Lignan

The lignans are a group of chemical compounds found in plants, particularly in flax seed. Lignans are one of the major classes of phytoestrogens, which are estrogen-like chemicals and also act as antioxidants. The other classes of phytoestrogens are the isoflavones, and coumestans. Plant lignans are polyphenolic substances derived from phenylalanine via dimerization of substituted cinnamic alcohols (see cinnamic acid), known as monolignols, to a dibenzylbutane skeleton 2. This reaction is catalysed by oxidative enzymes and is often controlled by dirigent proteins.

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Many natural products, known as phenylpropanoids, are built up of C6C3 units (a propylbenzene skeleton 1) derived from cinnamyl units just as terpene chemistry builds on isoprene units. Structure 3 is a neolignan.

Some examples of lignans are pinoresinol, podophyllotoxin, and steganacin.

When part of the human diet, some lignans are metabolized to form mammalian lignans known as enterodiol (1) and enterolactone (2) by intestinal bacteria. Lignans that can be metabolized to form mammalian lignans are pinoresinol, lariciresinol, secoisolariciresinol, matairesinol, hydroxymatairesinol, syringaresinol and sesamin.

Food sources

Flax seed and sesame seed are among the highest known sources of lignans. The principal lignan precursor found in flaxseed is secoisolariciresinol diglucoside. Other sources of lignans include cereals (rye, wheat, oat, barley), pumpkin seeds, soybeans, broccoli, beans, and some berries.

Secoisolariciresinol and matairesinol were the first plant lignans identified in foods. Pinoresinol and lariciresinol are more recently identified plant lignans that contribute substantially to the total dietary lignan intakes. Typically, Lariciresinol and pinoresinol contribute about 75% to the total lignan intake whereas secoisolariciresinol and matairesinol contribute only about 25%.^[1] This distribution may change as the contributions of syringaresinol and hydroxymatairesinol have not properly been quantified in foods.

Sources of lignans:^[2]

A recent study^[3] shows the complexity of mammalian lignan precursors in the diet. In the table below are a few examples of the 22 analyzed species and the 24 lignans identified in this study.

Mammalian lignan precursors as aglycones (µg / 100 g). Major compound(s) in bold.

References