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%.