rs6133175 — SLC23A2

Intronic variant in the tissue vitamin C transporter SVCT2 — GG homozygotes carry ~24% higher plasma vitamin C levels than AA homozygotes

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SLC23A2 rs6133175 — Your Tissue Vitamin C Transporter

Every cell in your body needs vitamin C, but not all cells are equal in their ability to acquire it. Once dietary ascorbate crosses the gut wall and enters the bloodstream, a second transporter system distributes it into tissues where it is needed most — the brain, adrenal glands, eyes, and metabolically active organs. The gene SLC23A2 encodes SVCT211 SVCT2
Sodium-dependent Vitamin C Transporter 2 — a high-affinity transporter expressed in metabolically demanding tissues including neurons, adrenal cortex cells, and the aqueous humor of the eye, a high-affinity transporter that pulls ascorbate from the bloodstream into these specialized tissues. The intronic variant rs6133175 sits within SLC23A2 and, despite lying outside the protein-coding sequence, influences circulating plasma vitamin C in a measurable way — with the GG genotype associated with approximately 24% higher plasma ascorbate than the common AA genotype.

The Mechanism

Unlike SLC23A1 (SVCT1), which handles intestinal absorption and renal reabsorption to maintain whole-body vitamin C homeostasis, SVCT2 operates in tissues with high metabolic demand. The brain accumulates vitamin C to concentrations roughly 10-fold higher than plasma — a feat achieved almost entirely by SVCT2 expressed on the blood-brain barrier and neuronal membranes. The adrenal glands similarly use SVCT2 to build the highest vitamin C concentration of any organ in the body, where ascorbate is required for cortisol and adrenaline synthesis.

The rs6133175 variant is an intron variant22 intron variant
Located within a non-coding intervening sequence of the gene; does not directly change the amino acid sequence but may affect gene expression, splicing, or regulatory element activity — its precise molecular mechanism has not been fully characterized. The most likely explanations are altered splicing efficiency33 splicing efficiency
The process by which intron sequences are removed from pre-mRNA; intronic variants near splice sites can shift the ratio of splice isoforms, changing how much functional protein is made or disruption of a transcriptional regulatory element within the intron. The fact that it influences plasma vitamin C at all — despite not changing the SVCT2 protein sequence — suggests it affects the amount of transporter protein expressed rather than its function.

Because SVCT2 handles redistribution of ascorbate from plasma into tissues, a variant that increases SVCT2 expression or activity could lower plasma levels by pulling more vitamin C into cells, or raise plasma levels by improving renal reabsorption secondary effects. The net effect observed in the EPIC cohort is higher plasma vitamin C in GG homozygotes, though the direction of causality through tissue distribution remains to be mechanistically confirmed.

The Evidence

The primary evidence comes from a nested case-control study in the European EPIC cohort44 nested case-control study in the European EPIC cohort
Duell EJ et al. Vitamin C transporter gene (SLC23A1 and SLC23A2) polymorphisms, plasma vitamin C levels, and gastric cancer risk in the EPIC cohort. Genes Nutr, 2013 involving 365 gastric cancer cases and 1,284 matched controls from 10 European countries. Among 311 controls with complete genotyping and plasma vitamin C data, genotype frequencies were AA 37%, AG 45%, and GG 12%. In a recessive model adjusted for age, sex, country, smoking, and season of blood draw, GG homozygotes had plasma vitamin C 24% higher than AA homozygotes (beta = 0.22, 95% CI: 0.029–0.40; P = 0.02). The raw plasma values were AA 39.1, AG 39.4, and GG 45.2 umol/L — a clinically meaningful spread given that adequate status is generally considered to be above 28 umol/L.

Importantly, both rs6133175 (SLC23A2) and rs33972313 (SLC23A1) independently predicted plasma vitamin C levels in multivariable models, suggesting the two genes tag non-overlapping mechanisms: SVCT1 controls gut absorption and renal reclamation, while SVCT2 variants apparently influence a separate step in vitamin C homeostasis.

A Chinese Han population study55 Chinese Han population study
Hou H et al. Impact of SLC23A1 and SLC23A2 Polymorphisms on the Risk for Preeclampsia in a Chinese Han Population. J Nutr Sci Vitaminol (Tokyo), 2022 found significant genotypic frequency differences for rs6133175 between preeclampsia cases and controls. Under a recessive model, the A allele (homozygous AA) was associated with protection against preeclampsia (OR = 0.71, 95% CI: 0.55–0.92; P = 0.01), while AG/GG genotypes showed elevated risk. This counterintuitive finding — where the lower-vitamin-C genotype appears protective — may reflect complex tissue-specific redox effects in pregnancy or confounding by population-specific factors.

A case-control study of chronic lymphocytic leukaemia66 case-control study of chronic lymphocytic leukaemia
Casabonne D et al. Fruit and vegetable intake and vitamin C transporter gene (SLC23A2) polymorphisms in chronic lymphocytic leukaemia. Eur J Nutr, 2017 found a log-additive association between the G allele and CLL risk (OR = 1.19, 95% CI: 1.00–1.41; P = 0.05), independent of fruit and vegetable intake.

