FADS2 rs3834458 — The Deletion That Blocks Your Omega-3 Pipeline
In the omega-3 synthesis pathway, alpha-linolenic acid (ALA) from flaxseed and
walnuts must pass through a series of enzymatic steps before becoming EPA and DHA
— the forms that actively protect the cardiovascular system and brain. The very
first step is catalyzed by delta-6 desaturase | FADS2 (Fatty Acid Desaturase 2),
the enzyme that introduces a double bond at the sixth carbon position of both
ALA (omega-3 pathway) and linoleic acid (omega-6 pathway), enabling all downstream
elongation steps. rs3834458 is a
single-nucleotide deletion in intron 3 of FADS2 that reduces this enzyme's
activity — creating a bottleneck where ALA accumulates while EPA and DHA production
falls. With a SNPedia magnitude of 4.0 and confirmed effects across a meta-analysis
of 7 trials, this is one of the more clinically significant variants in the FADS
gene cluster.
The Mechanism
The rs3834458 deletion removes a single thymine nucleotide from intron 3 of FADS2
at GRCh38 position chr11:61,827,449. Although it sits within an intron and does
not directly alter the protein sequence, intronic variants in the FADS cluster
frequently influence splicing enhancers and regulatory elements | Intronic variants
in FADS1/FADS2 alter transcription factor binding, enhancer activity, and
splice-site usage, as demonstrated for multiple FADS cluster variants in functional
studies. The deletion allele is in
high linkage disequilibrium with rs1535 (r²=0.96) and forms part of the ancestral
FADS haplotype associated with reduced delta-6 desaturase output.
Reduced FADS2 activity creates a functional impairment in both major unsaturated
fatty acid pathways simultaneously. In the omega-3 pathway: ALA → [delta-6
desaturase step] → stearidonic acid → eicosatrienoic acid → EPA. In the
omega-6 pathway: linoleic acid → [delta-6 desaturase step] → gamma-linolenic
acid (GLA) → DGLA → arachidonic acid. When this enzymatic step slows, ALA and
linoleic acid accumulate upstream while all downstream products — GLA, EPA, DHA,
and arachidonic acid — are produced in smaller quantities.
The Evidence
A meta-analysis of 7 trials | Chen et al. 2019, Prostaglandins Leukotrienes
Essential Fatty Acids directly
examined rs3834458 effects on n-3 LC-PUFA levels. Deletion carriers (-T and --)
had significantly higher circulating ALA (p<0.00001), lower EPA (p<0.00001), lower
docosapentaenoic acid (DPA, p=0.005), and lower DHA (p<0.00001) compared to TT
homozygotes. The meta-analysis concluded that the minor allele "may result in
lower activity of delta-6 desaturase leading to higher ALA and lower EPA, DPA
and DHA in blood" — a direct experimental confirmation of the pathway model.
Downstream effects extend to tissue-level fatty acid status. Cord blood analyses
| Conway et al. 2021, British Journal of Nutrition
found that minor allele homozygosity was associated with lower cord blood AA
(β=0.075, p=0.037) and reduced AA:linoleic acid ratio, indicating that the
enzymatic bottleneck is visible not just in maternal plasma but in fetal
circulation. Maternal minor allele homozygosity also associated with lower
cord blood DHA and sum of EPA+DHA, demonstrating that this variant shapes
fetal LC-PUFA exposure despite high dietary fish intake in the study population.
Breast milk PUFA composition is similarly affected. In 256 Chinese lactating
mothers | Ding et al. 2016, Prostaglandins Leukotrienes Essential Fatty Acids, a 2-locus haplotype including
rs3834458 was significantly associated with lower GLA and arachidonic acid in
breast milk, confirming that the omega-6 arm of FADS2 activity is impaired
alongside the omega-3 arm.
The cardiovascular implications cut in an unexpected direction. A large-scale
sequencing study | Shi et al. 2022, Journal of the American Heart Association of ischemic stroke in Han Chinese
found that each minor allele across correlated variants at the MYRF-FADS1-FADS2
locus (including rs3834458) conferred an OR of 0.83 (95%CI 0.78–0.88) for
decreased stroke risk. The authors concluded that "genetically elevated
polyunsaturated fatty acids may decrease ischemic stroke risk in East Asians"
— a paradox explained by the fact that the FADS cluster haplotype structure
means reduced desaturase activity increases precursor PUFAs (ALA, LA) which
can themselves modulate inflammation, while simultaneously altering eicosanoid
profiles. This association does not negate the functional EPA/DHA deficit but
highlights the complexity of how FADS genetics maps to clinical outcomes.
