If your SNiP report came back with FUT2 flagged in red, your first instinct is probably to assume something is wrong with the gene. That’s not what red means here.

One thing to say up front: SNiP is not a secretor-status test. We look at FUT2 for what it means for vitamin B12 absorption. This article explains the difference, because the two get mixed up constantly.

The question of secretor vs non-secretor comes down to one gene, FUT2, and it’s one of the few places on your report where a red result often means the gene is working exactly as designed. That sounds like a contradiction. It isn’t. FUT2 comes with real trade-offs, and the version that protects you in one area can leave you short in another. Knowing which side of that trade-off you landed on is one of the more useful things your DNA can tell you, and it’s exactly why your formula responds to it.

What Does the FUT2 Gene Do?

FUT2 stands for fucosyltransferase 2. It codes for an enzyme that attaches a sugar called fucose to the cells lining your mouth, gut, airways, and other mucosal tissue. That sugar becomes the H antigen, the foundation your ABO blood type markers are built on.

Most people know their blood type from a blood draw. Far fewer know that whether those same markers appear in saliva, tears, and digestive mucus comes down to FUT2. If your version of the enzyme is active, those sugars get secreted onto your mucosal surfaces. If it isn’t, they never show up there.

That one difference splits people into two groups: secretors and non-secretors. Roughly 80 percent of people are secretors. About 20 percent of people of European and African ancestry are not.

Secretor vs Non-Secretor: Reading the Genotypes

Two variants come up in this conversation, and it’s worth being precise about how they relate. They sit at different positions on the gene, and they aren’t interchangeable.

rs601338 is the causal variant. It’s a nonsense mutation, W143X, that cuts the FUT2 enzyme short and shuts it down. This is the textbook definition of secretor status:

• G/G is a full secretor
• G/A is a partial secretor
• A/A is a non-secretor

rs602662 is the variant SNiP tests and formulates around. It’s a different change to the same gene, a missense variant called S258G, located close enough to rs601338 that the two are almost always inherited as a package. Geneticists measure that with two numbers, and for these variants both are well established: D’ = 1 and r² = 0.76.

In plain terms, D’ = 1 means recombination hasn’t separated the two positions in the populations studied, so they travel together. The r² of 0.76 means knowing one tells you a great deal about the other, though not everything, because the two occur at somewhat different frequencies.

What that means for you: if your rs602662 result is G/G, you’re very likely a full secretor, since G is the secretor-associated allele at that position. Note: “likely” is not guaranteed. A minority of people carry combinations where the two do not line up, and the correlation is weaker outside European populations.

To be completely clear: SNiP does not test for secretor status, and your report does not tell you whether you are a secretor. SNiP doesn’t grade your FUT2 result by guessing your secretor status and working backward. The vitamin B12 association was measured at rs602662 itself, in large population studies that genotyped that exact position. Secretor status is the biology that explains why the association exists, not the thing being inferred.

FUT2 Has More Than One Secretor Variant

If you’ve searched this topic, you’ve probably run into several different rs numbers, all described as the FUT2 secretor SNP. They’re all real, and they’re all in FUT2. Public allele databases catalog more than 70 versions of the gene, and several independently switch the enzyme off. Which one matters most depends on ancestry.

• rs601338 (c.428G>A, W143X) is the predominant non-secretor variant in people of European, African, and Latin American ancestry.
• rs1047781 (c.385A>T, I129F) is the predominant one in East and Southeast Asian populations, where rs601338 is rare. It typically produces a weak or partial secretor rather than a complete non-secretor.
• rs200157007 (c.302C>T) and a whole-gene deletion known as sedel2 are additional inactivating variants used in comprehensive secretor typing.
• rs492602 sits in near-perfect linkage with rs601338 and is often genotyped as its stand-in.
• rs602662 (c.772G>A, S258G) is the marker SNiP tests.

What rs602662 does extremely well is the job SNiP reports it for. Your report labels FUT2 for vitamin B12 absorption, not for secretor typing, and that’s the right way to read it.

What Secretors Get

Those fucosylated sugars on your gut lining aren’t just identity tags. They’re food.

