Vitamin B12

FUT2 decides whether you are a "secretor" — whether the blood-group sugars on your cells are also secreted into saliva, tears and the mucus lining the gut. A large minority of people are non-secretors, and it is one of the most common functional differences in the human genome.

A 2008 genome-wide scan of 1,658 people, replicated in an independent 1,059, found a FUT2 variant associated with plasma vitamin B12 at p = 5.4 × 10⁻¹⁷. It is in strong linkage disequilibrium with the non-secretor change, which points at a mechanism rather than a coincidence: the gut lining is where B12 is absorbed, and it is also what secretor status changes.

So this is a real, replicated, mechanistically plausible effect on a blood level. What it is not is a reason to buy anything. B12 status is measured directly and cheaply, deficiency is diagnosed and treated on that measurement, and no guideline sets a dose from a genotype. The people who actually need B12 — after certain gut surgery, on long-term metformin, on a strict vegan diet, or with pernicious anaemia — need it whatever their FUT2 result says.

See also FUT2 secretor status, where the same gene turns up in resistance to norovirus.