Disregard of genetically-linked biological variation in the design and interpretation of clinical trials of marine omega-3 (n-3) highly unsaturated fatty acid (HUFA) supplementation and specifically limited inclusion of individuals carrying high-activity fatty-acid-desaturase (FADS1) alleles may have masked potential health benefits of individuals at elevated risk for cardiovascular and other chronic diseases. Our work over the past decade has revealed that genetic variation throughout the FADS gene cluster, and the resulting allele-frequency differences among population strata, are significant drivers of the effectiveness of n-3 HUFA supplementation. FADS1 are the rate-limiting steps in the conversion of dietary n-6 and n-3 polyunsaturated fatty acids (PUFA), linoleic acid (LA) and α-linolenic acid (ALA), respectively, to n-6 and n-3 HUFAs, including arachidonic acid (ARA), eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), and their biologically active oxylipin metabolites. Importantly, the FADS1/2 gene cluster variation was recently identified as the largest multimorbidity locus in the human genome. A central discovery from our work and others is the differential synthesis of HUFAs and metabolism to bioactive lipids across genotype-defined strata. Groups enriched for the high-activity FADS1 rs174537-GG allele exhibit efficient conversion of dietary LA to ARA and its pro-inflammatory/pro-thrombotic oxylipins. Evidence in support of this work comes from our genotype subgroup analysis of the VITAL n-3 HUFA supplementation trial affirming that carriers of the high-activity allele, but not lower-activity carriers, randomized to n-3 supplementation demonstrated an ~80 % reduction in myocardial infarction (MI). We then applied two machine-learning methods to matched VITAL cohorts and found strong evidence that n-3 supplementation significantly decreased MI only in high-activity allele carriers (odds ratio 0.178; 95 % CI 0.046–0.69). These data lead to this proposal’s central hypothesis that gene–diet interactions drive a pathogenic imbalance in ARA versus EPA and DHA and their oxylipin products, particularly in high-activity allele carriers. We postulate that n-3 HUFA supplementation will decrease the ratio of ARA to EPA and DHA, providing a major benefit to these individuals. We propose a two-site randomized, double-blind, placebo-controlled, crossover n-3 HUFA supplementation trial that leverages the varied allele-frequency distributions across individuals to test this hypothesis. Specifically, we will supplement participants stratified by FADS genotypes at rs174537 with EPA-enriched n-3 HUFAs and evaluate changes in the balance of HUFA-containing lipids and oxylipin products, as well as inflammatory and clinical biomarkers linked to chronic disease. This trial will provide critical mechanistic evidence for genotypelinked biological heterogeneity in differential biological response to n-3 HUFAs. Such mechanistic data are essential to the futu...