There’s a type of fatigue I see regularly in clinical practice that has a particular character. It’s not just tiredness. It’s a quality of exhaustion that sleep doesn’t touch, that is disproportionate to the activity that precedes it, and that often comes with a specific cluster of companions: brain fog that descends like a shutter, poor tolerance for exercise that previously wasn’t an issue, a flatness of mood and motivation that isn’t depression but resembles it, and a cold intolerance that wasn’t there before.
When I see this pattern, particularly in someone who has already had thyroid and iron checked and been told those are fine, the first test I reach for is the Organic Acids Test. Because this profile has a cellular signature — and the OAT reads it.
What Organic Acids Are and Why They Matter
Organic acids are metabolic byproducts — small molecules produced during normal cellular biochemistry that are excreted in urine. Measuring them isn’t new; organic acid testing has been used in paediatric metabolic medicine for decades to identify inborn errors of metabolism. What’s more recent is the application of the same technology to functional medicine, where patterns in organic acid levels reveal how well — or how poorly — specific metabolic pathways are running.
The Organic Acids Test measures around 70 markers from a single first-morning urine sample collected at home. No clinic visit, no blood draw, no fasting. The sample is sent to the lab, and what comes back is a map of five interconnected functional systems: mitochondrial energy production, neurotransmitter metabolism, B-vitamin status, gut microbial overgrowth, and detoxification and oxidative stress.
That scope in a single test is why the OAT is one of the five tests in the TDG programme. Blood chemistry, GI-MAP, and DUTCH each give you a detailed view of a specific territory. The OAT gives you something else — a window into what’s happening at the cellular level that none of the others can see.
The Mitochondria Problem
Every cell in your body (with the exception of red blood cells) contains mitochondria — organelles whose primary job is to convert glucose and fatty acids into ATP, the energy currency that powers everything from muscle contraction to neurotransmitter synthesis to DNA repair. When mitochondria function well, you have energy in proportion to what your lifestyle demands. When they don’t, the deficit shows up everywhere simultaneously.
The process that produces most of your ATP is the Krebs cycle — a series of eight sequential chemical reactions, each requiring specific enzymes and cofactors (mostly B vitamins and minerals), that take acetyl-CoA derived from glucose or fat and convert it into electron carriers that ultimately drive ATP synthesis in the mitochondrial membrane.
When the Krebs cycle is running efficiently, its intermediates — citrate, succinate, fumarate, malate, and others — are present at normal concentrations. When something disrupts it, those intermediates accumulate and spill into the urine in elevated quantities. The OAT measures exactly this. Elevated Krebs cycle acids in the urine are a direct marker of mitochondrial dysfunction — the biochemical equivalent of backed-up traffic on the motorway, telling you precisely where the blockage is.
Elevated Krebs cycle acids in the urine are the biochemical equivalent of backed-up traffic — telling you precisely where in the energy production pathway the blockage is.
What disrupts the Krebs cycle?
Several things, and this is important for understanding why mitochondrial dysfunction is so common in chronically unwell people:
B-vitamin deficiencies. Every enzyme in the Krebs cycle requires B vitamins as cofactors. B1 (thiamine), B2 (riboflavin), B3 (niacin), B5 (pantothenic acid), and alpha-lipoic acid are all direct participants. Deficiency in any one of them slows the cycle at a specific step and creates a recognisable pattern of backed-up intermediates. The OAT identifies which B vitamins are functionally deficient from the pattern of elevation — not just from a direct serum measurement, which often misses functional deficiency even when the serum level looks adequate.
Oxidative stress. Free radicals damage the mitochondrial membrane and impair the efficiency of electron transport. This is a common consequence of chronic inflammation, toxic load, and poor antioxidant status — all of which the OAT also measures.
Gut overgrowth. Certain gut bacteria and yeasts produce metabolic byproducts that directly inhibit mitochondrial function. Arabinose, produced by Candida species, inhibits the enzyme alpha-ketoglutarate dehydrogenase, stalling the Krebs cycle at a specific step. HPHPA, produced by Clostridia species, inhibits dopamine beta-hydroxylase and impairs neurotransmitter conversion. These organisms aren’t always showing symptoms in the gut itself — their effects often appear systemically as fatigue and brain fog.
Heavy metals and environmental toxins. Mercury, lead, arsenic, and various industrial chemicals preferentially accumulate in mitochondria and impair their function. This isn’t hypothetical — it’s documented occupational and environmental medicine. The OAT includes markers for several toxin-related pathway disruptions.
The Five Domains of the OAT
What makes the OAT genuinely unusual as a test is that it covers five functionally distinct areas from a single sample. In practice this matters because these systems don’t fail independently — they fail together, and the OAT captures the whole picture at once.
The Fatigue That Doesn’t Respond to Rest
Post-exertional malaise — PEM, the characteristic feature of ME/CFS — is essentially a mitochondrial problem. The working model is that mitochondrial dysfunction reduces ATP production below the threshold needed for normal cellular function. During exertion, the cell draws down its ATP reserves faster than it can replenish them. Recovery requires mitochondrial resynthesis of ATP, which in a dysfunctional system is impaired. So exertion produces fatigue that lasts significantly longer than the exertion itself — hours to days, disproportionate to the activity.
