Everyone Is Told X
"Take vitamin D for your immune system."
This is true as far as it goes. Vitamin D is deeply involved in immune regulation — it modulates both innate and adaptive immunity, reduces inflammatory cytokine production, and supports antimicrobial peptide synthesis. Deficiency is genuinely common in Scotland and across the UK. Supplementation is warranted for most people from October to April. None of that is the problem. The problem is what the advice leaves out: the conversion steps that can fail, the cofactors that must be present, the genetic variation that changes what the same blood level actually means, and the other systems that interact with vitamin D in ways a single supplement cannot address. Most people taking vitamin D for immunity are still getting ill. The question worth asking is why.
What the trilogy already covers
Before going further, there are three posts in the Detective Health vitamin D series that cover the foundational ground in detail. This post builds on them rather than repeating them.
If you haven't read those first, start there. What follows assumes the foundation and focuses on connections the trilogy doesn't make.
My own result — and what it didn't tell me alone
In March 2024 I ran my own Randox Signature blood panel. My 25-OH vitamin D came back at 68 nmol/L. By NHS standards, that's sufficient — the threshold for deficiency is 25 nmol/L, insufficiency 50 nmol/L. I was above both. No flag, no recommendation, no action required.
By functional medicine standards, 68 nmol/L is low-normal. The functional optimal range — the level associated with optimal non-skeletal outcomes including immune function, cardiovascular protection, and cognitive performance — is generally considered to be 100–150 nmol/L. At 68, I was sufficient but not optimal. And this was in March, after a Scottish winter.
Stephen Duncan · Randox Signature · March 2024
Vitamin D and the markers that contextualise it
25-OH Vitamin D
68
nmol/L · sufficient ⚠ not optimal
hsCRP
0.8
mg/L · optimal ✓
Magnesium
0.89
mmol/L · within range ✓
PTH
4.1
pmol/L · optimal ✓
Calcium
2.38
mmol/L · optimal ✓
Homocysteine
7.2
µmol/L · borderline functional ⚠
At 68 nmol/L, my vitamin D is sufficient by NHS criteria and low-optimal by functional criteria. But the surrounding context matters: low-normal inflammation (hsCRP 0.8) means inflammatory scavenging of NO is minimal. Magnesium adequate means the conversion steps for vitamin D can proceed. PTH appropriately suppressed means vitamin D signalling is working at the mineral axis level. The homocysteine at 7.2 µmol/L is slightly above my functional target of below 7 — a methylation signal worth monitoring. Reading 25-OH-D in isolation tells one part of the story.
The variables the supplement doesn't address
The gap between taking vitamin D and it actually working runs through six variables. Most people addressing only the first one — serum 25-OH-D level — and wondering why the results don't match expectations.
Variable 01
The magnesium conversion dependency
Both conversion steps that activate vitamin D — D3 to 25-OH-D in the liver, and 25-OH-D to active calcitriol in the kidney — require magnesium as a cofactor. High-dose D3 supplementation in someone who is magnesium-deficient can paradoxically worsen magnesium status by increasing demand. Symptoms: muscle cramps, poor sleep, irritability despite "normal" vitamin D levels. Estimated 50–80% of Western populations are suboptimal in magnesium.
Supplement forms →
Variable 02
The K2 calcium direction problem
Vitamin D increases calcium absorption. Without adequate vitamin K2 (MK-7 form), that calcium has no directional signal — it may deposit in soft tissue and arteries rather than bone and teeth. The D3/K2 combination is the clinically appropriate unit at doses above 2,000 IU/day. Supplementing D3 alone at high doses long-term without K2 is not a neutral intervention.
Mineral axis →
Variable 03
VDR genetic variation
The vitamin D receptor (VDR) — the protein through which active calcitriol signals in cells — has multiple polymorphisms that affect receptor sensitivity. Two people with identical serum 25-OH-D levels may have significantly different tissue-level vitamin D activity based on their VDR variants. The FokI, BsmI, ApaI, and TaqI polymorphisms are the most studied. Testing serum D without considering VDR function is measuring the key, not whether it opens the lock.
