SLC23A1: The Gene That Sets Your Vitamin C Set Point
Your body can't make vitamin C. This gene decides how much of it sticks around.
Humans can't make vitamin C. We eat it, and then a protein grabs it from the gut and the kidneys and hauls it back into the blood. That protein is SVCT1, and SLC23A1 is the gene that builds it. So this gene isn't about how much vitamin C you swallow. It's about how much of it actually ends up in your plasma. A common variant here, rs33972313, slows that transport a little, and the slowdown shows up as a real difference in blood levels between two people eating the same diet.
What SLC23A1 does
SLC23A1 codes for SVCT1, a transporter that actively pulls vitamin C (L-ascorbic acid) out of your gut and kidney tubules and back into your bloodstream.
Since you can't make vitamin C yourself, this transporter is a big part of what sets your plasma level for a given intake. It's the plumbing, not the diet.
The rs33972313 variant swaps a single building block in the protein, a valine for a methionine, and that version moves vitamin C a bit less efficiently.
Carry the lower-transport version and you tend to run lower blood vitamin C than someone without it, even eating the same food. That's part of why two people on the same diet land at different levels.
This is normal, common human variation. It points to a tuning knob on your vitamin C, not to deficiency or any disease.
Your variants, decoded
A single-letter change in SLC23A1 that slightly lowers how efficiently the SVCT1 transporter moves vitamin C into your blood. The T allele is the less common, lower-transport version. Note: this site is read on different strands by different labs, so a 23andMe export may print the complementary letters G and A instead of C and T (C reads as G, T reads as A). Same variant, mirrored letters.
| CC | Two standard copies. Your SVCT1 moves vitamin C the usual way, so on a given diet your plasma level lands in the typical range. The most common result. |
| CT | One standard copy and one lower-transport copy. On the same diet you tend to sit somewhat lower than CC. In the study below, each lower-transport copy tracked with about 6 micromol/L less plasma vitamin C, so a single copy is a modest shift. Food still matters most; this just nudges your baseline. |
| TT | Two lower-transport copies. The least common result, and the one that tracks with the lowest circulating vitamin C for a given intake. All the more reason to keep produce steady rather than sporadic. |
We show genotypes on the forward strand and ignore letter order. Your own export may print the complementary letters. Same result either way.
What the research suggests
If you carry the lower-transport T allele (CT or TT), your SVCT1 moves vitamin C into the blood a little less efficiently, so you tend to run lower than CC carriers on the same diet. A steady, modest vitamin C intake (food first, a small supplement if your produce is thin) helps hold a normal blood level without overshooting, since your blood saturates at modest intakes and dumps the rest.
PubMed 20519558 · (Timpson et al., Am J Clin Nutr 2010) — across 15,087 people in five cohorts, each copy of the rs33972313 minor (lower-transport) allele was associated with about 6 micromol/L lower circulating L-ascorbic acid (pooled estimate -5.98 micromol/L, 95% CI -8.23 to -3.73).
Educational only, not medical advice. “General evidence” means the finding is real, but the benefit isn’t specific to your genotype.
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Questions
Does a CT or TT result mean I'm vitamin C deficient?
No. The genotype shifts where your blood level tends to land for a given diet. It doesn't drop you below a deficiency line. Plenty of people with the variant have perfectly fine vitamin C status because they eat enough produce. If you want to know your actual status, a blood test measures it directly. The gene only tells you about the transport tendency.
My 23andMe file shows G and A, not C and T. Did I read the wrong gene?
No, you read the right spot. This variant sits on a stretch of DNA that different labs report from different strands. C and G are the same physical letter read from opposite directions, and so are T and A. So a C here equals a G in a G/A report, and a T equals an A. The genotypes line up: CC reads as GG, CT as GA, TT as AA.
If I have the lower-transport version, should I take huge doses of vitamin C?
Not huge ones. Your blood vitamin C saturates at fairly modest intakes, and once it's full the extra just leaves in your urine. The sensible move is to keep produce steady through the week, and add a small regular supplement if your diet is genuinely light on fruit and vegetables. Big spikes don't do more for your blood level.
Is this one of the variants I need to talk to a doctor about?
It's a nutrition variant, not a medication one, so there's no drug interaction to worry about here. That said, if you have a medical reason to track vitamin C, or you're considering high-dose supplements alongside any health condition, it's always reasonable to run it by a clinician. This page is educational and isn't medical advice.