Pathology explained · Vitamin D
The vitamin D receptor: why your blood level is only half the story.
A vitamin D test tells you how much is in the blood. It does not tell you whether your cells can use it. That job belongs to the vitamin D receptor, a protein inside nearly every cell that vitamin D has to unlock before anything happens. Gene variants, inflammation, low magnesium or zinc, and the state of your gut all change how well that lock turns. This page explains what the receptor does, what can quieten it, and what that means for the level you aim for.
The short version
Vitamin D is a hormone in waiting. The blood test measures the storage form. The active form has to bind a receptor inside the cell, and that receptor then switches genes on or off in bone, immune cells, the gut, the brain and beyond. Two people with the same test result can get very different effects, because the receptor is not the same in everyone. When the receptor is likely to be less responsive, the target level and the support around it both matter more.
genes answer to the vitamin D receptor, by some estimates. That is why one receptor reaches calcium, immunity, gut lining, mood and blood sugar all at once.
The pathway in four steps
Vitamin D from sun or food is not the vitamin D your cells use. It moves through a chain of steps, and each step can be the place where things go quiet.
Skin, food or a supplement supplies vitamin D3
Sunlight on skin turns a cholesterol relative into vitamin D3 (cholecalciferol). Oily fish, eggs and fortified foods add a little more, and supplements add the rest. At this stage it is a raw material. It does nothing on its own.
The liver makes the storage form
The liver adds one hydroxyl group and makes 25-hydroxyvitamin D, written 25(OH)D. This is the form that circulates for weeks, and it is the form your blood test measures. It is a good measure of supply. It is still not active.
The kidney and local tissues make the active hormone
A second enzyme adds another hydroxyl group and makes 1,25-dihydroxyvitamin D, also called calcitriol. The kidney does this for the whole body and keeps the blood level tightly controlled. Immune cells, gut cells, the brain and many other tissues also make their own active vitamin D on the spot, for local use. Both enzymes in this chain need magnesium to work.
The active hormone turns the key in the cell
Calcitriol enters the cell and binds the vitamin D receptor, usually shortened to VDR. The receptor pairs up with a partner called RXR, which depends on vitamin A, and the pair settles onto DNA and switches specific genes on or off. Only now does vitamin D actually do something. If the receptor is scarce, altered or blocked, a good blood level can still leave the cell waiting.
The picture
This is the diagram we draw in clinic. Green boxes are the active road. The grey box is what jams the lock. Everything above the lock is about supply. Everything below it is about effect.
The gene variants
The receptor is built from a gene called VDR on chromosome 12. Like most genes it comes in common versions, called polymorphisms or SNPs, that differ by a single letter. A handful have been studied for decades. It helps to separate them from the other vitamin D genes: variants in genes such as GC and CYP2R1 change how much 25(OH)D ends up in the blood. Variants in VDR change how well the cell responds to it, whatever the level.
| Variant | Where it sits | What it may change | Evidence |
|---|---|---|---|
| FokI (rs2228570) | The start of the gene, exon 2 | Shifts where the protein begins, so one version of the receptor is three amino acids longer. The longer form switches genes on less efficiently. This is the only common VDR variant that changes the protein itself. | Best studied |
| BsmI (rs1544410) | Near the end, intron 8 | Does not change the protein. It may alter how much receptor is made or how long its message lasts. Studied in bone density, autoimmune thyroid disease, type 1 diabetes and tuberculosis, with results that differ between populations. | Mixed results |
| ApaI (rs7975232) | Near the end, intron 8 | Usually inherited together with BsmI and TaqI, so it mostly marks the same stretch of the gene rather than adding a separate effect. | Mixed results |
| TaqI (rs731236) | Near the end, exon 9 | A “silent” change that leaves the protein sequence alone but travels with BsmI. Linked in some studies to bone density and autoimmune conditions. | Mixed results |
| Cdx2 (rs11568820) | The promoter, the gene’s on switch | Changes how much receptor the gut lining makes, which may affect how well calcium is absorbed from food. | Early evidence |
When the receptor goes quiet
Genes are only one reason the lock can be hard to turn. Several everyday states reduce how much receptor cells make, or get in the way of its partners. Any of these can leave a good blood level under-used.
The extreme case: inherited resistance
A very rare condition, hereditary vitamin D resistant rickets, is caused by a receptor that is broken outright. Children develop rickets and usually lose their hair, even though the level of active vitamin D in the blood is very high. It is the clearest demonstration of what the receptor does: without it, vitamin D in the blood cannot act. Everything else on this page is a milder, partial version of the same idea.