Full story →
Variable 04
The nitric oxide connection
VDR activation upregulates endothelial nitric oxide synthase (eNOS) — the enzyme that produces the nitric oxide governing vascular tone, platelet aggregation, and immune cell trafficking. Low vitamin D reduces eNOS expression, impairs vascular function, and reduces NO-mediated immune defence. The cardiovascular and immune effects of vitamin D are substantially mediated through NO — which means the full NO picture (homocysteine, BH4, oxidative stress) is part of the vitamin D conversation.
Nitric oxide →
Variable 05
Immune tolerance — not just immune activation
Vitamin D's most important immune function is often misframed. It is not primarily about making the immune system stronger — it is about making it more precise. Calcitriol reduces Th17 activity (the inflammatory arm implicated in autoimmunity), promotes Treg expansion (the regulatory arm that prevents the immune system attacking self-tissue), and suppresses inflammatory cytokine production. Low vitamin D doesn't just make you susceptible to infection — it shifts the immune system toward inflammatory and autoimmune patterns.
Variable 06
The UV skin-NO pulse
UVA light (which penetrates year-round in the UK, unlike UVB) releases stored nitrite from skin, producing a pulse of systemic nitric oxide independent of vitamin D synthesis. This is a distinct mechanism — it is why sunlight exposure has cardiovascular benefits that vitamin D supplementation alone cannot fully replicate. The NO released from skin by UV has measurable blood pressure effects. Supplementing D3 addresses the UVB pathway. It doesn't address the UVA-NO pathway at all.
UV and NO →
The immune system is not a single target
The framing "vitamin D for immunity" treats the immune system as a single lever. In reality, immune function depends on dozens of nutritional inputs operating simultaneously. Vitamin D is one of them — an important one — but it is neither sufficient alone nor always the limiting factor.
The limiting factor for someone who gets frequent upper respiratory infections might be vitamin D. Or it might be zinc — which is rate-limiting for T-cell maturation and thymulin production. Or selenium — the cofactor for glutathione peroxidase that protects immune cells from oxidative damage during the respiratory burst. Or vitamin A — which governs mucosal immunity, the first physical barrier the immune system deploys. Or inadequate sleep, which is when cytokine calibration and immunological memory consolidation occur. Or gut barrier compromise, which triggers chronic low-grade immune activation that exhausts the system before a pathogen arrives.
If you are taking vitamin D and still getting ill every winter, the question is not whether to take more. The question is what else is limiting your immune competence — and there are at least six other variables worth investigating before doubling the dose.
The single-supplement model produces single-supplement results. Which is often — not always, but often — disappointing in practice. The people who take vitamin D faithfully every winter and still get three colds a season are not taking the wrong supplement. They are applying a simple solution to a complex problem.
What the connection to methylation adds
There is a less obvious link between vitamin D function and methylation that runs through two mechanisms.
The first is direct: the VDR gene has methylation-sensitive promoter regions — meaning that epigenetic methylation patterns can suppress VDR expression regardless of how much vitamin D is in the blood. A poorly methylating genome may produce fewer VDR proteins, reducing tissue-level responsiveness to calcitriol even at adequate serum levels.
The second is through BH4. Calcitriol — the active form of vitamin D — upregulates eNOS via VDR. eNOS requires BH4 as its essential cofactor. BH4 synthesis is methylation-dependent. So: adequate vitamin D supports eNOS function, but only if BH4 is available, and BH4 availability depends on methylation cycle efficiency. An MTHFR carrier with impaired methylation and suboptimal vitamin D has two simultaneous hits on the same pathway.
Methylation connection
Vitamin D → VDR → eNOS → BH4 → nitric oxide — and where MTHFR fits in
Adequate vitamin D activates VDR → upregulates eNOS → eNOS requires BH4 → BH4 is methylation-dependent → MTHFR impairment reduces BH4 → eNOS uncouples, producing superoxide instead of NO. Testing vitamin D in isolation while ignoring methylation status misses the mechanism that determines whether the vitamin D signalling actually reaches its cardiovascular and immune endpoints.
