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.
Here's a fact people forget: humans can't manufacture vitamin C. We eat it, and then a protein has to grab it from the gut and the kidneys and haul it back into the blood. That protein is SVCT1, and SLC23A1 is the gene that builds it. So this gene isn't really 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, nudges that transport a little, and that nudge shows up as a difference in blood levels between people eating the exact same diet.
What SLC23A1 does
SLC23A1 codes for SVCT1, a transporter that actively pulls vitamin C (L-ascorbic acid) out of your gut and your kidney tubules and back into your bloodstream.
Because you can't synthesize vitamin C, this transporter is a big part of what sets your plasma level for a given intake. It's the plumbing, not the diet.
The variant rs33972313 swaps a single building block in the protein (a valine becomes a methionine), and the variant version moves vitamin C a bit less efficiently.
People carrying the lower-transport version tend to run lower blood vitamin C than people without it, even on the same food, which is why a genotype can explain part of the gap between two people's levels.
This is a normal, common bit of human variation. It tells you about a tuning point on your vitamin C, not about 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 transports vitamin C the usual way, so for a given diet your plasma vitamin C lands in the typical range. This is 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. More reason to keep produce steady rather than sporadic. |
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
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 supplement’s benefit isn’t unique to your genotype.
See this matched to your own DNA — free.
Upload your 23andMe or AncestryDNA file and get your actual SLC23A1 result, plus every other actionable variant — each line cited, your file never stored.
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 put you below a deficiency line. Plenty of people with the variant have perfectly fine vitamin C status because they eat enough produce. If you actually want to know your 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.