rs516246 — FUT2 FUT2 rs516246

Intronic FUT2 proxy variant tagging secretor status — whether ABO blood group antigens are secreted into body fluids; influences human milk oligosaccharide composition, infant gut microbiome colonisation, and mucosal immunity

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FUT2 Secretor Status — The Gene That Shapes What Passes Between Mother and Infant

Your FUT2 gene determines secretor status — one of the most consequential genetic traits in human biology, influencing not just your own gut immunity but the microbial world you pass on to your children. Secretors express ABO blood group antigens11 Secretors express ABO blood group antigens
H-type antigens built by alpha(1,2)-fucosyltransferase — the FUT2 enzyme — on mucosal surfaces and in saliva, tears, and breast milk. These fucosylated glycans are absent in non-secretors, who carry two loss-of-function copies of FUT2 in their intestinal mucus, saliva, and breast milk. rs516246 is an intronic proxy variant that tags this biological divide in European and African populations through strong linkage disequilibrium with the functional W143X nonsense mutation (rs601338). The T allele travels with the secretor phenotype; the C allele travels with the non-secretor phenotype.

This platform already catalogues rs601338 (the functional variant, focused on vitamin B12 metabolism) and rs492602 (a synonymous proxy focused on Crohn's disease and the gut-skin axis). rs516246 adds a distinct clinical dimension: the maternal-infant axis22 maternal-infant axis
The biological channel through which a mother's FUT2 genotype shapes the gut microbiome colonisation of her breastfed infant via the fucosylated oligosaccharides in her breast milk. This angle has direct implications for infant microbiome development and long-term immune programming.

The Mechanism

The FUT2 enzyme adds fucose residues to glycan chains on the intestinal epithelium and into secreted fluids, creating H-type 1 antigens33 H-type 1 antigens
The carbohydrate structure H type 1 is the intestinal and secretory form of the H antigen, built on Type 1 precursor chains; H type 2 is expressed on red blood cells. Only FUT2 generates H type 1 on mucosal surfaces and in milk. In breast milk, secretor mothers produce abundant 2'-fucosyllactose (2'-FL) and lacto-N-fucopentaose I (LNFP I)44 2'-fucosyllactose (2'-FL) and lacto-N-fucopentaose I (LNFP I)
The two most abundant human milk oligosaccharides (HMOs) produced exclusively by secretor mothers; 2'-fucosyllactose is the most prevalent HMO in secretor breast milk at concentrations of 1.5–4 g/L, collectively termed 2'-fucosylated human milk oligosaccharides (HMOs). Non-secretor mothers produce no 2'-fucosylated HMOs.

These fucosylated HMOs are the primary carbon source for Bifidobacterium species in the breastfed infant's gut — particularly B. longum subsp. infantis, which encodes a complete suite of fucosidase enzymes to harvest fucose from HMOs. Without 2'-FL and LNFP I in the milk, the infant's developing gut receives a fundamentally different microbial substrate, shifting which Bifidobacterium species flourish and which metabolic pathways dominate early colonisation.

The Evidence

The maternal secretor-infant microbiome connection was characterised by Lewis et al. 201555 Lewis et al. 2015
Lewis ZT et al. Maternal fucosyltransferase 2 status affects the gut bifidobacterial communities of breastfed infants. Microbiome, 2015: infants of secretor mothers established Bifidobacterium colonisation earlier and at significantly higher levels (p<0.001), with B. longum dominant in secretor-fed infants and B. breve more prevalent in non-secretor-fed infants — a difference in species composition with distinct metabolic and immune consequences.

Durham et al. 202166 Durham et al. 2021
Durham SD et al. A one-year study of human milk oligosaccharide profiles in the milk of healthy UK mothers and their relationship to maternal FUT2 genotype. Glycobiology, 2021 used rs516246 directly to genotype maternal secretor status in a longitudinal cohort, tracking HMO profiles across twelve months of lactation. The study confirmed that maternal FUT2 genotype at rs516246 reliably predicts 2'-fucosylated HMO composition, though it also noted occasional phenotype-genotype discordance, suggesting that enzyme expression variation can modulate predicted HMO output.

From the infant's own perspective, Thorman et al. 202377 Thorman et al. 2023
Thorman AW et al. Gut Microbiome Composition and Metabolic Capacity Differ by FUT2 Secretor Status in Exclusively Breastfed Infants. Nutrients, 2023 found that infant FUT2 secretor status was an even stronger driver of early microbiome composition than maternal secretor status. Full-secretor infants showed greater alpha diversity (p=0.049) and distinct metabolic profiles, with non-secretor infants displaying enriched sucrose-degradation pathways and different enterotypes.

Beyond infant biology, the broader FUT2 literature documents substantial effects on adult immunity. Non-secretors face elevated Crohn's disease risk88 Non-secretors face elevated Crohn's disease risk
McGovern DPB et al. Fucosyltransferase 2 (FUT2) non-secretor status is associated with Crohn's disease. Hum Mol Genet, 2010 at genome-wide significance (P=4.90×10⁻⁸) and increased type 1 diabetes susceptibility OR 1.29, 95% CI 1.20–1.3799 OR 1.29, 95% CI 1.20–1.37
Smyth DJ et al. FUT2 nonsecretor status links type 1 diabetes susceptibility and resistance to infection. Diabetes, 2011. The trade-off is near-complete protection against symptomatic norovirus GII infection — not a single non-secretor developed symptomatic norovirus1010 not a single non-secretor developed symptomatic norovirus
Thorven M et al. A homozygous nonsense mutation (428G→A) in FUT2 provides resistance to symptomatic norovirus (GGII) infections. J Virol, 2005 among outbreak-exposed adults in the Thorven 2005 study.

