rs3834458 — FADS2

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 desaturase¹¹ 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 elements²² 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 trials³³ 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 analyses⁴⁴ 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 mothers⁵⁵ 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 study⁶⁶ 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.