Origin of lignans in mammals and identification of a precursor from plants

Lignans are a class of compounds with a dibenzylbutane skeleton¹ which until recently had only been found in higher plants^2,3. The first lignans to be identified in humans and animals were trans-2,3-bis(3-hydroxybenzyl)-γ-butyrolactone (enterolactone)^4-6 and 2,3-bis(3-hydroxybenzyl)butane-1,4-diol (enterodiol)^4,5. These structures differ from those of plant lignans by having aromatic rings substituted only in the meta position. Enterolactone and enterodiol are excreted in the urine and bile, predominantly as glucuronide conjugates^7,8, and intestinal microflora is required for their formation^8,9. It is not known, however, whether they are synthesized de novo by microorganisms or are formed from dietary precursors. We report here evidence that these mammalian lignans are formed by microbial action on precursor lignans which are present as dietary constituents of plant origin. Precursors are found in seeds of different species, being particularly abundant in linseed. From this source, 2,3-bis(3-methoxy-4-hydroxybenzyl)butane-1,4-diol (secoisolariciresinol) has been identified as a glycoside.