Secoisolariciresinol diglycoside, or SDG, is a plant lignan most notably found in flaxseed (linseed). SDG is classified as a phytoestrogen since it is a plant-derived, nonsteroid compound that possesses estrogen-like activity. SDG has weak estrogenic activity. The level of SDG in flaxseed typically varies between 0.6% and 1.8%.
Lignans are one of the two major classes of phytoestrogens; the other class is the isoflavones. Plant lignans are polyphenolic substances derived from phenylalanine via dimerization of substituted cinnamic alcohols. Mammalian lignans are lignans derived from plant lignans. For example, following ingestion, SDG is converted to the aglycone secoisolariciresinol, which is then metabolized to the mammalian lignans enterolactone and enterodiol. Most of the effects of oral SDG are mediated by enterolactone and enterodiol.