What Your Surgeon Isn't Telling You · Part 4
This series applies functional nutritional assessment to medical contexts where biology is being treated without addressing the nutritional foundation that determines biological outcomes. Part 1: surgical recovery. Part 2: IVF. Part 3: dental implants. This post covers mental health — the clinical context where nutritional deficiencies are perhaps most consequential and least investigated.
The biological model of depression — the idea that depression is primarily caused by serotonin deficiency and corrected by drugs that increase serotonin availability — has been the dominant clinical framework for four decades. It has produced an antidepressant prescribing rate in the UK that has more than doubled since the mid-2000s, with over 8 million people now receiving antidepressant prescriptions annually.
The serotonin hypothesis has also, in recent years, come under serious scrutiny. A 2022 umbrella review published in Molecular Psychiatry (Moncrieff et al.) found no consistent evidence supporting the idea that depression is associated with lower serotonin activity or concentration. The review concluded that the popular idea that depression is caused by low serotonin is not supported by the evidence — which does not mean antidepressants don't work, but does mean the mechanism by which they work is not what patients are typically told.
What this opens is a question that should have been asked more systematically from the start: if depression is not primarily a serotonin deficiency, what is it? The answer, from the growing field of nutritional psychiatry, is that it is frequently — though not always — a multi-system biological problem with significant nutritional, inflammatory, methylation, and gut microbiome components that are specific to the individual and that nobody in the prescribing encounter is assessing.
This post covers what those components are, what the evidence says, and what an assessment that actually matched the biological reality would look like.
The nutritional drivers of depression and anxiety
Vitamin B12
Methylation · Myelin · Neurotransmitter synthesis
B12 deficiency produces neurological and psychiatric symptoms including depression, anxiety, cognitive impairment, and psychosis — before haematological changes appear. The serum B12 test used in standard blood panels measures total B12, not functional B12 — a significant proportion of people with "normal" serum B12 have functional deficiency detectable only through homocysteine and methylmalonic acid. B12 is essential for the methylation cycle that produces SAMe — the universal methyl donor that is required for serotonin, dopamine, and noradrenaline synthesis. B12 deficiency impairs neurotransmitter production upstream of the synapse — before any receptor-level drug intervention can be effective.
Functional B12 deficiency is significantly more common than serum testing suggests. Especially common in vegans/vegetarians, those on PPIs, those over 60, and those with gut dysbiosis affecting ileal absorption
Folate / Methylfolate
Methylation · Neurotransmitter cofactor · MTHFR
Folate deficiency is independently associated with depression — and the mechanism is specific: folate is required for the methylation cycle that drives SAMe production, which in turn drives monoamine neurotransmitter synthesis. Critically, the folate form matters profoundly. Up to 40% of the population have MTHFR variants that reduce conversion of dietary folate and folic acid to active methylfolate. These individuals cannot adequately support methylation-dependent neurotransmitter synthesis regardless of their dietary folate intake — unless they are supplementing methylfolate (5-MTHF) specifically. Antidepressant non-response is significantly more common in people with MTHFR variants.
L-methylfolate (Deplin) is FDA-approved in the US as an adjunctive treatment for depression specifically because of this mechanism. The evidence is there. The testing isn't happening in the prescribing encounter.
Zinc
NMDA receptor · Neuroplasticity · Antidepressant response
Zinc is one of the most extensively studied nutritional factors in depression. It modulates NMDA receptor activity — the same glutamate receptor system that ketamine and other novel antidepressants target. Zinc deficiency is associated with increased depression severity, and zinc supplementation has been shown in multiple RCTs to improve depressive symptoms both as a standalone intervention and as an augmentation strategy for antidepressants. The mechanism involves NMDA modulation, BDNF (brain-derived neurotrophic factor) regulation, and anti-inflammatory effects. Serum zinc is an insensitive marker — functional deficiency is significantly underdetected by standard testing.
Meta-analysis of 17 studies: significantly lower serum zinc in depressed vs non-depressed individuals. Multiple RCTs showing zinc augmentation improves antidepressant response in treatment-resistant cases.
Omega-3 (EPA specifically)
Neuroinflammation · Membrane fluidity · Serotonin system
Of the omega-3 fatty acids, EPA (eicosapentaenoic acid) has the strongest and most consistent evidence in depression — specifically at doses above 1g EPA daily. EPA reduces neuroinflammation via prostaglandin and cytokine pathways, supports serotonin receptor function by influencing neuronal membrane fluidity, and may directly modulate the HPA axis stress response. A 2022 meta-analysis in Translational Psychiatry found EPA supplementation significantly reduced depressive symptoms versus placebo, with larger effects in those with elevated inflammatory markers at baseline. This is clinically important: the anti-inflammatory mechanism means omega-3 is likely most effective in inflammatory subtype depression — identifiable by elevated CRP.