Practical Actions

For non-secretors (CC), the most targeted actions address the deficit in gut microbial diversity. Without host-derived fucosylated glycans in the gut lumen, dietary prebiotic fibers serve as an alternative carbon source for Bifidobacterium species. Targeted supplementation with multiple Bifidobacterium strains can partially compensate for reduced colonisation.

For secretor mothers (CT or TT) breastfeeding non-secretor infants, or non-secretor mothers who cannot produce 2'-fucosylated HMOs, awareness of the downstream infant microbiome implications is clinically relevant. Some infant formula products now supplement with 2'-fucosyllactose, and this genotype-driven difference in milk composition may influence breastfeeding supplementation decisions in consultation with a paediatrician.

Vitamin B12 monitoring is also relevant for non-secretors: standard total serum B12 may overestimate functional status because elevated haptocorrin-bound B12 inflates the total measurement without increasing bioavailable transcobalamin-bound B12.

Interactions

rs516246 is an intronic proxy in strong LD with rs601338 (FUT2 W143X, the functional variant) and rs492602 (another synonymous proxy) in European and African populations. Carrying the C allele at rs516246 very likely means also carrying the A (non-secretor) allele at rs601338. The three entries address distinct clinical dimensions of the same biology: rs601338 covers B12 metabolism, rs492602 covers Crohn's disease and the gut-skin axis, and this entry covers maternal-infant HMO biology.

In East Asian populations, the primary non-secretor determinant is rs1047781 (A385T), since the W143X-tagged haplotype is nearly absent in that ancestry.

Nutrient Interactions

vitamin B12 altered_metabolism

Genotype Interpretations

What each possible genotype means for this variant.

TT Secretor Normal

Full secretor — produces 2'-fucosylated milk oligosaccharides and mucosal glycans

The TT genotype at rs516246 tags the secretor haplotype via strong LD with the functional W143X secretor allele (G at rs601338). Your gut epithelium and mucosal secretions carry fucosylated H-type antigens that nourish Bifidobacterium species and shape mucosal immune responses.

If you breastfeed, your milk will contain the full complement of 2'-fucosylated HMOs — including 2'-fucosyllactose and LNFP I — which are primary carbon sources for B. longum subsp. infantis in your infant's developing gut. This substantially influences the speed and composition of early bifidobacterial colonisation.

The standard paradox of secretor status applies: your mucosal fucosylated glycans provide the attachment points that norovirus GII strains require to infect gut cells, so you have no genetic resistance to norovirus. This is the ancestral state, carrying normal infection susceptibility.

See rs601338 for the B12 metabolism angle and rs492602 for the gut-skin axis and psoriasis dimensions of this same biology.

CC Non-Secretor Absent

Non-secretor — no mucosal fucosylation, altered gut microbiome and breast milk composition

The CC genotype at rs516246 reliably tags FUT2 functional loss in European and African populations through tight LD with the W143X nonsense mutation (rs601338). With no functional enzyme, the downstream effects span four domains:

Infant microbiome (if breastfeeding): Lewis et al. 2015 showed infants of non-secretor mothers established Bifidobacterium later and at lower levels (p<0.001). Your breast milk lacks 2'-fucosylated HMOs (2'-FL and LNFP I), which deprives the infant gut of the primary substrate for B. longum subsp. infantis. The resulting microbiome tends toward B. breve dominance and different metabolic profiles.

Adult gut microbiome: Wacklin et al. 2011 found significantly reduced bifidobacterial richness and diversity in non-secretors (p<0.0001). Tong et al. 2014 showed functional reprogramming of the non-secretor microbiome with sub-clinical mucosal inflammation detectable before disease onset.

Autoimmune risk: GWAS data establish Crohn's disease association (P=4.90×10⁻⁸) and type 1 diabetes susceptibility (OR 1.29, 95% CI 1.20–1.37). The proposed mechanism runs through gut dysbiosis → impaired mucosal barrier → aberrant immune activation.

Norovirus resistance: The complete absence of mucosal H-type antigens means norovirus GII strains cannot bind gut epithelial cells. No symptomatic infections occurred among non-secretors in the Thorven 2005 outbreak cohort — a striking natural protection.

Vitamin B12: Total serum B12 is paradoxically elevated 16–18% in non-secretors, but this is in the biologically inactive haptocorrin-bound fraction. Holotranscobalamin (the active form cells actually use) is unaffected. Standard B12 tests can give falsely reassuring results.

CT Secretor (Carrier) Carrier

Secretor carrying one non-secretor proxy allele — full secretor function preserved

CT heterozygotes produce sufficient FUT2 enzyme from their single T-tagged functional allele to maintain secretor phenotype. Studies consistently group CT and TT individuals together as secretors because their microbial communities, HMO profiles, and autoimmune risk are essentially indistinguishable.

The clinical relevance of carrier status is primarily in reproductive planning: if both parents carry one C allele, each child has a 25% probability of being CC (non-secretor). In the context of infant feeding, this means a CC infant will not receive the benefit of your secretor-status 2'-fucosylated HMOs in breast milk, even though you produce them — it is the infant's own gut receptors and microbiota that respond to the HMOs, not just the HMO content of the milk.