Does the FUT2 rs602662 variant influence vitamin B12 levels and gut mucosal biology?

The FUT2 gene, specifically the rs602662 variant (and its linked functional variant rs601338), plays a critical role in determining an individual's "secretor" status. This status dictates whether H-antigens (fucosylated glycans) are present in bodily fluids and on the intestinal mucosa. Population studies and mechanistic research have established that these genetic differences significantly influence both vitamin B12 levels and the landscape of the gut microbiome.

Clinical and population evidence

The association between FUT2 variants and vitamin B12 levels is robust and has been replicated across multiple large-scale genome-wide association studies (GWAS).

• Statistical significance: Studies consistently identify rs602662 as a major genetic determinant of serum B12 levels, with high statistical significance (e.g., p-values reaching $1.83 \times 10^{-15}$).
• Allelic impact: The A allele, which is highly correlated with non-secretor status (the inability to secrete H-antigens), is associated with significantly lower circulating B12 concentrations compared to the G (ancestral) allele.
• Diversity of evidence: This association has been observed in diverse populations, including European, Indian, and Brazilian cohorts, suggesting a universal biological mechanism rather than a population-specific artifact.

Mechanistic explanations

The link between the FUT2 genotype and B12 levels is mediated by fundamental changes in gut mucosal biology and subsequent microbial interactions.

• Mucosal glycosylation: FUT2 encodes the $\alpha$-(1,2)-fucosyltransferase enzyme. In secretors, this enzyme adds fucose to the intestinal mucus. Non-secretors lack these terminal fucose residues, fundamentally altering the biochemical environment of the gut barrier.
• Microbial competition and niche construction: Host-derived fucose serves as a critical carbon source and adhesion site for specific beneficial bacteria, such as Bifidobacterium. Non-secretors often exhibit reduced microbial diversity and lower levels of these taxa, which may alter the competitive landscape for B12 availability.
• B12 absorption pathways: The absence of specific glycans in non-secretors may impair the efficiency of B12 binding to intrinsic factor or its subsequent uptake in the terminal ileum. Additionally, secretor status influences susceptibility to H. pylori—a known contributor to B12 malabsorption—further complicating the relationship between the host, the microbiome, and nutrient status.

Bottom line

The FUT2 rs602662 variant is a well-validated genetic marker for lower circulating vitamin B12 levels. This association is driven by the variant's impact on gut mucosal glycosylation, which shifts the microbiome composition and alters the physiological environment required for optimal B12 absorption. Individuals with the non-secretor genotype may have a higher risk for lower B12 status due to these inherent differences in gut-microbe-nutrient interactions.