Summary

Your VDR result determines how effectively your body responds to vitamin D — two variants (BsmI and FokI) together shape your bone density response, calcium absorption, and immune function, with FokI being the rare functional variant that actually changes the receptor protein itself.

Genotype spectrum

GG (normal)

Your vitamin D receptor expression is robust. Standard supplementation doses produce expected outcomes for bone density, immune function, and calcium absorption.

AG (intermediate)

You have flexibility — your response is slightly below optimal but responsive to dose adjustment. You'll likely notice improvement when you optimise vitamin D levels, which is itself useful feedback.

AA (reduced expression)

Vitamin D optimisation has outsized impact for you. Because your receptor expression is lower, moving from deficient to optimal produces a more dramatic improvement than it would for GG carriers.

CC (f/f — shorter, more active VDR)

You extract more value from every molecule of vitamin D that reaches your cells. Your VDR is a more efficient machine.

CT (F/f — intermediate)

Balanced receptor function. You respond predictably to vitamin D supplementation with good clinical outcomes at standard doses.

TT (F/F — longer, less active VDR)

Vitamin D is a higher-leverage target for you. Because your receptor is less efficient per molecule, optimising both vitamin D levels AND VDR expression (through exercise, which upregulates VDR) gives you a dual pathway to improvement.

Practical takeaway

For AA (BsmI) and/or TT (FokI) Carriers — Enhanced Vitamin D Strategy

Supplementation:
• Vitamin D3: 2000-4000 IU/day as a starting dose. Adjust based on blood levels after 2-3 months.
• Target 25(OH)D: 40-60 ng/mL (100-150 nmol/L). You need to sit higher in the range than average to compensate for lower VDR efficiency.
• Take with fat: Vitamin D is fat-soluble. Take with your largest meal or with a fat source (olive oil, avocado, nuts). Absorption increases 30-50% when taken with fat vs. on an empty stomach.
• Vitamin K2 (MK-7): 100-200 mcg/day alongside vitamin D. K2 directs calcium to bones (where you want it) and away from arteries (where you don't). Particularly important for you because you're supplementing at higher doses.
• Magnesium: 200-400 mg/day (glycinate or citrate forms). Vitamin D metabolism requires magnesium as a cofactor for both hydroxylation steps. Magnesium deficiency can blunt vitamin D response regardless of genotype — but it matters more when your VDR efficiency is already reduced.

Exercise — VDR upregulation:
• Weight-bearing exercise upregulates VDR expression in bone tissue. For you, this is a dual benefit: direct mechanical loading on bone + increased receptor density for vitamin D signalling.
• Resistance training 2-3x/week is particularly impactful for bone density in VDR-unfavourable genotypes.
• Running, jumping, stair climbing — anything that loads the skeleton — is your friend.

Monitoring:
• Test 25(OH)D after 2-3 months of supplementation. Adjust dose to reach 40-60 ng/mL.
• Retest annually (or seasonally if you live above 40°N latitude).
• If on higher-dose supplementation (>4000 IU/day), also monitor calcium levels to ensure you're not over-absorbing.

Calcium:
• Dietary calcium 1000-1200 mg/day from food first (dairy, sardines with bones, leafy greens, fortified foods).
• If diet falls short, supplement the gap — but don't exceed 1200 mg/day total (dietary + supplement).
• Calcium absorption is VDR-dependent, so optimising vitamin D helps calcium absorption too.

What "working" looks like:
• 25(OH)D maintained at 40-60 ng/mL
• Stable or improving bone density on DEXA scan (if being monitored)
• Reduced frequency of upper

Evidence detail

What This Gene Does

VDR encodes the vitamin D receptor — the nuclear receptor that vitamin D binds to in order to exert its effects on gene expression. Vitamin D is biologically inert until it docks with VDR inside cells, at which point VDR activates hundreds of target genes involved in calcium absorption, bone mineralisation, immune regulation, and cell differentiation. If your vitamin D levels are the supply, VDR is the demand side — it determines how efficiently your cells actually use the vitamin D that reaches them.

Two common polymorphisms shape VDR function through different mechanisms. BsmI (rs1544410) is a regulatory variant in the 3' region that affects VDR mRNA stability and expression levels — more receptor protein or less. FokI (rs2228570) is genuinely functional: the T→C change eliminates the first of two potential start codons, producing a shorter VDR protein (424 vs 427 amino acids) that is 1.7x more transcriptionally active. These are independent — you inherit BsmI and FokI separately, and their combination creates your vitamin D response profile.

Mechanism

The vitamin D signalling pathway works like this:

1. Vitamin D3 (cholecalciferol) is synthesised in the skin from UVB exposure or obtained from diet/supplements.
2. First hydroxylation in the liver converts D3 → 25(OH)D (calcidiol) — the form measured in blood tests.
3. Second hydroxylation in the kidneys (and locally in many tissues) converts 25(OH)D → 1,25(OH)₂D (calcitriol) — the biologically active hormone.
4. Calcitriol binds VDR inside target cells. The VDR-calcitriol complex partners with RXR (retinoid X receptor) to form a heterodimer that binds vitamin D response elements (VDREs) in DNA, activating transcription of target genes.

Where BsmI matters:

BsmI (rs1544410) is in the 3' untranslated region of VDR. It affects mRNA stability — the AA genotype produces less stable mRNA, resulting in lower VDR protein expression. Fewer receptors in the cell means less total vitamin D signalling capacity, even if each receptor works normally. Think of it as having fewer docking bays: same ship quality, fewer ports.

Where FokI matters:

FokI (rs2228570) is in exon 2 — it changes the protein itself. The C allele eliminates the first ATG start codon, so translation begins at the second ATG, producing a protein that's 3 amino acids shorter. This shorter protein (424aa vs 427aa) has enhanced interaction with the transcription factor TFIIB, making it 1.7x more transcriptionally active per molecule. This is receptor quality, not quantity.

Why both matter together:

VDR expression (BsmI) × VDR activity per molecule (FokI) = total vitamin D signalling capacity. An AA + TT carrier has fewer receptors AND each receptor is less active. A GG + CC carrier has more receptors AND each is more active. The difference in total signalling capacity between these extremes is substantial — enough to measurably affect bone density, calcium absorption, and immune function at the same circulating vitamin D level.

Why this matters for supplementation:

Standard vitamin D recommendations assume average VDR function. If your VDR is less efficient (AA, TT, or both), you need higher circulating 25(OH)D to achieve the same downstream biological effect. This is why some people supplement at 2000 IU/day and test at 45 ng/mL with great outcomes, while others supplement identically and still show suboptimal bone density — their VDR is working differently.

VDR Vitamin D

Summary

Genotype spectrum

Practical takeaway

Evidence detail

Sources (7)