rs174616 — FADS2 FADS2 Haplotype Block Variant

Intronic FADS2 variant tagging the FADS1/FADS2 haplotype block; A allele carriers have reduced delta-6 desaturase activity, impairing conversion of dietary plant-based omega-3 (ALA) to EPA/DHA and omega-6 (LA) to arachidonic acid

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FADS2 rs174616 — The Omega-3 Conversion Gatekeeper

The FADS2 gene on chromosome 11 encodes delta-6 desaturase11 delta-6 desaturase
The rate-limiting enzyme that performs the first desaturation step converting dietary short-chain fatty acids (plant-based ALA and LA) into longer, biologically active forms including EPA, DHA, and arachidonic acid, the enzyme that opens the gateway to all long-chain polyunsaturated fatty acid synthesis from plant precursors. rs174616 is an intronic FADS2 variant that tags a distinct linkage disequilibrium block within the FADS1/FADS2 cluster. Carriers of the A allele produce less delta-6 desaturase activity — the enzyme that catalyzes the first rate-limiting step in both omega-3 (ALA → EPA → DHA) and omega-6 (LA → arachidonic acid) synthesis.

The Mechanism

rs174616 lies within intron 7 of the FADS2 gene at chromosome 11 position 61,861,650 (GRCh38). Like other intronic variants across the FADS cluster, it acts through a regulatory rather than coding mechanism — influencing FADS2 expression levels, and in at least one study, through associated DNA methylation changes at a CpG site (cg07999042) in the FADS2 promoter region.

The A allele reduces delta-6 desaturase output, creating a characteristic substrate-accumulation, product-deficit pattern: the precursor linoleic acid (LA) and alpha-linolenic acid (ALA) build up, while the downstream products — gamma-linolenic acid (GLA)22 gamma-linolenic acid (GLA)
First omega-6 product after FADS2 acts on linoleic acid; further elongated to DGLA then arachidonic acid, dihomo-gamma-linolenic acid (DGLA)33 dihomo-gamma-linolenic acid (DGLA)
Omega-6 intermediate between GLA and arachidonic acid; anti-inflammatory precursor to series-1 prostaglandins, arachidonic acid (ARA), EPA, and DHA — are reduced. Because this is the first rate-limiting enzyme in the pathway, impairment ripples downstream to affect all long-chain PUFA products.

The Evidence

The foundational study for the FADS cluster is Schaeffer et al. 200644 Schaeffer et al. 2006
Schaeffer L et al. Common genetic variants of the FADS1 FADS2 gene cluster and their reconstructed haplotypes are associated with the fatty acid composition in phospholipids. Human Molecular Genetics, 2006, which genotyped 18 FADS1/FADS2 cluster SNPs including rs174616 in 727 German adults, demonstrating strong associations between FADS haplotypes and serum phospholipid fatty acid composition. The rs174616 A allele tagged a haplotype block independently associated with the substrate-accumulation, product-deficit PUFA signature.

Zec et al. 202055 Zec et al. 2020
Zec MM et al. FADS2 polymorphisms are associated with plasma arachidonic acid and estimated desaturase-5 activity in a cross-sectional study. Nutrition Research, 2020 found that the A allele was associated with lower plasma arachidonic acid and reduced estimated delta-5 desaturase activity, and identified a significant gene-diet interaction in which dietary carbohydrate percentage modified the effect on Δ6 desaturase activity.

In a case-control study of Han Chinese individuals66 case-control study of Han Chinese individuals
Yao et al. Polymorphisms of rs174616 in the FADS1-FADS2 gene cluster is associated with a reduced risk of type 2 diabetes mellitus in northern Han Chinese people. Diabetes Res Clin Pract, 2015 (618 cases, 618 controls), the A allele was associated with a decreased conversion rate of LA to arachidonic acid (AA/LA ratio) and a modestly reduced risk of type 2 diabetes, suggesting that lower arachidonic acid production — and the resulting reduction in pro-inflammatory eicosanoids — confers some metabolic benefit in high-carbohydrate dietary contexts.

A selection genetics analysis by Romero-Hidalgo et al. 202477 Romero-Hidalgo et al. 2024
Romero-Hidalgo S et al. Selection scan in Native Americans of Mexico identifies FADS2 rs174616: Evidence of gene-diet interactions affecting lipid levels and Delta-6-desaturase activity. Heliyon, 2024 found that rs174616 is the most highly differentiated FADS2 SNP across global populations, with the A allele approaching fixation (94%) in Native Americans of Mexico, occurring at ~47% in Europeans and only ~19% in East Asians. This extreme population differentiation and evidence of positive selection suggest the A allele conferred a metabolic advantage in populations subsisting on high-carbohydrate, low-marine-fat diets — contexts where lower arachidonic acid production may reduce inflammatory tone.