The ratio matters: high EPA relative to DHA is the most evidence-supported form for depression. Standard fish oil with equal EPA:DHA is less effective than high-EPA formulations at equivalent total dose.
Vitamin D
Neuroinflammation · Gene expression · Seasonal affective disorder
Vitamin D receptors are expressed throughout the brain — in the hippocampus, prefrontal cortex, and throughout the limbic system. Vitamin D regulates the expression of genes involved in neurotransmitter synthesis, neuroplasticity, and neuroinflammation. Low vitamin D is consistently associated with higher rates of depression, and the association is strongest for those in northern latitudes (Scotland included) where winter deficiency is endemic. The evidence for vitamin D supplementation as an antidepressant is modest but positive in those who are genuinely deficient — the effect size is larger in severely deficient populations than in those merely insufficient. The standard UK threshold of 50 nmol/L is insufficient for optimal neurological function — 100–150 nmol/L is the functional target.
Seasonal affective disorder is partly a vitamin D deficiency syndrome. The latitude-depression association tracks precisely with UVB availability and the resulting D3 synthesis capacity.
Magnesium
HPA axis regulation · NMDA · Stress resilience
Magnesium is a natural NMDA receptor antagonist — blocking NMDA overactivation that drives glutamate excitotoxicity and the neuroinflammatory cascade associated with chronic stress and depression. It also regulates HPA axis activity and cortisol secretion — magnesium deficiency is associated with elevated cortisol and increased stress reactivity. Chronic stress depletes magnesium via urinary excretion, creating a vicious cycle: stress depletes the mineral that buffers the stress response. Multiple RCTs show magnesium supplementation improves depression, particularly in those with high stress load and demonstrable deficiency. The UK population is chronically magnesium insufficient.
A 2017 RCT in PLoS ONE found magnesium chloride as effective as antidepressants for mild-to-moderate depression in a primary care population. This is not a fringe finding — it's published, replicated, and largely ignored clinically.
Iron / Ferritin
Dopamine synthesis · Energy · Cognitive function
Iron is required for tyrosine hydroxylase — the rate-limiting enzyme in dopamine and noradrenaline synthesis. Iron deficiency impairs dopaminergic neurotransmission before anaemia is detectable — a state of "iron-deficient non-anaemic" that is extremely common in premenopausal women and that produces fatigue, low mood, reduced cognitive function, and poor stress resilience. Ferritin below 30 µg/L is associated with depressive symptoms; below 50 µg/L with impaired cognitive performance. The NHS "normal" lower limit of 13–15 µg/L is a catastrophically low functional threshold for neurological health.
Ferritin is not routinely checked in psychiatric assessment. A woman with ferritin of 14 µg/L and depression has a correctable biological driver of her symptoms that her antidepressant prescription does not address.
Iodine
Thyroid function · Energy · Cognitive performance
Iodine deficiency impairs thyroid hormone synthesis — and hypothyroidism is one of the most common and most consistently overlooked causes of depression, particularly in women. The mood effects of subclinical hypothyroidism — TSH elevated within the "normal" range, or normal TSH with poor T4-to-T3 conversion — include low mood, cognitive slowing, fatigue, and anxiety. These symptoms are frequently attributed to depression rather than investigated for their thyroid origin. UK iodine insufficiency is increasing, driven by declining dairy consumption and low dietary diversity. Iodine is not measured in standard psychiatric assessments.
Every woman presenting with new-onset depression should have a full thyroid panel — TSH, free T3, free T4, TPO and TgAb antibodies — before an antidepressant is prescribed. This is not current standard practice.
The methylation-psychiatry connection
Methylation is the biochemical process by which a methyl group (CH3) is transferred from one molecule to another — and it is one of the most important processes in the entire body, occurring billions of times per second. The methylation cycle is the factory that produces SAMe (S-adenosylmethionine) — the universal methyl donor required for neurotransmitter synthesis, DNA methylation, phospholipid synthesis, and myelin production.
The clinical relevance to psychiatry is direct. If the methylation cycle is impaired — through B12 deficiency, folate insufficiency, MTHFR gene variants reducing enzyme activity, or elevated homocysteine — neurotransmitter synthesis is impaired upstream of the serotonin, dopamine, and noradrenaline systems that antidepressants target.