Beneficial bacteria, especially Bifidobacteria, feed on them and use them as anchor points to colonize the gut wall. A study in PLoS One found that secretor status strongly shapes which Bifidobacteria live in the intestine, and secretors tend to carry a richer community. Those bacteria help produce short-chain fatty acids such as butyrate, which support the integrity of the gut lining. Learn more in Got Butyrate? Here’s How to Get More of This Gut-Friendly Ally.

If you’re a secretor, your gut wall is running a full-service cafeteria for your resident microbes.

What Non-Secretors Get

The same sugars that feed beneficial bacteria are also a doorway for some of the most contagious pathogens on the planet.

Norovirus, the winter stomach bug, binds to the H1 antigen to enter cells in the small intestine. In a classic challenge study published in Nature Medicine, volunteers who were non-secretors resisted infection with Norwalk virus. Researchers have reported a similar pattern with rotavirus, where non-secretor children were largely protected from severe illness.

Helicobacter pylori, the bacterium that colonizes the stomach lining, also uses those secreted antigens to attach. Non-secretors are colonized far less often.

If you’re a non-secretor, your gut lining is simply a harder building to break into.

There Is No Winning Version of This Gene

Non-secretors tend to carry a less rich Bifidobacteria community, while secretors are more susceptible to the infections above and to one specific nutritional challenge.

Neither result is the good one. Each version is an advantage somewhere and a liability somewhere else, which is precisely why a single color on a report can mislead you if you don’t know what it’s measuring.

Why SNiP Flags G/G as Red

On your SNiP report, FUT2 rs602662 is labeled for B12 absorption, and a G/G result comes back red. Because red usually signals reduced function, the natural assumption is that G/G means your enzyme is broken.

It’s the opposite. G is the secretor-associated allele, so G/G points to a fully working enzyme. What’s flagged isn’t the gene’s general function. It’s one specific downstream consequence: vitamin B12.

The evidence is unusually strong. In a genome-wide study published in Nature Genetics that genotyped rs602662 directly, the G allele showed one of the most significant associations ever reported for a nutrient-related variant, reaching p = 1.35 × 10⁻³⁹ in the combined analysis. People carrying G had meaningfully lower circulating B12, and the finding has since replicated across European, American, and Indian populations.

The leading explanation runs through H. pylori. Secretor status makes the stomach lining easier for it to colonize. Long-term colonization can reduce the stomach’s output of intrinsic factor, the protein B12 has to bind to before it can be absorbed in the small intestine. Less intrinsic factor means less B12 crosses over, no matter how much you eat.

Red at FUT2 means one thing. Red at MTHFR means something else entirely. Each marker is graded against what that marker actually does.

Why One Lab Number Can Mislead

Most of the B12 in your blood rides on one of two carrier proteins. Holo-transcobalamin is the fraction your cells can take up and use. Holo-haptocorrin is taken up mainly by the liver and isn’t directly available to your tissues.

A 2017 study in Human Molecular Genetics found the FUT2 secretor variant moves those two fractions very differently. Non-secretors carried about 22 percent more holo-haptocorrin, while holo-transcobalamin was essentially unchanged. A meaningful share of the B12 gap between the groups sits in the fraction your cells can’t use.

So a total B12 result that looks fine doesn’t tell either group as much as it appears to. Your genotype tells you something the number alone can’t, and because your genes don’t change, that insight stays with you for life.

FUT2 Doesn’t Work Alone

B12 absorption at FUT2 is one input. Two other markers SNiP formulates around sit in the same pathway.

MTRR (rs1801394) affects how efficiently your body recycles B12 once it’s inside your cells. MTHFR (rs1801133 and rs1801131) affects how well you convert folate into its usable form. Someone with a red FUT2 plus an MTHFR variant has a very different B12 and folate picture than someone with a red FUT2 alone. We explain the folate side in B12 and Folate for Women: Why Your MTHFR Genes Decide Which Forms You Need.

Reading one gene in isolation gets people into trouble. Your report shows you the pathway, not a single dot on it.

Where DNA-Powered Nutrition Fits

This is where a formula built from your DNA does something a shelf multivitamin can’t. CODE Complex® is custom-compounded from your results, and FUT2 is one of the key genes it’s built around. The FUT2 blend combines SAMe, folate as Quatrefolic® (6S)-5-methyltetrahydrofolate, and vitamin B12 as methylcobalamin.

