FADS1

FADS1 rs174553 — When Your Fatty Acid Factory Runs Slow

Your body cannot synthesize omega-3 and omega-6 fats from scratch. It starts with
short-chain precursors — alpha-linolenic acid (ALA) from flaxseed and walnuts for
omega-3, linoleic acid (LA) from vegetable oils for omega-6 — and extends them
through a series of enzymatic steps into the longer-chain forms that actually drive
biology: EPA, DHA, and arachidonic acid (AA). The rate-limiting enzyme in this
pipeline is FADS1 | Fatty acid desaturase 1, also called delta-5 desaturase (D5D);
catalyzes the final desaturation step converting DGLA → AA in the omega-6 pathway
and ETA → EPA in the omega-3 pathway.
rs174553 is an intronic variant in the FADS1 gene cluster that alters how efficiently
this enzyme operates — and the difference shows up directly in your blood.

The Mechanism

As an intronic variant | a DNA change within a non-coding intron; can affect gene
expression through regulatory elements, splicing signals, or mRNA stability without
altering the protein sequence directly, rs174553 does not change the FADS1 protein.
Instead, the G allele sits in a regulatory region of the FADS1 locus that is part of
a tight haplotype block across the FADS1-FADS2 cluster on chromosome 11. Multiple
studies confirm that carrying the G allele — or the broader minor-allele haplotype
it tags — is associated with lower FADS1 desaturase activity across both the omega-6
and omega-3 pathways.

Practically, this means:
- DGLA → arachidonic acid (AA): reduced efficiency, lower circulating AA
- ETA → EPA: reduced efficiency, lower endogenous EPA from dietary ALA precursors
- Precursor accumulation: higher dihomo-gamma-linolenic acid (DGLA) and linoleic
acid (LA), which build up when the desaturation step is slow

The A allele, which corresponds to the GRCh38 plus-strand reference at this position,
is associated with normal or higher FADS1 activity.

The Evidence

The most rigorous evidence comes from a
study of 224 individuals in a geographically isolated founder population | Mathias
et al. FADS genetic variants and omega-6 polyunsaturated fatty acid metabolism in a
homogeneous island population. J Lipid Res, 2010.
Across 16 FADS cluster SNPs tested against 22 fatty acids, rs174553 G allele carriers
showed consistently lower omega-6 long-chain PUFAs. The FADS1 omega-6 activity ratio
(AA/DGLA) showed the strongest association of any fatty acid measure
(p = 2.11×10⁻¹³ to 1.8×10⁻²⁰) — an exceptionally strong signal for a common
intronic variant.

In pregnant and lactating women, Xie and Innis demonstrated | Xie L, Innis SM.
Genetic variants of the FADS1 FADS2 gene cluster are associated with altered (n-6)
and (n-3) essential fatty acids in plasma and erythrocyte phospholipids in women
during pregnancy and in breast milk during lactation. J Nutr, 2008
that GG homozygotes had lower arachidonic acid, lower EPA, and higher linoleic acid
in both plasma phospholipids and red blood cell membranes — confirming the
conversion impairment extends into tissue-level fatty acid composition, not
just circulating levels.

A key clinical implication emerged from an
infant RCT with 276 participants | Meldrum et al. Can polymorphisms in the FADS
gene cluster alter the effects of fish oil supplementation on plasma and erythrocyte
fatty acid profiles? Eur J Nutr, 2018
where minor G allele homozygotes across FADS1 SNPs, including rs174553, showed
significantly higher DHA levels after fish oil supplementation than other genotypes.
This counterintuitive finding — poor converters respond better to preformed DHA —
reflects that when endogenous conversion is impaired, exogenous EPA and DHA are
incorporated more efficiently because there is less competition from endogenously
synthesized product.

Population frequencies vary substantially: the G allele is rare in African populations
(~8%) but common in East Asian (~55%) and Latino (~49%) populations, suggesting
population-specific dietary adaptations to traditional marine food sources in some
ancestry groups.

Practical Actions

For G allele carriers, the core implication is that relying on plant-based omega-3
sources (ALA from flaxseed, chia, walnuts) is insufficient to maintain adequate EPA
and DHA levels, because the conversion machinery is slower. GG homozygotes in
particular should obtain preformed EPA and DHA directly from marine or algae-based
sources, bypassing the impaired desaturation step. Target 2–4 g combined EPA+DHA
daily for GG carriers; 1–2 g for AG heterozygotes. Algae-based DHA/EPA supplements
are equally effective and suitable for vegetarians.

On the omega-6 side, lower FADS1 activity means less AA is produced — which reduces
the substrate for pro-inflammatory eicosanoids. GG carriers may therefore have
a lower baseline inflammatory tone from the omega-6 pathway, but this comes at the
cost of the reduced EPA and DHA synthesis described above.

Interactions

rs174553 is in high linkage disequilibrium with rs174537, rs174547, and rs174546
in the same FADS1 haplotype block. These variants co-segregate, and carrying the
minor haplotype across multiple positions compounds the reduction in desaturase
activity. The nearby FADS2 gene (encoding delta-6 desaturase, which acts upstream
of FADS1) also contains functionally relevant variants — combined FADS1+FADS2
impairment more severely restricts the full ALA → EPA → DHA conversion pathway
than either gene alone.

The ELOVL2 gene (rs953413, rs2397142) encodes the elongase enzyme that operates
between FADS steps; ELOVL2 variants that reduce elongase efficiency interact with
FADS1 impairment to further reduce DHA synthesis capacity.