rs6596473 — SLC23A1 SLC23A1 variant

SLC23A1 rs6596473 — A Third Signal in Your Vitamin C Transporter Gene

The human body cannot synthesise vitamin C. Every milligram of ascorbate¹¹ ascorbate
The active, ionized form of ascorbic acid at physiological pH, the predominant form in blood and tissues in circulation arrived through an active transport mechanism: SVCT1²² SVCT1
Sodium-dependent Vitamin C Transporter 1 — encoded by SLC23A1 on chromosome 5q23.2, expressed primarily on the apical surface of small-intestinal enterocytes and renal proximal tubule cells extracts ascorbate from ingested food in the gut and conserves it in the kidneys before it can be lost in urine. rs6596473 is an intronic variant within the SLC23A1 gene — a third independent signal at this locus alongside the well-characterised rs33972313³³ rs33972313
Val264Met missense variant — the primary functional signal at SLC23A1, altering the SVCT1 protein structure and reducing both intestinal absorption and renal reabsorption efficiency (Val264Met) and rs11950646⁴⁴ rs11950646
An intronic regulatory variant in SLC23A1 independently predicting plasma vitamin C levels across European cohorts (a second regulatory variant). Together these three variants map the genetic architecture of vitamin C transport at a single gene locus.

The Mechanism

rs6596473 sits within an intron of SLC23A1 at GRCh38 chromosomal position 139,374,887. Intronic variants are not silent — they can harbour splice regulatory elements⁵⁵ splice regulatory elements
Intronic enhancer and silencer sequences that bind RNA-binding proteins, modulate the recognition of nearby splice sites, and alter both the efficiency and the pattern of exon inclusion in the mature mRNA, secondary promoters, or transcription factor binding sites. The molecular consequence of rs6596473 has not been characterised in published functional experiments. What the genetic data do establish is that the C allele is in strong linkage disequilibrium (D' = 0.94) with rs10063949, another intronic SLC23A1 variant, suggesting these two sites may be inherited together and tag the same regulatory effect on SVCT1 expression or function.

Unlike the missense variant rs33972313 (Val264Met), which directly alters the SVCT1 transporter protein, rs6596473 likely exerts its effect through regulatory means — influencing the quantity or splicing pattern of the SVCT1 transcript. This is consistent with the modest and somewhat variable effect sizes seen across cohorts.

The Evidence

The initial evidence for rs6596473 came from Timpson et al. 2010⁶⁶ Timpson et al. 2010
Genetic variation at the SLC23A1 locus is associated with circulating concentrations of L-ascorbic acid (vitamin C): evidence from 5 independent studies with >15,000 participants. Am J Clin Nutr, 2010, a staged meta-analysis beginning with the British Women's Heart and Health Study (BWHHS) discovery cohort. In that cohort, the C allele at rs6596473 (minor allele frequency 0.28) showed a nominally significant directional association with plasma vitamin C. The effect replicated directionally in EPIC-Norfolk (+1.01 µmol/L; p = 0.02), but rs6596473 was not taken forward into the three additional replication cohorts because rs33972313 emerged as the cleaner primary signal. This places the variant's vitamin C association at an emerging evidence level.

More consistent evidence comes from the de Jong et al. 2014 periodontitis study⁷⁷ de Jong et al. 2014 periodontitis study
SLC23A1 polymorphism rs6596473 in the vitamin C transporter SVCT1 is associated with aggressive periodontitis. J Clin Periodontol, 2014, which found enrichment of the rare C allele in aggressive periodontitis (AgP) cases in a multi-stage European cohort study. After pooling the German case cohort (674 cases, 2,891 controls), the C allele showed a statistically significant association with AgP (OR 1.35, p = 0.005 after adjustment for smoking and sex). Aggressive periodontitis is characterised by rapid alveolar bone destruction in younger patients — a condition in which vitamin C-dependent collagen synthesis and immune cell function in gingival tissue are thought to be pathophysiologically relevant.

Additional evidence comes from Crohn disease genetics. Shaghaghi et al. 2014⁸⁸ Shaghaghi et al. 2014
Polymorphisms in the sodium-dependent ascorbate transporter gene SLC23A1 are associated with susceptibility to Crohn disease. Am J Clin Nutr, 2014 genotyped 162 Crohn disease patients, 149 ulcerative colitis patients, and 142 controls from the Manitoba IBD Cohort Study. The C allele at rs6596473 forms part of the CGG risk haplotype (rs6596473-C, rs33972313-G, rs10063949-G) that is carried by 65.3% of Crohn disease patients versus 43.5% of controls (P < 0.0001), in strong LD with the primary driver rs10063949 (D' = 0.94). The biological rationale is that SVCT1 is the dominant vitamin C transporter in intestinal epithelial cells — reduced SVCT1 function would impair mucosal ascorbate concentrations, weakening the antioxidant defence of the gut lining.

Practical Actions

The C allele at rs6596473 is common in European populations (MAF ~31%) but is even more frequent in East Asian (~67%) and African (~64%) populations. In Europeans, approximately 43% of people are heterozygous CG carriers and 9% are CC homozygotes. This means the majority of CC homozygotes are relatively uncommon in Europe compared to East Asia, where CC is the most common genotype.

The clinical picture is one of subtly reduced SVCT1-mediated vitamin C absorption and/or renal reabsorption. The actionable response is the same as for other SLC23A1 variants: attention to consistent dietary vitamin C intake across the day, using multiple smaller servings rather than a single large dose to work around intestinal absorption saturation, and considering a modest daily supplement if dietary intake is inconsistent.

Interactions

rs6596473 acts at the same SLC23A1 locus as the stronger primary signals rs33972313 (Val264Met) and rs11950646 (a second regulatory variant). It is in strong LD with rs10063949 (D' = 0.94), meaning these two variants are frequently co-inherited. Individuals carrying risk alleles at multiple SLC23A1 positions face compounded reductions in SVCT1-mediated transport. Variants in SLC23A2 (encoding SVCT2, responsible for tissue-level vitamin C delivery to brain, adrenals, and immune cells) act in parallel — carrying risk alleles at both loci simultaneously reduces both whole-body availability and cellular delivery of ascorbate.