Walle et al. 201988 Walle et al. 2019
Walle P et al. Liver DNA methylation of FADS2 associates with FADS2 genotype. Clinical Epigenetics, 2019 identified a mechanistic link: the rs174616 genotype was significantly associated with DNA methylation at the FADS2 CpG site cg07999042 in liver tissue, with A allele carriers showing distinct methylation patterns, providing a plausible epigenetic mechanism for how this intronic variant regulates enzyme expression.

Practical Actions

The core implication of A allele carriage is consistent with the broader FADS2 picture: dietary plant-based omega-3 (from flax, chia, walnuts) supplies ALA that requires FADS2 to begin converting toward EPA and DHA. With reduced FADS2 activity, this first conversion step is rate-limited and less EPA/DHA reaches circulation, regardless of ALA intake. Preformed EPA and DHA from fatty fish, fish oil, or algae-based supplements bypass this bottleneck entirely.

The omega-6 side matters too: reduced arachidonic acid production from dietary LA changes the balance of eicosanoid production, favoring less inflammatory signaling. Limiting excess omega-6 from refined seed oils (soybean, corn, sunflower) reduces the LA load on an already constrained FADS2 enzyme, supporting a better omega-3:omega-6 balance even when EPA/DHA are in short supply.

For vegetarians and vegans, the implications are particularly pronounced: without marine food sources, the only pathway to adequate EPA/DHA is through algae-based supplements that provide these fatty acids in preformed bioavailable form.

Interactions

rs174616 is in linkage disequilibrium with other FADS2 variants (rs174575, rs1535) and with the FADS1 delta-5 desaturase variants (rs174547, rs174537). The FADS cluster forms two linked haplotype blocks: carriers of risk alleles at rs174616 often also carry risk alleles at other cluster variants, compounding the PUFA synthesis deficit. When both FADS2 (delta-6) and FADS1 (delta-5) variants are present, the upstream deficit from FADS2 reduces substrate available for FADS1, compounding the reduction in EPA and arachidonic acid. Any APOE or cardiovascular risk variants that increase inflammatory sensitivity make adequate omega-3 status even more critical.

Nutrient Interactions

alpha-linolenic acid (ALA) impaired_conversion
linoleic acid (LA) impaired_conversion
EPA increased_need
DHA increased_need
arachidonic acid (ARA) impaired_conversion

Genotype Interpretations

What each possible genotype means for this variant.

GG Normal Desaturase Activity Normal

Normal delta-6 desaturase activity — efficient omega-3 and omega-6 conversion

You carry two copies of the G reference allele at rs174616, associated with normal FADS2 delta-6 desaturase activity. Your body converts dietary plant-based omega-3 (ALA) and omega-6 (LA) precursors into their active long-chain forms — EPA, DHA, and arachidonic acid — with typical efficiency. About 31% of people of European descent share this genotype, though it is much more common (~64%) in people of East Asian descent and relatively rare (~6%) in Native American populations.

AG Intermediate Desaturase Activity Intermediate

Moderately reduced delta-6 desaturase activity — partial impairment of omega-3 and omega-6 conversion

The A allele reduces FADS2 expression through regulatory mechanisms that include altered DNA methylation at the FADS2 promoter region (cg07999042), slowing the first desaturation step in both the omega-3 (ALA → stearidonic acid → EPA → DHA) and omega-6 (LA → GLA → DGLA → arachidonic acid) pathways. With one A allele, the effect is intermediate — precursor fatty acids accumulate and downstream EPA, DHA, and arachidonic acid are moderately reduced. The Zec et al. 2020 study (PMID 33011673) identified that dietary carbohydrate intake modifies the expression of this variant's effect on Δ6 desaturase activity, suggesting that high-carbohydrate diets may amplify the functional impact of the A allele.

AA Poor Desaturase Activity Poor Converter

Substantially reduced delta-6 desaturase activity — plant omega-3 is unreliable for EPA/DHA

With two A alleles at rs174616, delta-6 desaturase activity is substantially reduced, creating a cascade deficit across both major PUFA pathways. In the omega-3 pathway, impaired ALA-to-stearidonic acid conversion reduces EPA and DHA yield. In the omega-6 pathway, less LA converts to GLA, reducing downstream DGLA and ultimately arachidonic acid. The Yao et al. 2015 study (PMID 25981324) found AA genotype carriers had significantly lower AA/LA ratios, confirming reduced flux through the delta-6 gating step.

The Romero-Hidalgo et al. 2024 genetic selection analysis (PMID 39166092) found that the A allele is under positive selection in populations with historically low marine fat intake (Native American, agricultural populations), suggesting that this reduced desaturase phenotype was adaptive in high-carbohydrate, low-fish dietary environments — possibly because lower arachidonic acid production reduces inflammatory eicosanoid tone.

However, in modern dietary contexts where the omega-6:omega-3 ratio from processed seed oils is already elevated (10:1 or higher), reduced FADS2 activity limits the ability to mount adequate anti-inflammatory EPA and DHA responses. The Walle et al. 2019 liver epigenetics study (PMID 30654845) indicates the mechanism involves altered DNA methylation at the FADS2 promoter, providing a durable regulatory suppression of enzyme expression.