Practical Actions
The core clinical consequence for deletion carriers is a reduced ability to
convert dietary ALA into EPA and DHA. This is particularly significant for:
- People who rely primarily on plant-based omega-3 sources (flaxseed, chia, walnuts)
- Pregnant and lactating women, where fetal and infant DHA depend substantially on
maternal conversion capacity
- Infants whose DHA status is shaped by both maternal genetics and breast milk
fatty acid composition
The therapeutic response is straightforward: supplementing with preformed EPA and
DHA from marine or algae-based sources bypasses the impaired FADS2 conversion step
entirely. The meta-analysis results confirm that the deficit is real and measurable
in blood; the solution is to provide the downstream products directly rather than
relying on the impaired enzyme to synthesize them.
For -- homozygotes (two deletion copies), both the omega-3 and omega-6 arms are
substantially impaired. Consider monitoring GLA through supplementation (evening
primrose or borage oil) if inflammation markers are elevated, as arachidonic
acid synthesis is also reduced.
Interactions
rs3834458 is in very high linkage disequilibrium with rs1535 (r²=0.98 in
European populations per Harsløf et al. 2013), meaning these variants nearly
always co-occur and tag the same underlying FADS2 expression phenotype. The
variant is also part of the broader FADS cluster haplotype that includes rs174568,
rs174575, rs99780, and rs174553 on chromosome 11q12.2.
The FADS1 variant rs174537 (also on the platform) encodes delta-5 desaturase —
the enzyme that acts after FADS2 in the omega-3 pathway (stearidonic acid →
EPA involves FADS2 first, then ELOVL for elongation, then FADS1). A user carrying
both the rs3834458 deletion and FADS1 risk alleles faces impairment at two
sequential steps in the ALA→EPA→DHA pathway, compounding the EPA/DHA deficit.
ELOVL2 (rs17606561), the elongase enzyme that converts EPA to DHA, forms a third
sequential block. A user with impaired FADS2 (rs3834458), impaired FADS1, and
impaired ELOVL2 has virtually zero endogenous DHA synthesis capacity and depends
entirely on preformed DHA from diet or supplements.
Alla genotyper
Two T alleles — normal FADS2 delta-6 desaturase activity
You carry two copies of the reference T allele at rs3834458. This is the common genotype associated with normal FADS2 (delta-6 desaturase) activity, meaning your body converts plant-based omega-3 precursors (ALA) through to EPA and DHA at the population-typical rate. About 51% of people globally share this genotype; in European populations it is approximately 42% (given a deletion allele frequency of ~34.5% among Europeans). With normal FADS2 function, dietary ALA from flaxseed, chia, and walnuts can contribute meaningfully to your EPA status, though marine sources remain the most efficient route to adequate EPA and DHA.
One deletion allele — moderately reduced FADS2 activity with lower EPA and DHA synthesis
You carry one copy of the deletion allele at rs3834458. This heterozygous state partially reduces FADS2 (delta-6 desaturase) activity — the enzyme responsible for the first committed step in converting plant-based ALA to EPA and linoleic acid to GLA and ultimately arachidonic acid. The meta-analysis by Chen et al. (2019) grouped -T and -- carriers together and found collectively lower circulating EPA, DPA, and DHA and higher ALA compared to TT homozygotes (all p<0.00001), with the effect scaling with allele dose. About 37% of people globally carry one deletion allele. In European populations, the deletion allele frequency is approximately 34.5%, making the -T genotype common. Plant-based omega-3 sources such as flaxseed and chia contribute less to your EPA and DHA status than they would for TT individuals — the delta-6 desaturation step is a rate-limiting bottleneck for this conversion.
Two deletion alleles — substantially impaired FADS2 activity with blocked omega-3 and omega-6 conversion
You carry two copies of the deletion allele at rs3834458, the homozygous state associated with the most substantially reduced FADS2 (delta-6 desaturase) activity. The meta-analysis by Chen et al. (2019) showed that deletion carriers (-T and -- combined) have significantly higher circulating ALA (p<0.00001), lower EPA (p<0.00001), lower DPA (p=0.005), and lower DHA (p<0.00001) versus TT homozygotes — with -- homozygotes representing the most impaired end of this spectrum. About 12% of people globally carry two deletion alleles. In East Asian populations the deletion allele is more common (~55%), making the -- genotype substantially more prevalent in these populations (~30%). Your body's conversion of dietary ALA to EPA is substantially impaired, and the omega-6 pathway is similarly affected — GLA, DGLA, and arachidonic acid are all produced at reduced rates from dietary linoleic acid. Relying on plant-based omega-3 sources is insufficient to maintain EPA and DHA at optimal tissue levels for this genotype.