Does the FUT2 rs602662 variant lower circulating vitamin B12 concentrations?

The association between the FUT2 rs602662 genetic variant and circulating vitamin B12 concentrations has been well-characterized in genetic epidemiology and biochemical research.

Clinical and effectiveness evidence

• Robust Association in GWAS: Large-scale genome-wide association studies (GWAS) and candidate gene analyses consistently identify the FUT2 locus, specifically the rs602662 variant (along with its proxy rs601338), as a major genetic determinant of serum vitamin B12 levels.
• Effect Sizes: The rs602662 variant is associated with statistically significant shifts in total serum B12 concentrations, typically explaining approximately 1% to 5% of the variance in circulating B12 levels in populations of European and Asian descent. Carriers of the G allele (or non-secretor-associated alleles) frequently exhibit lower baseline circulating B12 compared to those with secretor-associated genotypes.

Mechanistic explanations

• Fucosyltransferase 2 and Secretor Status: FUT2 encodes alpha-(1,2)-fucosyltransferase, an enzyme that regulates the secretion of ABH blood group antigens into bodily fluids (such as saliva and gastric mucus). The rs602662 variant acts as a strong marker for "secretor" vs. "non-secretor" phenotypes.
• Haptocorrin Glycosylation: Rather than affecting active cellular uptake via transcobalamin II (the clinically active carrier), FUT2 variants primarily influence the glycosylation and clearance of haptocorrin (also known as transcobalamin I). Haptocorrin binds the majority of circulating B12, and alterations in its glycosylation modify how rapidly it is cleared by the liver, leading to lower total circulating levels of haptocorrin-bound B12 without necessarily indicating a tissue-level B12 deficiency.
• Gut Microbiota and Mucosal Interactions: Non-secretor status alters the glycan presentation on the gastrointestinal mucosa, which in turn shapes the composition of the gut microbiota. Certain commensal bacteria rely on mucosal glycans as carbon sources, and variations in these glycans modify the abundance of B12-synthesizing or B12-consuming microbes, directly affecting B12 bioavailability.

Clinical implications and limitations

• Active vs. Inactive B12: Because the FUT2 variant predominantly influences haptocorrin-bound B12 (the clinically inactive fraction), individuals carrying the low-B12 allele may present with lower total serum B12 measurements while maintaining normal tissue-level activity and normal levels of active holotranscobalamin.
• Biomarker Interpretation: For clinicians evaluating B12 status, a borderline-low serum B12 measurement in an individual carrying the rs602662 risk variant may not represent physiological deficiency. Assessing secondary functional markers—such as methylmalonic acid (MMA) or homocysteine—provides a more accurate representation of true cellular B12 status.

Bottom line

The FUT2 rs602662 variant is a robust genetic marker associated with lower circulating vitamin B12 concentrations. This effect is driven by alterations in haptocorrin glycosylation and mucosal-microbial interactions rather than impaired active B12 absorption, meaning total serum B12 levels may be lowered without a corresponding intracellular deficiency.