Lignans are composed of two phenylpropanoid units joined together to form an 18-carbon skeleton. Many other functional groups can then be added by the plant to modify this base structure. Generally, these molecules are lipophilic and function within plant cell membranes to provide rigidity, strength, and water impermeability. Most lignans are relatively safe. Few generalizations can be made about this class of compounds beyond these statements, in part because of the lack of research. As interest in lignans grows, more information will surely become available.

Several lignans have demonstrated intriguing and important clinical activity. Podophyllotoxin from Podophyllum peltatum (mayapple) acts as a cathartic laxative that is distinct from the anthraquinone glycosides; it inhibits human papillomavirus (HPV) and is antineoplastic. The semisynthetic chemotherapy drugs teniposide and etoposide are derived directly from this molecule.

The lignans in Linum usitatissimum (flax), such as sec-oisolariciresinol, are transformed by the gut flora to enterodiol and enterolactone, known phytoestrogenic constituents that clearly are active in vivo. Flax seeds have definite anticancer effects, as has been documented in the results of clinical trials. Flaxseed oil (which is low in lignans) has not shown these same benefits as consistently; this supports the contention that the lignans are critical to these effects when flax seeds are consumed.

Lignins are larger phenylpropanoid polymers that are a component of dietary fiber and thus may provide some benefits in this form, as was discussed earlier under polysaccharides. Otherwise, they are considered relatively unimportant as botanical constituents (Select Lignan-Rich Herbs).

Select Lignan-Rich Herbs
Linum usitatissimum (flax)
Podophyllum peltatum (mayapple)
Schisandra chinensis (schisandra, wu wei zi)