This isn’t weakness. It isn’t deconditioning. It’s cellular energy bankruptcy. And it explains why telling someone with this pattern to exercise more — the standard medical advice for fatigue — makes them worse. You can’t run a car faster when the fuel supply is impaired.
The OAT doesn’t diagnose ME/CFS — nothing does, currently, in a definitive sense. But it maps the metabolic underpinnings that are almost always present, identifies which specific disruptions are driving the pattern in this person, and points toward what can actually be done. The intervention for arabinose-driven mitochondrial inhibition (antifungal protocol, gut repair) is different from the intervention for B-vitamin-driven enzyme deficiency (targeted repletion, cofactor support) which is different from the intervention for oxidative stress (antioxidant support, toxin reduction). The OAT tells you which of these you’re dealing with — often all of them simultaneously, in different proportions.
Elevated Krebs cycle acids (succinate, fumarate, malate): Mitochondrial dysfunction, B-vitamin cofactor deficiency, possible toxin exposure
Elevated pyruvate/lactate ratio: Impaired entry into the Krebs cycle; thiamine deficiency or pyruvate dehydrogenase dysfunction
Elevated arabinose: Yeast/Candida overgrowth in the gut, direct mitochondrial inhibition
Elevated HPHPA: Clostridia overgrowth, dopamine pathway disruption, often associated with mood and concentration problems
Elevated methylmalonic acid: Functional B12 deficiency, even when serum B12 appears normal
Elevated xanthurenate/kynurenate: B6 functional deficiency; also seen in chronic inflammation (tryptophan shunting)
Low pyroglutamate: Glutathione depletion; impaired detoxification and antioxidant capacity
Why Serum B-Vitamin Tests Often Miss This
One of the most practically important things the OAT reveals is functional B-vitamin deficiency — and this is worth being specific about, because it surprises people.
Serum B12, for example, measures how much B12 is circulating in the blood. What it doesn’t tell you is how much is getting into the cell and being used. B12 enters the cell via specific transport proteins; if those are impaired, serum B12 can be entirely normal while the cell is functionally deficient. Methylmalonic acid — an OAT marker — rises when B12 is not available intracellularly for a specific metabolic conversion. It’s a functional marker of B12 status, not a serum level. It catches deficiency that the serum level misses.
The same logic applies to B6 (measured via xanthurenate and kynurenate) and folate (measured via FIGLU). These OAT markers reflect what the B vitamin is actually doing inside the cell — whether it’s functionally available at the enzyme level. For anyone with chronic fatigue, depression, cognitive impairment, or peripheral nerve symptoms, these markers are significantly more informative than a serum panel.
The Neurotransmitter Connection
Brain fog, low mood, poor motivation, anxiety, difficulty concentrating — these aren’t always psychological. They often have a metabolic signature, and the OAT can read it.
HVA (homovanillic acid) is the primary urinary metabolite of dopamine. Low HVA suggests reduced dopamine turnover — a finding associated with low motivation, poor concentration, and the cognitive symptoms of chronic fatigue. 5-HIAA (5-hydroxyindoleacetic acid) is the primary metabolite of serotonin. VMA (vanillylmandelic acid) reflects norepinephrine metabolism.
What the OAT reveals here isn’t a diagnosis of depression or ADHD. It’s a map of the biochemical terrain that symptoms are arising from. Low dopamine metabolites in the context of elevated HPHPA (Clostridia toxin) tells a specific story: a gut organism is producing a compound that inhibits the enzyme that converts dopamine to norepinephrine, backing up dopamine in a dysfunctional form. That’s an addressable gut problem, not a psychiatric one — and the intervention is completely different.
How the OAT Fits in the TDG Framework
In the TDG five-test programme, the OAT sits alongside blood chemistry, GI-MAP, DUTCH hormone panel, and food sensitivity panel. It covers the Energy and Nervous system domains of the HIDDEN framework — the two that conventional blood testing almost completely ignores.
The integration between the OAT and the other tests is where it gets particularly useful. Elevated arabinose on the OAT points me toward the GI-MAP for confirmation and more detailed characterisation of what’s in the gut. Low neurotransmitter metabolites in the context of elevated cortisol on the DUTCH tells me the adrenal picture is driving neurotransmitter depletion — address the HPA axis first. B-vitamin functional deficiency on the OAT changes how I read the blood chemistry methylation markers and homocysteine. These tests don’t just sit beside each other — they read each other.
If you’re dealing with fatigue that doesn’t improve with rest, brain fog that persists regardless of sleep quality, or a pattern of symptoms that nobody has been able to explain satisfactorily, the cellular level is worth looking at. Not because it’s exotic — it isn’t. Because it’s where the answer often is.
The companion diagram for this post maps the difference between optimal and dysfunctional mitochondrial function, including the Krebs cycle intermediates the OAT measures and what their elevation means. It’s the clearest visual explanation I know of for why fatigue can be cellular rather than systemic.