MTHFR and methylation →
Nitric oxide and eNOS →
The testing picture — what to measure alongside 25-OH-D
A serum 25-OH-D measurement tells you the circulating level of the storage form. It does not tell you how well it is being converted to active calcitriol, whether the VDR is responding appropriately, whether magnesium is sufficient for the conversion steps, or what the downstream effects on NO and immune function look like. The markers that contextualise it:
Randox Signature — Vitamin D contextual markers
What to measure alongside 25-OH-D for a complete picture
25-OH Vitamin D
Functional optimal: 100–150 nmol/L
The starting point — but not the complete picture. Sufficient by NHS criteria (≥50 nmol/L) does not mean optimal for immune, cardiovascular, or cognitive endpoints.
Magnesium
Functional optimal: 0.85–0.95 mmol/L
Required for both D3 conversion steps. Low magnesium = impaired activation regardless of serum D3 level. Also required for PTH regulation and VDR expression.
PTH (Parathyroid Hormone)
Functional: 1.6–4.2 pmol/L
Elevated PTH with adequate 25-OH-D suggests either impaired D3 to calcitriol conversion or calcium insufficiency. PTH is the most sensitive functional indicator that vitamin D signalling at the mineral axis is working — or not.
Calcium (adjusted)
Functional: 2.25–2.50 mmol/L
High calcium with high vitamin D supplementation: check K2 status. Low calcium despite supplementation: check absorption, magnesium, PTH.
hsCRP
Functional optimal: <1.0 mg/L
Chronic inflammation suppresses VDR expression and impairs vitamin D signalling. Elevated hsCRP with low vitamin D is a bidirectional relationship — inflammation worsens D insufficiency and D insufficiency worsens inflammation.
Homocysteine
Functional optimal: <7.0 µmol/L
Marker of methylation status — contextualises VDR methylation sensitivity and BH4 availability for the eNOS pathway that vitamin D activation feeds into.
Zinc
Functional optimal: 12–15 µmol/L
Zinc is rate-limiting for VDR function (zinc finger protein) and is independently required for T-cell immunity. Low zinc + low vitamin D = compounded immune compromise that supplementing D3 alone cannot address.
The functional medicine argument in miniature
Vitamin D is a useful case study in why functional medicine exists as a discipline. The single-nutrient single-target model — take D3 for immunity — is not wrong. It is incomplete. And the incompleteness has real clinical consequences: people supplementing faithfully, testing periodically, and still not reaching functional optimal levels because their magnesium is insufficient for the conversion. Or reaching adequate serum levels but getting no downstream benefit because their VDR expression is suppressed by chronic inflammation. Or addressing the D3/immunity story while the zinc, selenium, sleep, and gut barrier picture goes unexamined.
This is not a counsel of perfection or an argument for complexity for its own sake. It is an argument for precision. Testing the right markers, reading them together, and building an intervention that addresses the actual limiting factors rather than the most popular one.
In 37 years of clinical practice, the clients who benefit most from addressing vitamin D are invariably those where it is addressed in context — alongside magnesium, K2, the mineral axis, the inflammatory picture, and the methylation status that determines how well the vitamin D signal reaches its cellular targets. The supplement alone helps some people some of the time. The full picture helps more people more consistently.
The honest position on supplementation
D3 supplementation is warranted for most UK adults from October to April — this remains true regardless of everything above. The argument here is not against vitamin D supplementation. It is against vitamin D alone as a complete immune strategy. For most adults: 2,000–4,000 IU D3 daily October to April, with magnesium glycinate or malate (300–400mg elemental), and K2 as MK-7 (100–200mcg) if supplementing above 2,000 IU. Test in March (seasonal trough) and September (seasonal peak). Target 100–150 nmol/L on functional criteria.
Know your full vitamin D picture
The Randox Signature blood panel includes 25-OH vitamin D, PTH, adjusted calcium, magnesium, hsCRP, homocysteine, and zinc — all the markers needed to read vitamin D in context rather than in isolation. Interpreted alongside your full clinical picture.
Book a conversation →
The vitamin D trilogy — read in full
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