In vitro activity: Sesamin (Fagarol) is a lignan isolated from the bark of Fagara plants and from sesame oil. It has been used as a dietary fat-reduction supplement. Sesamin truly increases fat metabolism. Sesamin specifically inhibits delta 5 desaturase at low concentrations in polyunsaturated fatty acid biosynthesis in both microorganisms and animals, but does not inhibit delta 6, delta 9 and delta 12 desaturases. Dietary sesamin greatly increases the hepatic activity of fatty acid oxidation enzymes, including carnitine palmitoyltransferase, acyl-CoA dehydrogenase, acyl-CoA oxidase, 3-hydroxyacyl-CoA dehydrogenase, enoyl-CoA hydratase, and 3-ketoacyl-CoA thiolase. Dietary sesamin also increases the activity of 2,4-dienoyl-CoA reductase and delta3,delta2-enoyl-CoA isomerase, enzymes involved in the auxiliary pathway for beta-oxidation of unsaturated fatty acids dose-dependently. Sesamin is also a novel inhibitor of pregnane X receptor (PXR) and may be useful for modulating CYP3A4 and drug efficacies.