Does FADS1 rs174547 TT lower delta-5 desaturase activity and reduce long-chain PUFA synthesis?

The FADS1 gene encodes delta-5 desaturase (D5D), a critical rate-limiting enzyme in the pathway that converts short-chain polyunsaturated fatty acids (PUFAs) into biologically active long-chain PUFAs (LC-PUFAs). While genetic variation at the rs174547 locus profoundly impacts this biosynthetic pathway, clinical evidence directly refutes the claim that the TT genotype reduces enzymatic function.

Genotype-specific enzymatic activity

• The high-activity variant: Large-scale cohort evaluations and meta-analyses demonstrate that the FADS1 rs174547 TT genotype represents the high-activity reference variant.
• C-allele reduction: Conversely, the minor C allele (TC and CC genotypes) is the variant associated with reduced D5D activity, resulting in compromised enzymatic conversion.

Mechanistic and metabolic impacts

• Efficient precursor conversion: Because the D5D enzyme converts dihomo-gamma-linolenic acid (DGLA) to arachidonic acid (AA), individuals with the highly active TT genotype exhibit rapid conversion rates. This is marked by significantly higher AA-to-DGLA ratios.
• Distinct lipid profiles: In vivo lipid synthesis studies using stable isotope tracers show that TT homozygotes possess approximately double the biosynthetic capacity to convert precursor linoleic acid into AA compared to CC homozygotes. Consequently, TT carriers present with elevated tissue levels of downstream LC-PUFAs (like AA) and depleted upstream precursors (like DGLA).

Bottom line

• The claim is scientifically unsupported; the FADS1 rs174547 TT genotype is characterized by increased delta-5 desaturase activity and enhanced conversion of short-chain precursors into long-chain PUFAs, whereas the minor C allele is responsible for reduced synthesis.