MTHFR and Psychiatric Medication — The Clinical Picture
What MTHFR does
MTHFR (methylenetetrahydrofolate reductase) converts dietary folate and folic acid to active methylfolate (5-MTHF) — the form that enters the methylation cycle. Without adequate active methylfolate, the cycle cannot produce sufficient SAMe for neurotransmitter synthesis.
Prevalence of variants
C677T homozygous (TT genotype): approximately 10% of the population, reduces MTHFR activity by ~70%. C677T heterozygous (CT genotype): ~40% of population, reduces activity by ~35%. A1298C variants add additional impairment. Combined, over 40% of the population has some degree of impaired MTHFR function.
Antidepressant non-response
Multiple studies have found higher rates of antidepressant non-response in MTHFR variant carriers — particularly C677T homozygous. The mechanism: reduced methylfolate availability impairs the neurotransmitter synthesis that antidepressants depend on having adequate substrate to modulate. Prescribing an SSRI to someone with severe MTHFR impairment and folate insufficiency is like trying to raise the water level in a bucket with a significant hole in it.
The folic acid problem
Folic acid — the synthetic form added to foods and found in most supplements — requires MTHFR to convert it to active methylfolate. For MTHFR variant carriers, folic acid supplementation may actually worsen the situation — unmetabolised folic acid competes with active methylfolate for folate receptors, potentially blocking the small amount of methylfolate being produced. The correct intervention is methylfolate (5-MTHF) directly, bypassing the impaired enzymatic step entirely.
Homocysteine as the marker
Elevated homocysteine is the measurable downstream consequence of impaired methylation — detectable from a standard blood draw, directly modifiable with methylfolate, B12, and B6, and independently associated with depression, cognitive impairment, and dementia risk. It is not routinely checked in psychiatric assessment. It should be.
Inflammation — the most important mechanism nobody's treating
The inflammatory model of depression — the idea that at least a significant subset of depression is driven by or maintained by chronic systemic inflammation — has accumulated substantial evidence over the past 15 years. It is now the most active area of depression research, generating more published studies annually than the serotonin hypothesis at its peak.
The clinical picture is consistent across multiple research groups: elevated inflammatory markers (particularly CRP, IL-6, and TNF-alpha) are found in a significant proportion of people with depression. The relationship is bidirectional — inflammation drives depressive symptoms, and depressive states produce inflammatory signals. But critically, the inflammatory subtype appears to be a genuinely distinct biological presentation that responds differently to treatment.
SSRIs produce minimal anti-inflammatory effects. They are the wrong tool for inflammatory-subtype depression. The interventions that reduce systemic inflammation — omega-3 EPA, zinc, vitamin D, gut dysbiosis treatment, dietary anti-inflammatory approaches — address the biology that SSRIs don't reach.
A CRP above 1 mg/L in someone presenting with depression is a clinical signal that the inflammatory pathway is relevant to their presentation — and that anti-inflammatory nutritional intervention should be part of the treatment approach. Nobody in the prescribing encounter is measuring CRP. Nobody is asking about diet quality, gut health, toxic burden, or stress physiology as drivers of the inflammatory load.
The gut-brain axis — why the gut is a psychiatric organ
The Gut-Brain Connection in Psychiatry
Approximately 90–95% of the body's serotonin is produced in the gut — by enterochromaffin cells in the intestinal lining, regulated by the gut microbiome. The assumption that psychiatric serotonin is primarily a brain phenomenon is anatomically incorrect.
The gut microbiome produces neurotransmitter precursors — tryptophan (serotonin precursor), tyrosine (dopamine precursor), and GABA — that influence brain neurochemistry via the gut-brain axis. Dysbiosis alters this production significantly.
Intestinal permeability (leaky gut) allows bacterial endotoxins (LPS) to enter systemic circulation, triggering the inflammatory cascade that drives neuroinflammation and depressive symptoms. GI-MAP testing measures this directly via LPS antibodies.
The vagus nerve carries bidirectional signals between gut and brain — gut bacterial metabolites influence vagal tone, and vagal tone influences HPA axis regulation, stress resilience, and mood. Vagal stimulation is an approved treatment for treatment-resistant depression — less invasive approaches include addressing the gut dysbiosis that impairs vagal signalling.
Short-chain fatty acids (SCFAs) produced by gut bacteria cross the blood-brain barrier and influence neuroinflammation, microglial function, and BDNF production. Butyrate — produced by Faecalibacterium prausnitzii and other key commensals — has documented antidepressant effects in animal models and is being investigated clinically.
