rs174561 — FADS1

Intronic variant in the FADS1 gene cluster that tags a haplotype block controlling delta-5 desaturase activity; the C allele reduces conversion of omega-6 and omega-3 precursors to long-chain PUFAs (AA, EPA) and also increases miR-1908-5p expression, independently lowering LDL cholesterol

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FADS1 rs174561 — A Fatty Acid Bottleneck With a Cardiovascular Twist

The FADS gene cluster on chromosome 11 encodes the two rate-limiting enzymes of long-chain PUFA synthesis11 long-chain PUFA synthesis
polyunsaturated fatty acids (PUFAs) are essential fats. The short-chain forms (ALA, LA) come from plants; the long-chain forms (EPA, DHA, AA) are what the brain, heart, and immune system actually use. The FADS enzymes convert short-chain to long-chain forms. rs174561 is an intronic variant within FADS1 that tags a haplotype block governing delta-5 desaturase22 delta-5 desaturase
the enzyme encoded by FADS1 that catalyses a key step in converting fatty acid precursors: DGLA→AA in the omega-6 pathway and ETA→EPA in the omega-3 pathway activity. The C allele, which occurs in about 7% of Africans, 31% of Europeans, and over 50% of East Asians, is consistently associated with reduced enzymatic throughput — and carries an unexpected protective side effect: it independently lowers circulating LDL cholesterol through a separate microRNA pathway.

The Mechanism

rs174561 does not alter the FADS1 protein sequence (it sits in an intron), but it tags a haplotype block in strong linkage disequilibrium with regulatory variants that control FADS1 and FADS2 transcription. C allele carriers produce less delta-5 desaturase, slowing the final steps of long-chain PUFA synthesis in both the omega-6 and omega-3 pathways:

  • Omega-6 arm: DGLA (dihomo-gamma-linolenic acid) accumulates; less converts to arachidonic acid (AA), the precursor to inflammatory prostaglandins.
  • Omega-3 arm: ETA (eicosatetraenoic acid) is less efficiently converted to EPA; alpha-linolenic acid (ALA) from plant foods accumulates rather than flowing through to EPA and DHA.

A separate and distinct mechanism operates in parallel. rs174561 acts as a strong microRNA quantitative trait locus33 microRNA quantitative trait locus
a variant that controls how much of a specific microRNA is produced in a tissue for miR-1908-5p: the C allele significantly increases miR-1908-5p abundance. This microRNA suppresses TGFB1 expression, which reduces BMP1 protein levels, which in turn decreases proteolytic cleavage of the LDL receptor — leaving more receptors intact on cell surfaces to clear LDL particles from the blood. The net effect: C allele carriers tend to have lower circulating LDL cholesterol, lower fasting glucose, and lower HbA1c — benefits that are mechanistically independent of the desaturase activity reduction.

The Evidence

Mathias et al. 201044 Mathias et al. 2010
Mathias RA et al. FADS genetic variants and omega-6 polyunsaturated fatty acid metabolism in a homogeneous island population. Journal of Lipid Research, 2010 studied 224 individuals from the Tangier island population and found rs174561 minor allele carriers consistently had lower omega-6 PUFAs including AA (p = 5.8×10⁻⁷ to 1.7×10⁻⁸) and substantially reduced FADS1 enzymatic activity (p = 2.11×10⁻¹³ to 1.8×10⁻²⁰). The exception was DGLA, which increased in minor allele carriers — consistent with substrate accumulation upstream of the blocked desaturation step.

Muzsik et al. 201855 Muzsik et al. 2018
Muzsik A et al. Associations between Fatty Acid Intake and Status, Desaturase Activities, and FADS Gene Polymorphism in Centrally Obese Postmenopausal Polish Women. Nutrients, 2018 measured red blood cell fatty acid composition in 128 women. Minor C allele carriers at rs174561 had 9.7% lower AA concentrations and 15–18% reduced delta-5 desaturase activity compared to TT homozygotes.

Al-Hilal et al. 201366 Al-Hilal et al. 2013
Al-Hilal M et al. Genetic variation at the FADS1-FADS2 gene locus influences delta-5 desaturase activity and LC-PUFA proportions after fish oil supplement. Journal of Lipid Research, 2013 examined 310 healthy subjects in the MARINA study and confirmed rs174561 minor allele association with reduced D5D activity (p = 4.5×10⁻¹⁸) and lower LC-PUFA proportions. Crucially, fish oil supplementation significantly increased D5D activity (p = 4.0×10⁻⁹) and improved PUFA profiles — demonstrating that preformed EPA/DHA supplementation can partly compensate for the reduced synthesis capacity.

Beehler et al. 202177 Beehler et al. 2021
Beehler K et al. A Common Polymorphism in the FADS1 Locus Links miR1908 to Low-Density Lipoprotein Cholesterol Through BMP1. Arteriosclerosis, Thrombosis, and Vascular Biology, 2021 established rs174561 as a strong miR-1908-5p QTL, with the C allele associating with higher miR-1908-5p, lower LDL-cholesterol, lower fasting glucose, and lower HbA1c. This LDL-lowering effect is independent of the PUFA conversion pathway.