Inflammation
In laboratory studies, inflammatory signals lower how much receptor cells make and interfere with the way it settles onto DNA. This is one reason low vitamin D and inflammation travel together in so many conditions, and why a healthy result on paper does not always deliver a healthy effect during long-running inflammation.
Some infections
Certain microbes appear to switch the receptor down as a survival tactic, since the receptor drives the antimicrobial defences that would otherwise clear them. This has been shown in laboratory work for tuberculosis, Epstein-Barr virus, HIV and the Lyme bacterium, among others. How much it matters in everyday practice is still being worked out.
Low magnesium, zinc or vitamin A
The receptor grips DNA with two “zinc fingers”, and its partner RXR needs vitamin A to do its half of the job. Magnesium is needed by the enzymes that make the active hormone in the first place. Low levels of any of these leave the lock stiffer, and low magnesium in particular is common.
Ageing
Receptor levels fall in the gut and in muscle with age in some studies. This is part of why older adults absorb calcium less well, lose muscle more easily, and may need a higher blood level to get the same effect a younger person gets from less.
A disrupted gut
Gut bacteria and the receptor talk in both directions. Butyrate, made when bacteria ferment fibre, raises receptor levels in the gut wall, and certain probiotic strains do the same. In turn, the receptor keeps the gut lining sealed and the microbial mix balanced. A disrupted gut can therefore mean a quieter receptor, and a quieter receptor a more disrupted gut.
What the receptor reaches
Because the receptor sits in nearly every tissue, poor vitamin D signalling does not show up as one symptom. It shows up as a pattern. The table lists the main systems, what the receptor does there, and what tends to happen when the signal is weak. Most of the “when poor” column comes from observational studies, which show association rather than cause.
| System | What the receptor does there | When the signal is poor |
|---|---|---|
| Bone and calcium | Drives calcium absorption in the gut, tells the parathyroid glands to settle, and steers bone building and remodelling. | Less calcium absorbed, parathyroid hormone climbs, bone thins over time, and older adults have more falls and fractures. |
| Immune defence | Immune cells make their own active vitamin D and use the receptor to switch on antimicrobial peptides such as cathelicidin, the body’s own antibiotic. | More frequent or lingering respiratory infections. Trials show the clearest benefit from supplementing people who start out deficient. |
| Immune balance | Nudges T cells away from the aggressive Th1 and Th17 types and toward the regulatory T cells that keep tolerance. | Higher rates of autoimmune conditions in association studies: multiple sclerosis, type 1 diabetes, Hashimoto’s, Graves’ disease and rheumatoid arthritis. |
| Gut lining and microbiome | Maintains the tight junctions between gut cells, supports the cells that make gut antimicrobials, and shapes which bacteria thrive. | A leakier gut, dysbiosis and more inflammation in the gut wall. See rebuilding a leaky gut. |
| Brain and mood | Found in the hippocampus and other mood centres. Switches on the enzyme that makes serotonin in the brain and supports nerve growth factors. | Low mood, seasonal dips and slower thinking are linked with low vitamin D signalling in observational work. |
| Blood sugar and heart | Supports insulin release from the pancreas and insulin sensitivity in muscle and fat. Quietens renin, part of the blood pressure system. | Association with insulin resistance, type 2 diabetes and higher blood pressure. Supplement trials in people who were not deficient have been mostly disappointing, which suggests the receptor and the wider picture matter as much as the level. |
| Thyroid and hormones | Works alongside the thyroid hormone receptor and shares the same RXR partner, so the two signals lean on each other. | Autoimmune thyroid disease is more common with some VDR variants, and low vitamin D blunts the cell’s response to T3. See how T4 becomes T3. |
| Skin and hair | Controls how skin cells mature and runs the hair follicle cycle. | Psoriasis (the receptor is the target of calcipotriol cream), and hair loss in the rare inherited resistance. |
| Cell growth | Slows cell division, encourages cells to mature, and helps clear damaged cells. | Colorectal, breast and prostate cancer risk track with low vitamin D and some VDR variants in observational studies. Large trials have not shown that supplements prevent cancer in the general population. |
| Muscle | Supports protein building and the fast-twitch fibres that catch you when you trip. | Weakness and, in older adults, more falls. Resistance training raises receptor levels in muscle. |
What this means for your target level
The test measures supply. It does not measure whether the receptor is turning that supply into action. So the question “what level should I aim for?” has two parts: what the ranges say, and how responsive your receptor is likely to be.
| Band | 25(OH)D, nmol/L | Who uses it |
|---|---|---|
| Deficient | Below 50 | Australian laboratories and the Australian and New Zealand position statement. Below 30 is moderate, below 12.5 severe. |
| Sufficient for bone | 50 and above at the end of winter | The Australian and New Zealand position statement. Levels swing with the seasons, so the end of winter is the low point to check. |
| Sufficient, broader view | 75 and above | The US Endocrine Society guideline, on the basis that parathyroid hormone keeps falling until about this point. |
| Functional target | 100 to 150 | Many nutrition and functional medicine practitioners, especially where immune, autoimmune or receptor concerns are in play. This is a clinical judgement rather than a guideline. |
| Caution | Above 250 | Most laboratories flag this. Toxicity is rare and shows as high blood calcium, but it is real, so levels this high need review. |
Australian results are reported in nmol/L. Overseas articles often use ng/mL. Multiply ng/mL by 2.5 to convert.
