FADS1

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 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 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 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. 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. 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. 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. 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. 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.