Psychobiotic interventions — specific probiotic strains with documented effects on mood and anxiety — include Lactobacillus rhamnosus, Lactobacillus helveticus, and Bifidobacterium longum. These are not replacements for clinical treatment but adjunctive interventions supported by growing evidence that are not discussed in standard psychiatric care.
Drug-nutrient interactions — what the prescription depletes
The nutritional psychiatry conversation is complicated by the fact that many psychiatric medications deplete the very nutrients that support the biological processes they're attempting to modulate.
SSRIs — particularly those with significant cytochrome P450 involvement — affect the metabolism of multiple nutrients. Melatonin production is affected by some SSRIs. Lithium, used in bipolar disorder, depletes sodium and affects thyroid function. Antipsychotics deplete B vitamins and increase homocysteine. Valproate (commonly used as a mood stabiliser) depletes folate, B12, and zinc, and impairs carnitine metabolism. Carbamazepine depletes folate, B12, vitamin D, and calcium.
The drug-nutrient depletion issue means that people on long-term psychiatric medication may be accumulating the very nutritional deficiencies that worsen the condition being treated — in addition to any pre-existing deficiencies present at the time of prescribing. Nobody monitors this. Nobody assesses nutritional status at the point of prescription or at subsequent medication reviews.
What a pre-prescription assessment would look like
A functional nutritional assessment before a first antidepressant prescription — or as part of a review for treatment-resistant depression — would include:
Minimum Assessment Before Psychiatric Medication
Full thyroid panel — TSH, free T3, free T4, TPO antibodies, TgAb antibodies. Subclinical hypothyroidism and Hashimoto's are common causes of depression that antidepressants don't address
Ferritin — not just haemoglobin. Ferritin below 30 µg/L with depressive symptoms is a correctable biological driver
B12 — functional assessment where possible; homocysteine and methylmalonic acid more sensitive than serum B12 alone
Homocysteine — downstream methylation marker, independently associated with depression and dementia risk
Folate — with MTHFR status where possible; flags those who need methylfolate rather than folic acid
Vitamin D — with 100–150 nmol/L as functional target, not the NHS 50 nmol/L threshold
Zinc — ideally red blood cell zinc rather than serum zinc for better functional accuracy
CRP — identifies inflammatory-subtype presentation where anti-inflammatory interventions are the primary clinical need
Fasting glucose and insulin — metabolic dysregulation is an independent driver of mood disorder and cognitive impairment
Omega-3 index — identifies EPA/DHA insufficiency where supplementation has documented evidence of effect
None of these are exotic. All are measurable from a blood draw. Most are available through the NHS with appropriate clinical justification. The cost of running this panel is a fraction of the cost of a year's antidepressant prescription plus subsequent medication reviews. The clinical yield — identifying correctable biological drivers that change the treatment approach — is substantial.
This is not an argument against antidepressants. For many people, SSRIs and other psychiatric medications are appropriate, effective, and sometimes life-saving interventions. The argument is that prescribing them without first assessing the nutritional and biological foundation is starting in the middle of the clinical picture — and that the deficiencies identified above, if present, need to be addressed regardless of whether antidepressants are also used.
A woman with ferritin of 14, vitamin D of 38, homocysteine of 18, and significant gut dysbiosis has multiple correctable biological drivers of her depression. Her SSRI prescription addresses none of them. Both could be true simultaneously — and addressing the nutritional foundation often changes the medication picture significantly.
The broader pattern — and where functional investigation sits
This is the fourth post in this series and the pattern is completely consistent. Surgical outcomes, IVF success, implant osseointegration, psychiatric medication response — all are substantially influenced by a nutritional and inflammatory biological foundation that the clinical system does not assess before intervening.
The psychiatric context is in some ways the most important of the four — because mental health presentations carry the most stigma, the most misattribution of biological problems to psychological ones, and the least investigation of the physical substrate before treatment begins. And because the consequences of undertreated depression — for quality of life, for relationships, for physical health through the HPA-immune connection — are profound and long-lasting.
The functional investigation approach doesn't replace psychiatric care. It adds the biological layer that psychiatric care currently lacks — mapping the nutritional, inflammatory, methylation, and gut microbiome picture that determines whether someone has the substrate to respond to treatment, and what correctable factors are maintaining the presentation regardless of what's being prescribed.
Concerned about the nutritional picture behind your mental health?
The DH Clinical Concierge can help you understand what a functional assessment would look like for your specific situation — and what investigation would tell you that your current clinical care isn't measuring.
Talk to the Concierge
If you are currently experiencing a mental health crisis or suicidal thoughts, please contact the Samaritans on 116 123 (free, 24/7) or your GP as an emergency.
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