This study investigates the mechanism by which dietary docosahexaenoic acid (DHA) downregulates its own synthesis in the liver. Using compound-specific isotope analysis (CSIA), the researchers found that the brain receives DHA synthesized from α-linolenic acid (ALA) when dietary DHA is low. They also showed that dietary DHA increases EPA levels in mouse liver and serum, which is dependent on ALA. However, the source of increased EPA is slowed EPA metabolism, not increased DHA retroconversion. DHA feeding alone or with ALA lowered liver elongation of very long-chain (ELOVL2) enzyme activity, despite no change in protein content. Enzyme competition assays suggested that DHA inhibits EPA elongation uncompetitively and noncompetitively, depending on DHA levels. In a human supplementation study, a SNP (rs953413) in the ELOVL2 gene was identified as a predictor of increased EPA levels following DHA supplementation. The findings identify a novel feedback inhibition pathway where dietary DHA downregulates its liver synthesis by inhibiting EPA elongation.This study investigates the mechanism by which dietary docosahexaenoic acid (DHA) downregulates its own synthesis in the liver. Using compound-specific isotope analysis (CSIA), the researchers found that the brain receives DHA synthesized from α-linolenic acid (ALA) when dietary DHA is low. They also showed that dietary DHA increases EPA levels in mouse liver and serum, which is dependent on ALA. However, the source of increased EPA is slowed EPA metabolism, not increased DHA retroconversion. DHA feeding alone or with ALA lowered liver elongation of very long-chain (ELOVL2) enzyme activity, despite no change in protein content. Enzyme competition assays suggested that DHA inhibits EPA elongation uncompetitively and noncompetitively, depending on DHA levels. In a human supplementation study, a SNP (rs953413) in the ELOVL2 gene was identified as a predictor of increased EPA levels following DHA supplementation. The findings identify a novel feedback inhibition pathway where dietary DHA downregulates its liver synthesis by inhibiting EPA elongation.