SLC19A1 — Reduced Folate Carrier
The gatekeeper that pulls folate into your cells
Folate doesn't drift passively through cell membranes. It needs a dedicated transporter to get inside, and SLC19A1 encodes the main one: the reduced folate carrier, or RFC. This protein grabs the folate forms found in food, principally 5-methyltetrahydrofolate (5-MTHF), and shuttles them across the cell membrane into circulation. The rs1051266 variant is a single base change in the coding region of SLC19A1, designated c.80G>A. One study in healthy young adults from Northern Ireland found that this variant was associated with red blood cell folate concentrations in women. Not in men. Not with serum folate. Not with homocysteine. Just RBC folate, in women, explaining about 5% of the variation in that one biomarker. That leaves 95% of the picture to diet, intake, and everything else.
What SLC19A1 does
SLC19A1 encodes the reduced folate carrier (RFC), a membrane protein responsible for moving natural folates into cells. Under normal physiological conditions, RFC is the dominant mechanism for getting folate from the bloodstream into your tissues.
Folic acid is a poor substrate for RFC. The synthetic form in most supplements and fortified foods doesn't fit the transporter nearly as well as the reduced folate forms that occur naturally in food: 5-methyltetrahydrofolate (5-MTHF) in leafy greens and legumes, or 5-formyltetrahydrofolate. Worth knowing regardless of which genotype you carry.
Your variants, decoded
SLC19A1 sits on the minus strand of chromosome 21, so VCF allele labels and coding-strand notation run in opposite directions. T (VCF reference) = c.80A on the coding strand; C (VCF alternate) = c.80G. In the one population study with data on rs1051266, women homozygous for c.80G (that is, CC in VCF terms) had lower RBC folate than women carrying one or two A alleles. The variant accounted for roughly 5% of the RBC folate variation among women. No association appeared in men, and serum folate and homocysteine were unaffected in either sex.
| TT (homozygous reference; coding AA) | Associated with higher red blood cell folate in women in the studied population. The A allele looks like the higher-folate form, at least in one Northern Irish cohort. Not a diagnosis of anything good, just a modestly favorable signal on one biomarker. |
| TC (heterozygous; coding AG) | Women with one copy of each allele had higher RBC folate than CC women, similar in direction to TT. Treat this as a single data point from one study. The difference is real enough to be statistically significant, but the biology here is not fully mapped. |
| CC (homozygous alternate; coding GG) | Lower RBC folate in women. This is the at-risk group in the one study that measured it. Even so, the SNP explains only 5% of variance in that biomarker, so your actual folate intake is doing far more work than your genotype. |
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
RFC preferentially transports reduced folate forms like 5-MTHF, which dominate in whole foods. Adequate dietary folate supports RBC folate status, the biomarker associated with the rs1051266 genotype in women.
PubMed 19650776 · rs1051266 (c.80G>A) explained ~5% of RBC folate variance in women; no serum folate or homocysteine association; association absent in men
RFC is the primary cellular folate transporter, and folic acid is a poor substrate for it. 5-MTHF is the physiologically preferred RFC substrate. This is general transporter biology, not a finding specific to any rs1051266 genotype; no clinical trial has tested supplement form stratified by this SNP.
PubMed 40811598 · folic acid is a poor RFC substrate; 5-methylTHF and 5-formylTHF are the physiologically preferred forms for RFC-mediated uptake (from GeneReviews RFC biology section, not rs1051266-specific)
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
Why does this variant seem to matter in women but not men?
The evidence doesn't tell us. The sex difference was observed in one study in young adults from Northern Ireland, but the mechanism wasn't established. Hormonal differences, different reproductive demands on folate stores, or sex-specific variation in folate metabolism are all plausible reasons, but none have been confirmed for this SNP.
Should CC carriers take a different form of folate supplement?
No study in the available evidence tested 5-MTHF versus folic acid outcomes split by rs1051266 genotype. So the honest answer is: we don't know. What RFC biology does tell us is that the carrier works better with reduced folates than with synthetic folic acid, regardless of genotype. Whether that matters more for CC individuals than TT individuals is still an open question.