A client brings in blood results from their GP. Vitamin D: 48 nmol/L. Reported as normal. They have fatigue, low mood, recurrent infections, muscle aching, and difficulty concentrating. Their GP has suggested they may be slightly stressed.
48 nmol/L is above the conventional deficiency threshold of 25–30 nmol/L. It therefore clears the reference range and is reported as normal. But 48 nmol/L is not functional sufficiency. It is the lower edge of a range that was set using populations in which widespread insufficiency had already shifted the average downward — the same methodological problem we have discussed in the context of ferritin.
More fundamentally: 48 nmol/L of 25-hydroxyvitamin D (the storage form measured by a standard blood test) tells you what is in storage. It tells you nothing about conversion to the active hormonal form, nothing about whether that conversion is proceeding normally, nothing about how sensitive the vitamin D receptor is in this person's tissues, and nothing about the magnesium and K2 status that determines whether the converted D is being directed appropriately. It is one data point in a system with multiple variables, and it is being treated as the whole story.
What 25-OH-D actually measures — and what it doesn't
Vitamin D operates through a two-step conversion sequence before it has any biological effect. Understanding this sequence is not an optional detail. It is the clinical argument for why a blood test that returns a "normal" 25-OH-D result can coexist with genuine functional insufficiency.
Why vitamin D does far more than bones
The public understanding of vitamin D is almost entirely framed around bone health — calcium absorption, rickets, osteoporosis prevention. This is accurate but represents perhaps 20% of what vitamin D actually does in the body. The discovery of VDR expression in nearly every cell type has expanded the clinical picture considerably.
The reference range problem — again
The conventional lower threshold for vitamin D sufficiency in the UK is typically 50 nmol/L, with deficiency defined as below 25–30 nmol/L. These thresholds were set primarily with bone health endpoints in mind — the level at which rickets and osteomalacia risk is minimised. They were not set with immune function, neurotransmitter synthesis, muscle function, or glucose metabolism as endpoints. When those systems are the clinical concern, the threshold is insufficient.
Functional ranges — what the evidence supports for non-bone endpoints
| 25-OH-D Level | Conventional Status | Functional Assessment | Clinical Significance |
|---|---|---|---|
| Below 25 nmol/L | Deficient | Clinically deficient | Risk of rickets, osteomalacia, severe immune impairment. Immediate clinical priority. |
| 25–50 nmol/L | Low-normal | Functionally insufficient | Passes conventional threshold. Immune function, neurotransmitter synthesis, muscle function, and glucose metabolism likely impaired. Symptom cluster consistent with insufficiency common in this range. |
| 50–75 nmol/L | Normal | Borderline functional | Adequate for bone endpoints. Evidence suggests non-bone systems benefit from higher levels. GrassrootsHealth data shows health outcomes improving up to 100–125 nmol/L in most populations. |
| 75–150 nmol/L | Normal | Functional optimal range | Range where immune, neurological, metabolic, and musculoskeletal endpoints all appear well supported. Target for repletion in most clinical contexts. Individual variation within this range. |
| Above 150 nmol/L | High | Caution warranted | No established benefit above this level for most people. Risk of hypercalcaemia with prolonged very high supplementation. Requires clinical justification and monitoring. |
What needs to be in place for vitamin D to work properly
Vitamin D does not act alone. The full mineral regulatory system — calcium, magnesium, phosphate, PTH, and K2 — must be operating correctly for vitamin D to be converted efficiently and directed appropriately. Two cofactors deserve particular attention.
Magnesium is required for vitamin D activation (both hydroxylation steps), for VDR function, and for the enzymes involved in vitamin D metabolism. Supplementing vitamin D without adequate magnesium is like pressing the accelerator with the engine oil warning light on. The conversion happens poorly, and some of the downstream signalling is impaired. Most people supplementing vitamin D are also magnesium-insufficient — because the food supply depletes both, and because the stress-demand relationship amplifies magnesium deficiency.
Vitamin K2 directs calcium to bone and teeth rather than to soft tissues and arterial walls. High-dose vitamin D increases calcium absorption significantly. Without adequate K2, that calcium may deposit in arteries rather than bones — the mechanism behind concerns about vitamin D supplementation in isolation. Food-based K2 (fermented dairy, natto) or K2 as MK-7 supplementation alongside D3 is the clinical standard for anyone supplementing vitamin D.
"Vitamin D is a hormone precursor operating within a mineral regulatory system. Treating it as a standalone supplement — a number to raise, a capsule to take — misses the clinical picture almost entirely."
The individual variation principle
Every principle in this post comes with the same caveat: individual variation is real, significant, and clinically relevant. Two people with identical 25-OH-D results may have very different functional vitamin D status depending on their VDR genetics, their kidney conversion capacity, their magnesium status, their inflammatory load, and their baseline requirements. A result that represents functional sufficiency for one person may represent functional insufficiency for another.
This is why the clinical position on vitamin D is not "everyone needs a specific dose" or "everyone needs a specific level." It is: test first, address the cofactors, supplement to functional sufficiency confirmed by retesting, and adjust based on individual response. The target range is a guide. The individual data is the clinical reality.
Vitamin D is one of 150+ markers on the TDG blood chemistry panel
Interpreted alongside magnesium, calcium, PTH, inflammatory markers, and the full metabolic picture — so the number makes clinical sense rather than being read in isolation. The full vitamin D story, including what drives insufficiency in this specific person, requires the full clinical picture.
See the TDG Programme →