VDR: The Receptor That Decides How Much of Your Vitamin D Actually Counts
Same dose, different blood level. The receptor is part of the reason.
VDR is the gene for your vitamin D receptor, the protein that vitamin D has to dock into before it can do anything. You can have plenty of vitamin D in your blood, but if the receptor is built a little differently, your body reads the same dose its own way. That's the whole story here. It's also why two people taking an identical 2,000 IU can land at different numbers on a lab report. We look at two spots in the gene. FokI sits right at the start codon, so it actually changes the shape of the receptor protein that gets built. BsmI sits in an intron, a non-coding stretch, so it doesn't change the protein; it's more of a marker that has traveled alongside vitamin D differences in some groups. The honest part: FokI is the spot with a real supplementation trial behind it, while BsmI flips direction depending on which population you read. We won't pretend the second one gives you a clean dosing answer when it doesn't.
What VDR does
Builds the vitamin D receptor, the docking protein that vitamin D (as its active form, calcitriol) binds to so it can switch genes on and off.
Once vitamin D is docked, the receptor controls how your body handles calcium, bone, and a long list of vitamin D-dependent genes.
The FokI variant changes the receptor's start point, producing a slightly longer or shorter protein with a different ability to turn those genes on.
Because the receptor is the middleman, VDR helps explain why the same vitamin D dose can land two people at different blood 25(OH)D levels.
Your variants, decoded
This one sits right at the spot where the receptor protein starts being built, so it actually changes the protein. In a year-long trial on a fixed daily dose of vitamin D, the genotype groups raised their blood levels by different amounts, and one group came out clearly at the bottom.
| AA | Largest rise among the groups. Sits at the opposite end of the gradient from the lowest responders, so on the same daily amount this group tends to move its blood level the most. |
| AG | In between. One copy of each, an intermediate response. |
| GG | Smallest rise per dose. In the trial this was the lowest-responding group on the same daily amount and may need a higher dose to reach the same blood level. Educational only, not a dosing instruction. |
This one sits in a non-coding stretch, so it doesn't change the receptor protein itself. It's a marker that's been linked to baseline vitamin D differences, but studies disagree on the direction across populations, so we won't hand you a dosing rule from it.
| CC | Common version. Linked to vitamin D variation in some studies, but the direction isn't consistent across groups. |
| CT | One copy of each. Intermediate, with the same caveat about mixed evidence. |
| TT | The other version. Some studies tie it to lower baseline vitamin D, others to higher; the signal isn't settled. |
Genotypes are shown order-insensitively and on the forward strand; your own export may print the complementary letters — the meaning is the same.
What the research suggests
VDR builds the receptor your vitamin D has to bind to, and the FokI variant changes how much a given dose moves your blood level. In a 12-month trial on 2,000 IU/day, the GG carriers showed the smallest rise in serum 25(OH)D of any genotype group, so GG tends to convert a dose into blood level less efficiently and may need more to reach the same number. That trial enrolled people with type 2 diabetes, so the exact response may not transfer one-to-one to everyone, but the genotype pattern is the point. This is about hitting a target lab value, not treating anything. Test your 25(OH)D and let a doctor or pharmacist set the dose; never self-adjust off a genotype alone.
PubMed 28811597 · (Scientific Reports, 2017; 204 patients with type 2 diabetes, 2,000 IU/day vitamin D3 for 12 months) — the lowest increment in serum 25(OH)D was seen in the FokI lowest-responder genotype (reported as Fok-I CC, which is GG on the dbSNP forward strand), p<0.0001; the paper did not publish per-genotype ng/mL change values.
Educational only — not medical advice. “General evidence” means the finding is real but the supplement’s benefit isn’t unique to your genotype.
See this matched to your own DNA — free.
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Questions
Does the GG version mean I'm vitamin D deficient?
No. It doesn't say anything about whether you're deficient right now. It's about efficiency: in the trial, GG carriers raised their blood level less per dose than the higher-responding groups on the same amount. You could carry GG and have perfectly good levels, or carry AA and be low. The only way to know your status is a blood test for 25(OH)D.
My 23andMe file shows C and T, not A and G. Did I read the wrong gene?
You read it right. We report this spot on the forward DNA strand as A/G, which is how dbSNP lists it. A lot of vitamin D research and some raw-data exports read the opposite strand and print C/T (the older FokI 'F/f' naming). It's the same variant in a mirror: our A is their T, and our G is their C. So a C/T export flips cleanly to our A/G.
Should I just take a megadose of vitamin D if I have the GG version?
No, and please don't self-adjust based on a genotype. The trial finding is that GG carriers raised their blood level the least per dose, not that they should take huge amounts. Too much vitamin D has its own problems. The right path is a blood test plus a doctor or pharmacist who can set a dose for your actual level.
What about the BsmI result, can it tell me my dose?
Not reliably. BsmI sits in a non-coding part of the gene and doesn't change the receptor protein. Studies have linked it to baseline vitamin D differences, but the direction flips between populations, so we treat it as context rather than a dosing signal. FokI is the more actionable of the two.
Can my genotype replace a vitamin D blood test?
No. Genetics can nudge your starting guess for how a dose might behave, but your actual 25(OH)D level depends on sun, diet, body weight, skin tone, and more. The blood test is the real answer; the gene is just one input your clinician can factor in.