When the receptor is likely to be less responsive
A known VDR variant, an autoimmune condition, ongoing inflammation, recurrent infection, low magnesium, older age or a disrupted gut all make it reasonable to aim toward the upper part of the sufficient range rather than settling at the cut-off, while staying well under the caution line. Whether that is right for you depends on your history, your calcium and your other results, and it is a decision to make with your practitioner, not a rule.
What the second test does and does not tell you
Laboratories can also measure the active hormone, 1,25(OH)2D. It is not a status test, because the kidney holds it steady even when stores are low. It is useful in specific situations, such as granulomatous conditions, kidney disease or some chronic infections, where the active form runs unusually high or low. For most people the storage form, 25(OH)D, is the right test.
Helping the receptor do its job
Raising the blood level is the easy part. Getting the cell to hear it is the rest. These are the levers that have reasonable support, roughly in order of how often they matter.
Get the supply right, then retest
Sensible sun on skin when the UV index allows, oily fish, eggs and fortified foods cover the base. A supplement is often needed in winter or with darker skin, indoor work or higher body weight, and the dose should be set from a test rather than guessed. Recheck about three months after any change, because that is how long the storage form takes to settle.
Magnesium
Both enzymes that activate vitamin D need magnesium, and so does the binding protein that carries it. People low in magnesium often respond poorly to vitamin D until the magnesium is corrected. Green leafy vegetables, nuts, seeds, legumes and cacao are the food sources. A red cell magnesium test tells you more than a standard serum level.
Zinc
The part of the receptor that grips DNA is built around two zinc atoms. Low zinc means a receptor that binds less firmly. Oysters, red meat, pumpkin seeds and legumes are the main sources, and plasma zinc is easy to test.
Vitamin A, in balance
The receptor cannot act alone. Its partner RXR is a vitamin A receptor, so adequate vitamin A is part of vitamin D working. Balance is the word: very high vitamin A intakes compete with vitamin D rather than help it. Eggs, dairy, liver in small amounts, and orange and green vegetables for beta-carotene cover most people.
Vitamin K2 and calcium handling
Vitamin D raises calcium absorption. Vitamin K2 activates the proteins that direct that calcium into bone rather than soft tissue. It is sensible company for anyone raising their vitamin D intake, particularly at higher targets. Natto, hard cheeses, egg yolks and butter from grass-fed animals are the food sources.
Feed the receptor from the gut
Butyrate, made when gut bacteria ferment fibre, raises receptor levels in the gut wall, and certain Lactobacillus strains have done the same in studies. That means fibre, resistant starch, polyphenol-rich plants and fermented foods are receptor support as much as gut support. See resistant starch for the practical list.
Move against resistance
Resistance exercise raises receptor levels in muscle, which is part of why vitamin D and strength training work better together than either alone for muscle and falls in older adults. Two or three sessions a week is enough to matter.
Deal with the blockers
Long-running inflammation, an unresolved gut infection or dysbiosis, poor sleep and unmanaged stress all lean on the receptor. A good level on paper does little while the lock is jammed. This is often the piece that explains why “my vitamin D is fine” and “I still feel low” can both be true.
What we look at in clinic
A standard check measures 25(OH)D alone and calls anything above the cut-off fine. That answers the supply question only. When symptoms and the result do not agree, or when the history suggests the receptor is under strain, we look at the whole chain.
The level
25(OH)D, ideally timed for the end of winter, with a repeat about three months after any change. Calcium and parathyroid hormone when the level has been high, when bone is a concern, or when calcium supplements are involved.
The helpers and the blockers
Red cell magnesium, plasma zinc and a marker of inflammation such as CRP show whether the receptor has what it needs and what is working against it. Thyroid antibodies and other autoimmune markers are added when the history points that way.
The genes
VDR variants can be read from a genetic panel and sit naturally alongside the methylation genes we discuss on our MTHFR and methylation page. They do not change the treatment on their own. They change how thorough we are about the rest.
If low vitamin D is one thread in a bigger picture of fatigue, autoimmunity or infections that keep returning, our complex chronic patterns page describes how we work through it.
