess: Energy (sustained or dropping?), Hunger (satisfied or still wanting more?), Mental clarity (clear or foggy?), Mood (stable or irritable?), Cravings (free of them or wanting something sweet?). Use these signals to refine your macro ratios over 2–3 weeks.
When feedback is inconsistent
If post-meal signals are unpredictable — feeling great after the same meal some days and poorly on others — consider: stress load (higher stress increases protein/fat need), sleep quality (poor sleep disrupts glucose metabolism and alters apparent nature), gut dysfunction (infections impair digestion and absorption, masking true nature), hormonal status (cortisol and sex hormone fluctuations alter macronutrient processing).
Where Metabolic Nature Fits — The Nutritional Foundation, Not the Whole Picture
Metabolic Nature profiling answers the question "what should I eat?" — but it doesn't answer every clinical question. This is why it is Module 10 in this manual, not Module 1. You might be eating perfectly for your Metabolic Nature yet still struggle because of:
Gut dysfunction affecting absorption (Module 4) — you can eat correctly for your nature but fail to absorb the nutrients if digestive function is compromised. H. pylori suppressing stomach acid, dysbiosis producing inflammatory metabolites, intestinal permeability triggering food reactions — all of these impair the results of even a perfectly nature-matched diet.
Blood sugar dysregulation (Module 6) — reactive hypoglycaemia, insulin resistance, or cortisol-driven glucose instability can produce symptoms that mimic the wrong nature and undermine any dietary approach regardless of macronutrient ratios.
Hormonal imbalances (Module 5) — HPA axis dysfunction changes macronutrient requirements. A Calibrated type under significant cortisol dysregulation may temporarily need substantially more protein and fat than their nature would indicate. Addressing the HPA axis often restores the original Metabolic Nature expression.
Sleep disruption (Module 8) — one week of poor sleep produces metabolic changes that significantly alter glucose handling, macronutrient tolerance, and appetite hormone signals — potentially making a Calibrated type temporarily appear to be Kinetic in their hunger and craving patterns.
Metabolic Nature provides the nutritional foundation. Functional testing reveals what's blocking progress. Nature tells you what to eat. Testing tells you what's preventing the results.
Eat according to your Metabolic Nature. Investigate the obstacles through the five-test programme. Address dysfunction in the clinical sequence (HPA → blood sugar → gut → thyroid → sex hormones). The cumulative effect of all three components — nature-matched nutrition, targeted testing, clinical intervention in sequence — is transformational in a way that any single component alone cannot produce.
What this module establishes
- There is no universally healthy diet. Roger Williams documented in 1956 that nutritional needs can vary by 700%+ between individuals. Generic dietary guidelines are optimal for virtually no one
- Two physiological systems determine Metabolic Nature: autonomic dominance (sympathetic vs parasympathetic) and cellular oxidation rate (fast vs slow). Their interaction produces six distinct profiles
- The six Metabolic Natures are: Kinetic (fast oxidiser, sympathetic dominant — needs richest fuel), Grounded (slow oxidiser, parasympathetic dominant — substantial but paced), Catalyst (mixed oxidiser — needs balance, fails at extremes), Endurance (balanced autonomic, slow-moderate — most metabolically flexible), Adaptive (sympathetic dominant, variable — nutrition must track stress cycle), Calibrated (parasympathetic dominant, slow — lightest fuel requirements)
- Every diet works — for someone. Keto works brilliantly for strong Kinetic types and often fails Calibrated types. High-carb plant-based works well for Calibrated types and can be actively harmful for Kinetic types. Neither diet is wrong; they're wrong for specific people
- The post-meal check — energy, hunger, clarity, mood, cravings assessed 30–90 minutes after eating — is the most accurate Metabolic Nature refinement tool available. Your body provides real-time feedback when you know what to observe
- Inconsistent post-meal signals indicate an obstacle: stress load, poor sleep, gut dysfunction, or hormonal dysregulation masking the true Metabolic Nature. Address the obstacle first, then reassess the nature
- Metabolic Nature is Module 10, not Module 1 — because eating correctly for your nature while gut infections, blood sugar instability, or HPA axis dysfunction are unaddressed will produce incomplete results. The nature provides the nutritional foundation; the clinical programme addresses what's blocking it from working
- The Metabolic Nature Profiling tool in the TDG clinical platform integrates questionnaire results with test findings across all five panels to produce a cross-validated nature profile — accounting for the ways that gut dysfunction, cortisol patterns, and thyroid status can mask the constitutional nature