Last reviewed: May 12, 2026 Last updated: May 12, 2026

Written by: Jay Hastings , CEO of PlexusDx

Jay Hastings is the CEO of PlexusDx, a precision health company focused on genetic testing, blood biomarker insights, and personalized wellness recommendations. He has more than 20 years of experience across healthcare innovation, genomics, laboratory operations, healthcare investing, and strategic finance. His work has included scaling healthcare startups, leading CLIA lab integrations, and helping expand consumer access to precision health tools.

Medically reviewed by: Jayden Lee, PharmD, EMBA

Jayden Lee, PharmD, EMBA, is the PlexusDx Medical Science Liaison with a PharmD and MBA specializing in pharmacogenomics and clinical product development, with a proven ability to bridge the gap between genomic research and practical patient outcomes. Dr. Lee has more than 10 years of professional experience in clinical pharmacy, academia, and research.

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Most conversations about vitamin D stop at serum levels. Is your 25(OH)D above 30 ng/mL? Above 50? The number gets treated as the outcome — as if measuring the nutrient in your blood is the same as confirming it’s working in your muscle tissue. It isn’t. Your serum vitamin D level is the supply. The VDR gene — which encodes the vitamin D receptor — determines the demand sensitivity: how effectively your muscle cells read and respond to that supply. The Precision Peptide Genetic Test analyzes VDR variants as one of 15 Muscle Growth insights across 14 pathways, 49 peptides, and 150+ genetic insights.

What VDR Does in Muscle Tissue

VDR encodes a nuclear receptor — a class of proteins that act as transcription factors, entering the cell nucleus and directly switching target genes on or off. When the active form of vitamin D (1,25-dihydroxyvitamin D3, or calcitriol) binds to VDR, the receptor-ligand complex moves into the nucleus, binds to vitamin D response elements (VDREs) in the genome, and alters the transcription of hundreds of target genes. In skeletal muscle specifically, VDR activation regulates four interconnected processes:

Protein synthesis: VDR target genes include regulators of the mTOR-dependent protein synthesis pathway — the same downstream route that IGF-1 and the growth hormone axis engage. VDR-driven transcription amplifies the anabolic signal capacity available to muscle fibers independently of GH axis stimulation.

Calcium handling: VDR regulates the calcium channel and pump genes that govern intracellular calcium flux in muscle cells. Calcium is the trigger for muscle contraction — binding troponin C to initiate the actin-myosin cross-bridge cycle. VDR genotype therefore influences both contractile force and the speed of force development.

Satellite cell activity: Muscle satellite cells — the stem-cell-like precursors that add myonuclei to growing fibers — express VDR. Activation of VDR in satellite cells promotes their proliferation and differentiation, supporting the hypertrophic adaptation that follows resistance training.

Fiber type composition: Research has linked VDR genotype to Type II (fast-twitch) fiber proportion, creating an intersection with ACTN3 that compounds across both genes. Higher VDR activity is associated with a more favorable fast-twitch fiber ratio in some populations, adding a fiber architecture dimension to the receptor’s muscle role.

FokI and the VDR Haplotype: The Variants That Matter

The VDR gene has four commonly studied polymorphisms, and they tend to be analyzed together as a haplotype because their combined effect on receptor function is larger than any single variant in isolation:

FokI (rs2228570) is the most functionally significant. It creates a start-codon variation that produces either a longer (f allele) or shorter (F allele) VDR protein. The shorter FF form is more transcriptionally active — it interacts more efficiently with transcription factor IIB (TFIIB), generating stronger downstream gene expression from the same ligand signal. FF genotype carriers effectively get more VDR activity per unit of calcitriol. ff genotype carriers produce a less active receptor and may need higher circulating vitamin D levels to achieve equivalent downstream effects.

BsmI (rs1544410), ApaI (rs7975232), and TaqI (rs731236) are located in the 3′ end of the gene and influence VDR mRNA stability — how long the receptor transcript survives before degradation. These variants affect how much VDR protein is produced in the first place, adding a quantity dimension to the quality effect of FokI. Specific haplotype combinations have been associated with differences in muscle strength, mass, and injury risk across multiple independent study populations.

What Your VDR Genotype Means for Muscle Performance

The practical implications of VDR genotype span the full arc of muscle performance — from acute force production to long-term hypertrophy to injury resilience. FF genotype carriers tend to show better calcium-dependent contractile efficiency, stronger satellite cell response to resistance training stimulus, and in some studies, a higher Type II fiber proportion that compounds with ACTN3 RR genotype when both are present. ff genotype carriers aren’t disadvantaged across the board — but their vitamin D status becomes more consequential. The same serum level that produces adequate VDR-driven gene expression in an FF individual may be insufficient for an ff individual, whose receptor requires a stronger signal to generate equivalent output.

This is the practical value of knowing your VDR genotype: it reframes the vitamin D conversation from a binary (“deficient or not”) to a personalized question (“what level does my receptor require to operate efficiently?”). Genetics as a guide, not a guarantee — but a guide that meaningfully changes the target.

