What FUT2 Means in Your 23andMe Raw Data

FUT2 in a 23andMe raw data file tells you whether you are a secretor of ABO/H blood group substances into saliva, mucus and the gut. FUT2 controls secretion of the H antigen that carries A and B blood group substances, not your ABO blood type itself. If your raw file shows the non-secretor genotype at rs601338, often AA on the forward strand or TT on the reverse strand, you have a genotype linked to strong resistance to GII.4 norovirus and some other norovirus strains, but that does not make you immune to gastroenteritis as a whole. FUT2 has also been associated with B12 levels in population studies, but a genotype alone should not be used to predict your B12 level.

What does FUT2 mean in my 23andMe raw data?

FUT2 is a gene on chromosome 19. It codes for an enzyme called alpha-1,2-fucosyltransferase 2. That enzyme adds a fucose sugar to the end of oligosaccharide chains on mucosal surfaces. The result is the H antigen, which is the basis for the A and B blood group antigens. People with a working FUT2 enzyme put these ABO/H antigens into saliva, tears, gut mucus and other body fluids. They are called secretors. People with two non-working copies of FUT2 have little or no secreted H antigen in these fluids. They are called non-secretors. FUT2 controls whether these antigens are secreted, not which ABO blood type you have.

Non-secretor status is common, not a rare mutation. In people of European ancestry, roughly 1 in 5 are non-secretors. The frequency varies by ancestry, and the exact non-working variant can vary too. In many European-ancestry raw files, the common marker to check is rs601338. That marker is not the universal FUT2 answer. Other FUT2 loss-of-function variants exist, and rs601338 alone cannot rule out non-secretor status in all ancestries.

A common marker to check: rs601338

What does FUT2 mean in my 23andMe raw data? For many people of European ancestry, the practical answer sits in one marker.

• How to find it: Search your raw data file for rs601338. Raw files may report either DNA strand, so compare your genotype with both forward-strand and reverse-strand rows in the table.
• What each genotype means: See the table below for secretor status and norovirus expectations.
• Limits of the result: One working copy of FUT2 is enough to be a secretor. You only become a non-secretor when both copies carry a loss-of-function variant. rs601338 alone does not capture all non-secretor FUT2 genotypes, especially in non-European populations.

How secretor status changes your gut

The reason this matters for norovirus is mechanical, not metabolic. Norovirus, and especially the GII.4 genotype that causes many outbreaks, uses H antigen as a docking point in the gut. A secretor's gut mucus is rich in secreted H antigen, so the virus has plenty to grab. A non-secretor's gut mucus has little or no secreted H antigen, so GII.4 has fewer binding sites.

This is why non-secretor status is a strong resistance factor against GII.4 and some other norovirus strains. It is not immunity to stomach flu, and it does not protect you from every gastroenteritis pathogen.

FUT2 non-secretor status is a strong resistance factor against GII.4 norovirus and some other strains, but it is not immunity to stomach flu, and its B12 effect is not a diagnosis.

Norovirus is not the whole stomach flu story

The common mistake is to read 'norovirus resistance' as 'I will not get the stomach flu.' Norovirus is not influenza, and most people who say 'stomach flu' are describing a syndrome, not a specific virus. Rotavirus, adenovirus, astrovirus, bacterial food poisoning, Campylobacter, Salmonella and many other causes of vomiting and diarrhoea do not rely on FUT2 status.

Even within norovirus, protection is strongest against GII.4 and some other strains. Other norovirus genotypes can have different binding preferences, and some can infect non-secretors. The simple answer is still useful for many winter outbreaks, but it is not the whole answer for a vomiting illness.

Does FUT2 make my B12 high or low?

FUT2 is associated with B12 levels in population studies. The HealthOS report may show a high or low tendency for B12 based on your genotype, but that tendency is not a lab result. A genotype is not enough to turn a clearly deficient B12 into a normal value, or to make a normal value abnormally high. Do not skip a B12 blood test because your genotype says 'higher' or 'lower.' A lab value beats a genotype for your actual status.

What to do with this

If your raw file says you are a non-secretor, you have a genotype linked to strong resistance to GII.4 norovirus and some other strains. You should still wash your hands, you should still treat any vomiting illness as contagious, and you should not assume a high or low B12 reading is explained by this genotype. If you are a secretor, the finding is less exciting; it means your gut has the secreted H antigen that GII.4 norovirus uses. That is the normal, common state. It does not predict that you will be deficient in B12, and it does not call for a special supplement.

healthosx.com/dna

The download steps for 23andMe, on the page itself.

healthosx.com/dna

The short version: what is worth acting on, first.

If you want to see this in your own numbers, the free DNA tool at https://healthosx.com/dna reads a raw 23andMe, AncestryDNA, MyHeritage or annotated whole-genome VCF in the browser. The FUT2 trait and health story appears in the browser with nothing asked, and the free full report opens after a one-time code sent to your email. FUT2 appears under 'built-in resistance to the winter stomach bug' and 'whether your B12 tends to run high or low.' Whole-genome VCFs without rsID annotations cannot produce health findings, so FUT2 reporting requires an annotated VCF.