FUT2: Secretor Status, Vitamin B12, and Your Gut
One gene decides whether your blood B12 reads high for a reason that has nothing to do with how much you absorb.
FUT2 is the gene behind something called secretor status. The enzyme it makes sticks a sugar (fucose) onto the linings of your gut, saliva, and other body fluids, which sets the stage for which microbes get to settle in there and how some of your blood proteins get decorated. About one in five people of European ancestry carry two broken copies and make no working enzyme at all. They're called non-secretors, and they're perfectly healthy. The reason FUT2 shows up on a B12 page is one of the most replicated findings in nutrition genetics: your FUT2 type is the single strongest common-gene predictor of how much total vitamin B12 shows up in a blood test. Here's the twist worth understanding before you panic at a lab result. The B12 that moves with this gene is mostly the parked, liver-bound kind, not the form your tissues actually use. So a "high" B12 in a non-secretor can be a bit of a mirage.
What FUT2 does
FUT2 codes for an enzyme that adds fucose sugars to the surfaces of your gut lining and secretions, which sets your ABO blood-group secretor status.
Secretor status shapes which gut bacteria can attach and feed, so it's one input into the makeup of your microbiome.
The same enzyme decorates haptocorrin, a protein that ferries vitamin B12 in the blood, which is why the gene tracks so tightly with your total B12 reading.
The non-secretor version (two A alleles at rs601338) is tied to a higher total serum B12 number, but the extra is mostly the liver-bound, less-usable fraction rather than the active form.
Your variants, decoded
This is the switch that decides whether you make a working FUT2 enzyme. The G allele builds a functional enzyme (secretor). The A allele inserts a premature stop, so the enzyme comes out truncated and dead (non-secretor). Two A copies means no working enzyme at all. Note on reading your own data: this site is reported on one DNA strand, but a 23andMe or AncestryDNA export may print the complementary letters for this position (C instead of G, T instead of A). The biology is identical. If your file shows C/C, T/T, or C/T, read those as G/G, A/A, and G/A here.
| GG | Secretor. Both copies of FUT2 work, so you make the enzyme normally. Tends to go with a more average total B12 reading. About 80% of Europeans carry at least one G. |
| GA | One working copy, one broken. Still a secretor in practice, since one good copy is enough to make the enzyme. B12 reading lands in the typical range. |
| AA | Non-secretor. Both copies carry the stop, so you make no functional FUT2 enzyme. Studies link this to a total serum B12 reading about 16 to 18 percent higher than secretors, but the extra is mostly the parked haptocorrin-bound form, not the active form your cells use. A high number here doesn't guarantee you're well-stocked. |
Genotypes are shown order-insensitively and on the forward strand; your own export may print the complementary letters — the meaning is the same.
What the research suggests
In non-secretors (rs601338 AA), the higher total serum B12 rides almost entirely on the haptocorrin-bound fraction, which the liver clears and tissues don't use, while the bioactive transcobalamin-bound fraction was unaffected by FUT2 type. So for an AA person, the total B12 number is a poor read on true status, and an active-B12 or MMA test gives a clearer picture. This is a testing-strategy point, not a supplement to buy off a shelf; bring it to a clinician.
PubMed 29040465 · found rs601338 AA (non-secretor) had 16 to 18 percent higher total serum B12 and higher holo-haptocorrin, while holo-transcobalamin (the bioactive carrier) was not influenced by the variant.
Educational only — not medical advice. “General evidence” means the finding is real but the supplement’s benefit isn’t unique to your genotype.
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Questions
I'm a non-secretor (AA) with high B12 on my blood test. Am I good?
Not necessarily. The FUT2 effect mostly bumps up the haptocorrin-bound fraction of B12, which the liver takes up and your tissues don't use much. So a high total serum number in an AA person can overstate how much usable B12 you actually have. If you have symptoms that fit low B12, an active-B12 (holoTC) or methylmalonic acid test cuts through the noise. Talk to your doctor about which test makes sense for you.
Does being a non-secretor mean I absorb less B12 from food or supplements?
The genetics don't say that cleanly. What's well-established is that your FUT2 type shifts how B12 distributes across carrier proteins in your blood, which changes the total number you see on a test. One study in an Indian cohort even found non-secretors responded more to B12 supplementation. The honest answer is that the effect on the lab reading is solid; the effect on real absorption is less settled. Don't change your intake based on this alone.
What does secretor status have to do with my gut?
The FUT2 enzyme decorates your gut lining with fucose sugars that certain bacteria latch onto and feed on. Non-secretors have a measurably different microbiome makeup than secretors because that food source isn't there. It's one of several inputs into your gut community, not a verdict on gut health.
My 23andMe file shows C/C or T/T for rs601338. What does that mean?
It means your raw file is reporting the opposite DNA strand from this page. C lines up with our G (secretor), and T lines up with our A (non-secretor). So C/C reads as G/G, T/T reads as A/A, and C/T reads as G/A. The result is the same either way. The letters just got flipped by which strand the test chose to print.
Should I take a B12 supplement because of my FUT2 result?
Not based on this gene by itself. FUT2 changes how to read your B12 number, not whether you're deficient. If you're worried about your B12 status, the move is a proper test (active B12 or methylmalonic acid) and a conversation with your doctor or a registered dietitian, who can factor in your diet, age, and any gut conditions.