Practical Implications

The key finding for most carriers is straightforward: AA homozygotes — about 50% of the global population — run plasma vitamin C levels roughly 6 umol/L lower than GG homozygotes on the same diet. This is a consistent genetic baseline effect that dietary choices can compensate for, but cannot eliminate. If your dietary vitamin C intake is adequate (above 75–90 mg/day), the genotype effect is unlikely to push you into frank deficiency. But if your diet is limited in vitamin C-rich foods — especially common in winter months or during food restriction — the AA genotype adds a structural disadvantage.

The G allele is notably more common in East Asian populations (~64%) than in Europeans (~37%), and quite rare in African populations (~17%). This means the GG "high-ascorbate" genotype affects about 40% of East Asians compared to roughly 14% of Europeans.

Interactions

This variant operates through a different biological step than rs3397231377 rs33972313
SLC23A1 Val264Met — reduces intestinal and renal vitamin C transport capacity in SLC23A1 (SVCT1). The Duell 2013 EPIC study demonstrated that both variants independently predicted plasma vitamin C in the same multivariable model, indicating additive rather than redundant effects. A person carrying the reduced-function SLC23A1 variant (rs33972313 CT/TT) alongside the low-vitamin-C SLC23A2 genotype (rs6133175 AA) faces a dual disadvantage: both absorbing less vitamin C from food and having less favorable tissue distribution.

The closely located variant rs605300588 rs6053005
SLC23A2 intronic variant, ~66 kb downstream of rs6133175, also associated with 24% higher plasma vitamin C in TT homozygotes vs CC homozygotes in the same EPIC cohort in SLC23A2 (approximately 66 kb downstream within the same gene) showed nearly identical effects in the EPIC cohort (TT: +24%, beta = 0.21, 95% CI: 0.058–0.37, P = 0.007). These two SLC23A2 variants likely tag the same haplotype block and may not represent fully independent signals.

Nutrient Interactions

vitamin C altered_metabolism

Genotype Interpretations

What each possible genotype means for this variant.

GG Higher Vitamin C Baseline Beneficial

GG genotype — plasma vitamin C ~24% higher than AA carriers on the same diet

In the Duell 2013 EPIC analysis (n=311 controls), GG homozygotes had a mean plasma vitamin C of 45.2 umol/L versus 39.1 umol/L for AA homozygotes — a 15.6% absolute difference that achieved statistical significance under the recessive model (beta = 0.22, 95% CI: 0.029–0.40; P = 0.02). This effect was independent of dietary fruit and vegetable intake, confirming a genetic rather than behavioral origin.

The elevated baseline provides a wider buffer before plasma levels fall into the suboptimal range (below 28 umol/L). For most GG carriers, standard dietary vitamin C recommendations (75 mg/day for women, 90 mg/day for men) are sufficient for optimal ascorbate status.

One note of context: a preeclampsia study in a Chinese Han population found that AG/GG genotypes were associated with slightly elevated preeclampsia risk compared to AA, suggesting the picture during pregnancy may be more complex. This does not alter general recommendations but warrants monitoring during pregnancy.

AG Intermediate Vitamin C Profile Intermediate

One G copy — plasma vitamin C between the AA and GG extremes

Under the recessive model that best fit the EPIC data, the AG genotype clusters with AA rather than GG in terms of plasma vitamin C. This means heterozygotes do not show a clear intermediate effect — the G allele appears to exert its vitamin C-elevating effect primarily in the homozygous GG state. The mean plasma values in EPIC were AA 39.1, AG 39.4, GG 45.2 umol/L, confirming the recessive pattern.

As with AA, adequate dietary intake of vitamin C keeps plasma levels well within normal reference ranges for most AG carriers. The functional distinction from AA is minimal based on available data.

AA Lower Vitamin C Baseline Decreased

Common genotype — plasma vitamin C runs ~24% lower than GG carriers

The A allele at rs6133175 represents the ancestral, common state of this variant. Despite being the "common" genotype, it is associated with lower plasma vitamin C compared to GG homozygotes in a recessive genetic model (beta = 0.22 per GG vs AA comparison, P = 0.02; Duell 2013, EPIC cohort). The difference in absolute terms was approximately 6 umol/L.

For context, the general population range for adequate vitamin C is approximately 28–85 umol/L, with frank deficiency below 11 umol/L and optimal antioxidant status often cited above 50 umol/L. An AA genotype with mean values around 39 umol/L sits comfortably in the adequate range, but the buffer before hitting suboptimal territory is smaller than for GG carriers.

In populations with lower dietary vitamin C intake, or in physiological states that increase vitamin C turnover (infection, inflammation, smoking, pregnancy), the genotype effect becomes more clinically relevant. Smokers independently require an extra 35 mg/day of vitamin C, and the AA genotype narrows the safety margin further.