FADS1

FADS1 rs174548 — The Omega-3 Bottleneck

Your ability to build the long-chain omega-3s that your brain, heart, and
immune system rely on is not just about what you eat — it depends on how
efficiently your body can convert short-chain fatty acids into their active
forms. FADS1 encodes delta-5 desaturase | The rate-limiting enzyme that
adds a double bond at the delta-5 position of the carbon chain, catalysing
the conversion of DGLA (an n-6 fatty acid) to arachidonic acid (AA) and
dihomo-gamma-linolenic acid to EPA in the omega-3 pathway, the enzyme
that sits at the critical bottleneck where plant-derived short-chain fatty
acids are elongated into the biologically potent long-chain PUFAs | Long-chain
polyunsaturated fatty acids (LC-PUFAs) include arachidonic acid (AA, C20:4n-6),
EPA (eicosapentaenoic acid, C20:5n-3), and DHA (docosahexaenoic acid, C22:6n-3)
— the forms actually used in cell membranes and signalling that your cells
actually use.

rs174548 is an intronic variant in FADS1 that influences how much of this enzyme
gets made. It sits in high linkage disequilibrium with a cluster of functionally
related FADS1 variants, making it one of the top GWAS signals for plasma PUFA
levels across multiple large studies. The G allele reduces FADS1 gene expression,
dampening delta-5 desaturase activity and impairing the conversion of dietary
omega-6 linoleic acid (LA) to arachidonic acid and dietary omega-3
alpha-linolenic acid (ALA) to EPA.

The Mechanism

In the n-6 pathway: LA → GLA → DGLA → [delta-5 desaturase] → AA. In the n-3
pathway: ALA → SDA → ETE → [delta-5 desaturase] → EPA. When delta-5 desaturase
activity is reduced, both pathways back up at the same step. The G allele at
rs174548 is associated with lower FADS1 mRNA expression in liver and blood
| Wang et al. Metabolome-wide association study identified the association between
a circulating polyunsaturated fatty acids variant rs174548 and lung cancer.
Carcinogenesis, 2017, translating
directly to measurable changes in fatty acid profiles: higher precursor levels
(LA, ALA, DGLA) and lower product levels (AA, EPA). This is not a rare mutation —
the G allele is carried by roughly 30% of Europeans and nearly 60% of Latino
populations, reflecting differential evolutionary selection pressure associated
with ancestral dietary patterns.

The Evidence

A genome-wide association study in the InCHIANTI cohort | Tanaka T et al.
Genome-wide association study of plasma polyunsaturated fatty acids in the
InCHIANTI Study. PLoS Genet,
2009 of 1,075 Italian adults
identified the FADS1 locus as the single strongest genetic determinant of plasma
PUFA levels, with the lead SNP explaining 18.6% of additive variance in
arachidonic acid — a remarkably large effect for a common variant. Replication
in 1,076 subjects from the GOLDN study confirmed the signal.

A CHARGE consortium meta-analysis | Smith CE et al. Dietary fatty acids modulate
associations between genetic variants and circulating fatty acids in plasma and
erythrocyte membranes: meta-analysis of nine studies. Mol Nutr Food Res,
2015 of 11,668 participants across
nine cohorts specifically examined rs174548 alongside rs174538 in FADS1. The
study found compartment-specific gene-diet interactions: dietary alpha-linolenic
and linoleic acid intake modified the genetic association with DHA and DPA in
plasma versus erythrocyte membranes, underscoring that the G allele's impact
depends partly on what you eat.

A metabolome-wide association study | Wang C et al. Metabolome-wide association
study identified the association between a circulating polyunsaturated fatty acids
variant rs174548 and lung cancer. Carcinogenesis,
2017 validated the rs174548–PUFA
link in 253 Chinese subjects (beta = −0.57, P = 1.68 × 10⁻³⁾ and additionally
showed the G allele associated with reduced FADS1 gene expression (beta = −0.84,
P = 6.49 × 10^−3). The same study found the G allele was associated with
reduced lung cancer risk (OR_meta = 0.87, P = 1.76 × 10⁻¹⁵ across 32,292
Europeans and Asians), suggesting PUFA pathway modulation has broader tissue-level
consequences.

Practical Actions

For GG homozygotes — roughly 9% of Europeans — the impairment in PUFA conversion
is substantial. Eating flaxseed, chia, or walnuts for their omega-3 content will
raise ALA in the blood but will not efficiently translate to EPA or DHA, the forms
that reduce cardiovascular risk and support brain function. These individuals benefit
most from direct preformed EPA/DHA supplementation from marine sources (fish oil or
algae). A supplementation trial | Meldrum SJ et al. Can polymorphisms in the fatty
acid desaturase gene cluster alter the effects of fish oil supplementation on plasma
and erythrocyte fatty acid profiles? Eur J Nutr,
2018 found minor homozygous carriers
of FADS1 cluster SNPs (including rs174548) showed significantly greater DHA
increases with fish oil supplementation than other genotypes — consistent with
greater baseline deficit and more room for improvement.

For CG heterozygotes, the conversion impairment is partial but meaningful, especially
for vegetarians, vegans, and those who rarely eat fish.

Interactions

rs174548 is in high linkage disequilibrium with rs174547, rs174546, and rs174537
within the FADS1–FADS2–FADS3 gene cluster on chromosome 11q12. These variants
often travel together as a haplotype, and GWAS signals for PUFA levels frequently
colocalize across this region. rs17606561 in the neighboring ELOVL2 gene (elongase
2, which extends EPA to DHA) interacts with FADS1 variants to further influence
DHA synthesis capacity — individuals with impaired function in both enzymes may have
particularly pronounced DHA deficiency.

Dietary context matters: the CHARGE consortium analysis showed gene-diet interactions,
with the G allele's impact on circulating DHA modified by dietary ALA intake. Even
poor converters can partially compensate through very high fish intake or direct
supplementation.