Conway et al. 202188 Conway et al. 2021
Conway MC et al. Maternal and child fatty acid desaturase genotype as determinants of cord blood long-chain PUFA concentrations in the Seychelles Child Development Study. British Journal of Nutrition, 2021 demonstrated in 1,088 infants that rs174561 minor allele carrier mothers had lower cord blood AA:LA ratio and lower fetal LCPUFA concentrations — showing the impaired conversion is relevant even during fetal development.

Practical Actions

For C allele carriers (TC and CC), the impaired conversion of ALA to EPA means plant-based omega-3 sources (flaxseed, chia, walnuts) cannot reliably supply adequate EPA. Direct supplementation with preformed EPA and DHA from marine or algae sources bypasses the blocked desaturation step entirely. CC homozygotes show the strongest impairment and benefit most from higher doses (2–4 g EPA+DHA daily); TC heterozygotes typically need 1–2 g daily.

The miRNA-driven LDL benefit in C allele carriers is a passive biological effect requiring no additional action — but it does mean the net metabolic picture for C carriers is mixed: impaired PUFA synthesis coupled with modestly better LDL handling. Monitoring omega-3 index (RBC EPA+DHA%) is more useful than LDL alone for tracking whether supplementation is adequate.

For TT homozygotes (the most common genotype globally), desaturase activity is normal — but this higher enzymatic throughput also drives more AA production from dietary omega-6 precursors. Balancing the omega-6:omega-3 dietary ratio remains important to avoid excess AA-derived inflammatory signalling.

Interactions

rs174561 is in strong linkage disequilibrium (r² > 0.7) with rs174537, rs174547, rs174575, and rs3834458 within the FADS1 haplotype block. These variants co-segregate and amplify one another's effects on desaturase activity. Carriers who also have the low-activity alleles at multiple FADS1 cluster SNPs may have more pronounced impairment of PUFA synthesis than any single SNP predicts.

Dietary omega-6 intake modifies the impact of this variant: high linoleic acid intake (from seed oils) coupled with normal TT desaturase activity drives substantial AA production; in CC carriers, the same diet produces less AA but also less EPA — a different risk profile that specifically requires marine omega-3 compensation rather than omega-6 reduction.

Nutrient Interactions

alpha-linolenic acid (ALA) impaired_conversion
arachidonic acid (AA) altered_metabolism
EPA increased_need
DHA increased_need

Genotype Interpretations

What each possible genotype means for this variant.

TT Full Desaturase Activity Normal

Normal delta-5 desaturase activity — efficient PUFA conversion

The TT genotype is associated with full delta-5 desaturase throughput at this haplotype block. Your cells efficiently convert DGLA to arachidonic acid (AA) in the omega-6 pathway, and ETA to EPA in the omega-3 pathway. Baseline circulating AA and EPA levels will track with your dietary intake rather than being significantly bottlenecked at the enzymatic step.

Note that higher desaturase activity also means dietary omega-6 oils (linoleic acid from sunflower, corn, soybean oil) convert more efficiently to AA — the precursor to pro-inflammatory eicosanoids. The TT genotype carries modestly higher LDL compared to C allele carriers, since the miR-1908-5p LDL-lowering effect is absent here.

The other FADS1 cluster SNPs (rs174537, rs174547) are in partial LD with rs174561. Your full PUFA picture requires examining those variants as well.

CT Reduced Desaturase Activity Intermediate

One C allele — moderately reduced conversion of fatty acid precursors to EPA and AA

With one C allele, your FADS1 enzymatic throughput is reduced at this haplotype. The reduction is intermediate: you convert DGLA to AA and ETA to EPA less efficiently than TT individuals, but not as severely as CC homozygotes. Plasma ALA tends to be modestly elevated while EPA and AA are somewhat lower.

The miR-1908-5p effect is partially active in CT heterozygotes — you carry one copy of the allele that increases miR-1908-5p, providing a partial LDL benefit compared to TT homozygotes. This is a passive effect from the variant and does not require supplementation to activate.

Plant-based omega-3 sources (flaxseed, chia, walnuts) will supply ALA, but the conversion rate to EPA is reduced. Relying on plant sources alone is not ideal for maintaining adequate EPA status with this genotype.

CC Poor Desaturase Activity Poor Converter

Two C alleles — substantially impaired PUFA synthesis requiring preformed EPA and DHA

The CC genotype produces the lowest FADS1 throughput at this locus. Both major PUFA pathways are affected: DGLA accumulates upstream of the blocked omega-6 step (producing less AA), and ALA accumulates in plasma when dietary intake is high because it cannot efficiently proceed to EPA. This "ALA backing up" effect was directly observed in a study of pregnant women where CC homozygotes showed the highest plasma ALA concentrations at the highest ALA dietary intake — a functional marker of conversion failure.

Fish oil supplementation studies confirm that CC carriers can effectively utilize preformed EPA/DHA — they respond well to supplementation even though endogenous synthesis is impaired. This means the nutritional gap is addressable with targeted supplementation.

The miR-1908-5p LDL effect is maximal in CC carriers (two copies of the allele that elevates miR-1908-5p), providing the strongest passive LDL benefit in this group. However, this does not offset the PUFA synthesis deficit, which requires dietary or supplemental compensation.

Fetal development may be particularly sensitive: maternal CC genotype is associated with lower cord blood AA:LA ratios and lower fetal LCPUFA concentrations even in a high-fish-eating population.