VDR, IGF-1, and Growth Hormone Axis Pathways

VDR activity and the growth hormone axis don’t operate in parallel silos — they intersect at the muscle fiber level. Vitamin D receptor activation in skeletal muscle has been documented to upregulate IGF-1 receptor expression, increasing the density of IGF-1 binding sites on muscle cell surfaces. More IGF-1 receptors means greater sensitivity to circulating IGF-1, which means the anabolic signal generated by the GH→IGF-1 cascade lands on a more receptive target. Your VDR genotype therefore shapes the cellular environment that growth hormone axis pathway signals enter — amplifying or dampening their downstream effect based on receptor activity.

This intersection makes VDR genotype especially relevant for individuals exploring growth hormone axis pathway protocols with a healthcare provider. Knowing that your VDR function may be attenuating IGF-1 receptor density — or amplifying it — adds a layer of biological context that informs both vitamin D optimization and realistic expectations for anabolic protocol response.

The Full Muscle Growth Genetic Panel

VDR sits at the intersection of multiple systems within the Precision Peptide Genetic Test’s 15 Muscle Growth insights. Each other gene in the panel targets a mechanism that VDR activity influences, supports, or compounds with:

ACTN3 (R577X) — fast-twitch fiber composition; VDR genotype may modulate Type II fiber proportion, creating compounding effects when read alongside ACTN3 findings.

MSTN (myostatin) — the hypertrophy ceiling gene; VDR activation may influence myostatin expression, adding a regulatory interaction between vitamin D status and muscle mass constraints.

IGF1 — growth hormone axis downstream mediator; VDR activation upregulates IGF-1 receptor expression, shaping how sensitively muscle responds to circulating IGF-1.

GHSR — ghrelin receptor; governs the upstream GH pulses that drive IGF-1 production that VDR receptor density amplifies or dampens.

GHR — growth hormone receptor; determines how sensitively the liver reads GH signals to produce the IGF-1 that VDR receptor density then handles.

ACE — the endurance-versus-power split; VDR’s fiber type influence intersects with ACE’s I/D variant to shape overall athletic physiology.

IL-6 — post-exercise inflammation; VDR has documented immunomodulatory effects that influence the inflammatory-recovery balance IL-6 governs.

Beyond Muscle Growth, VDR findings cross into 9 Tissue Repair insights (wound healing and connective tissue recovery both depend on vitamin D signaling) and 11 Immunity insights (VDR is one of the most important immune-regulatory nuclear receptors in the genome). Your VDR genotype is genuinely a cross-pathway finding.

The Precision Peptide Genetic Test analyzes how your genes influence muscle growth pathways. It does not recommend, prescribe, or determine which peptides you should use. Consult a qualified healthcare provider before beginning any peptide protocol.

Ready to see how your VDR genotype shapes your muscle performance baseline? Take the Precision Peptide Genetic Test

Frequently Asked Questions About VDR and Muscle Performance

What does the VDR gene reveal about muscle performance?

VDR encodes the vitamin D receptor — the protein that translates vitamin D signals into cellular action in muscle tissue. Variants including FokI (rs2228570) influence receptor activity and downstream effects on protein synthesis, calcium regulation, and satellite cell function. The Precision Peptide Genetic Test analyzes VDR as part of 15 Muscle Growth insights.

Does VDR genotype affect how growth hormone axis pathways work?

VDR and the growth hormone axis intersect because vitamin D receptor activation in muscle upregulates IGF-1 receptor expression — amplifying sensitivity to anabolic signals. Your VDR genotype shapes the cellular environment that growth hormone axis pathway signals enter. Knowing both pieces helps your healthcare provider build a more complete picture of your muscle growth biology.

What other genes are tested alongside VDR in the muscle growth panel?

The Precision Peptide Genetic Test analyzes 15 Muscle Growth insights — including ACTN3 (fiber type), MSTN (myostatin ceiling), IGF1 (growth hormone axis signaling), GHSR (ghrelin receptor), GHR (growth hormone receptor), ACE (endurance vs power), and IL-6 (inflammation and recovery). VDR is one gene in a multi-gene muscle performance profile.

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Medical and Editorial Standards

Medical review process: This article was reviewed for medical accuracy, scientific clarity, evidence alignment, and appropriate discussion of genetics, medications, supplements, biomarkers, and health-related claims.

Sources and evidence: PlexusDx educational content is developed using peer-reviewed research, clinical literature, reputable medical references, and, where applicable, public health or regulatory guidance. References are included at the end of the article when scientific, medical, or health-related claims are discussed.

Commercial transparency: PlexusDx offers genetic testing, blood biomarker testing, personalized supplement recommendations, and related precision wellness services. Product mentions are intended to help readers understand available options and should not be interpreted as medical advice.

Important disclaimer: PlexusDx educational content is for informational purposes only and should not be used as a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider before making decisions about medications, supplements, genetic testing, lab testing, or health-related care.