Sources
Haussler MR, et al. Molecular mechanisms of vitamin D action. Calcif Tissue Int. 2013.
Bouillon R, et al. Skeletal and extraskeletal actions of vitamin D: current evidence and outstanding questions. Endocr Rev. 2019.
Uitterlinden AG, et al. Genetics and biology of vitamin D receptor polymorphisms. Gene. 2004.
Jurutka PW, et al. The polymorphic N terminus in human vitamin D receptor isoforms influences transcriptional activity by modulating interaction with transcription factor IIB. Mol Endocrinol. 2000.
Carlberg C, Haq A. The concept of the personal vitamin D response index. J Steroid Biochem Mol Biol. 2018.
Malloy PJ, Feldman D. Genetic disorders and defects in vitamin D action. Endocrinol Metab Clin North Am. 2010.
Liu PT, et al. Toll-like receptor triggering of a vitamin D-mediated human antimicrobial response. Science. 2006.
Patrick RP, Ames BN. Vitamin D hormone regulates serotonin synthesis. FASEB J. 2014.
Wu S, et al. Intestinal epithelial vitamin D receptor deletion leads to defective autophagy in colitis. Gut. 2015.
Uwitonze AM, Razzaque MS. Role of magnesium in vitamin D activation and function. J Am Osteopath Assoc. 2018.
Feng M, et al. Polymorphisms in the vitamin D receptor gene and risk of autoimmune thyroid diseases: a meta-analysis. Endocrine. 2013.
Jolliffe DA, et al. Vitamin D supplementation to prevent acute respiratory infections: a systematic review and meta-analysis of aggregate data from randomised controlled trials. Lancet Diabetes Endocrinol. 2021.
Manson JE, et al. Vitamin D supplements and prevention of cancer and cardiovascular disease. N Engl J Med. 2019.
Makanae Y, et al. Acute bout of resistance exercise increases vitamin D receptor protein expression in rat skeletal muscle. Exp Physiol. 2015.
Nowson CA, et al. Vitamin D and health in adults in Australia and New Zealand: a position statement. Med J Aust. 2012.
Holick MF, et al. Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2011.
Frequently asked questions
What is the vitamin D receptor?
The vitamin D receptor, or VDR, is a protein found inside nearly every cell in the body. Active vitamin D has to bind to it before anything happens. Once bound, the receptor pairs with a partner called RXR, settles onto DNA and switches specific genes on or off in bone, immune cells, the gut, the brain and elsewhere. Without a working receptor, vitamin D in the blood cannot act.
Can my vitamin D level be normal and still not be working?
Yes. The standard blood test measures 25-hydroxyvitamin D, the storage form, which shows supply rather than effect. Gene variants in the receptor, ongoing inflammation, some infections, low magnesium or zinc, older age and a disrupted gut can all reduce how well cells respond. Research also shows that people differ naturally in how strongly their genes respond to vitamin D, regardless of their blood level.
What are VDR gene variants and do they matter?
VDR gene variants are common single-letter differences in the vitamin D receptor gene. The best known are FokI, BsmI, ApaI, TaqI and Cdx2. FokI changes the receptor protein itself and makes a slightly less efficient version. The others may change how much receptor is made. Studies link them to small shifts in risk for bone loss, autoimmune conditions and infections, but results vary between populations. A variant is a tendency, not a diagnosis, and nutrient status and lifestyle can matter more.
What vitamin D level should I aim for?
Australian laboratories call anything below 50 nmol/L deficient and 50 or above sufficient for bone health. The US Endocrine Society uses 75 nmol/L. Many nutrition and functional medicine practitioners aim for around 100 to 150 nmol/L where immune, autoimmune or receptor concerns are involved, while keeping well below the 250 nmol/L level that laboratories flag for possible toxicity. The right target depends on your history, calcium and other results and is best set with a practitioner.
Which nutrients help the vitamin D receptor work?
Magnesium is needed by the enzymes that activate vitamin D. Zinc forms part of the section of the receptor that grips DNA. Vitamin A is needed by the receptor's partner RXR, though very high intakes compete rather than help. Vitamin K2 directs the extra calcium that vitamin D absorbs into bone. Fibre and fermented foods raise receptor levels in the gut wall through butyrate, and resistance exercise raises receptor levels in muscle. Whether any supplement is appropriate depends on testing and should be discussed with a practitioner.
Reviewed by Rohan Smith, BHSc Nutritional Medicine · Elemental Health & Nutrition, Adelaide. Last reviewed 7 September 2026.
Important: This summary is general information, not personalised medical advice, diagnosis, or a treatment protocol. Speak with a qualified practitioner about your individual situation. Book a consultation →