The forms matter as much as the ingredients. Methylcobalamin is an active, ready-to-use form of B12, so it doesn’t depend on the conversion steps that cyanocobalamin requires. Quatrefolic® is already-active folate, which matters because B12 and folate work as a pair in the methylation cycle. Our article Vitamin B12: Optimizing Genetic Expression and Ensuring Safety explains why the form of B12 matters, and Natural vs. Synthetic Folic Acid covers folate. Laboratory research published in 2020 showed that supplying methylfolate directly bypasses a reduced MTHFR step, while folic acid does not.

Because the formula responds to your actual result, a red FUT2 receives different support than a green one. That’s the point of building from DNA instead of a label.

Everyday Ways to Support B12 and Your Gut

How much your body gets from these foods starts with your FUT2 result, so read it first and let it set the priority.

• Keep B12-rich foods in rotation. Clams, sardines, salmon, eggs, and beef are among the densest dietary sources.
• Eat folate-rich foods alongside them. Leafy greens and legumes support the same methylation cycle. Our guide How to Support Methylation With Food lists the best options.
• Feed your microbiome. Fiber and fermented foods support the environment B12 travels through. See Biohacking With Fermentation.
• Be consistent. Absorption happens in a living system, and steady daily support beats occasional large doses.

Common Questions About FUT2 and Secretor Status

What is the difference between a secretor and a non-secretor?
Secretors produce an active FUT2 enzyme and release ABO blood group antigens into saliva, digestive mucus, and other bodily fluids. Non-secretors do not. Around 80 percent of people are secretors.

Are rs601338 and rs602662 the same thing?
No. They are two different variants in the FUT2 gene that are almost always inherited together. rs601338 is the causal nonsense mutation that defines secretor status. rs602662 is a nearby missense variant strongly correlated with it, at D’ = 1 and r² = 0.76 in European populations. Your result at one usually predicts the other, but not in every case. SNiP tests rs602662, and the vitamin B12 research behind SNiP’s report was measured at rs602662 itself.

Are there other FUT2 SNPs besides rs602662?
Yes, many. More than 70 FUT2 alleles are catalogued, and several independently inactivate the enzyme. rs601338 is the main non-secretor variant in European, African, and Latin American populations. rs1047781 is the main one in East and Southeast Asian populations. rs200157007 and the sedel2 deletion are others. For definitive secretor typing, rs601338 and rs1047781 are the more direct markers. rs602662 is the marker with the strongest direct evidence for vitamin B12, which is what SNiP reports it for.

Does a red FUT2 result mean my gene is broken?
No. At rs602662, G is the secretor-associated allele, so a G/G result points to a functioning enzyme. The red designation is specific to vitamin B12 absorption, not to the gene’s overall function.

Which FUT2 genotype has lower vitamin B12?
Research consistently associates the G allele at rs602662, and G/G in particular, with lower circulating vitamin B12. The A allele tracks with non-secretor status and higher measured B12.

Is it better to be a secretor or a non-secretor?
Neither. Secretors tend to have a more diverse gut microbiome but greater susceptibility to norovirus, rotavirus, and H. pylori. Non-secretors are more resistant to those infections but show lower Bifidobacteria richness and higher observed rates of certain inflammatory and autoimmune conditions.

How do I find out my secretor status?
Secretor status is determined by DNA testing, most directly at rs601338 and, for Asian ancestry, rs1047781. SNiP’s DNA test reports FUT2 rs602662 alongside 102 other markers and labels it for vitamin B12 absorption, and FUT2 is one of the 22 markers your CODE Complex^® formula is built around.

Can I just take a B12 supplement?
The form matters. Methylcobalamin is the active form and does not require the conversion steps that cyanocobalamin does. Because B12 and folate work together, most people benefit from supporting both.

The Takeaway

Your FUT2 result is a good reminder that genes aren’t good or bad. They’re context. The same variant that makes you resistant to one of the most contagious viruses on earth may be the reason a standard B12 supplement never quite moved the needle. Our genes don’t change. What changes is how precisely your nutrition matches them.

Ready to see your own FUT2 result? Get started with SNiP. Already have DNA data from 23andMe or AncestryDNA? Enter your results to receive your personalized CODE Complex® formula.

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