The lignans are a group of chemical compounds found in plants. 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.
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, a structure formed by joining the two propylbenzene residues at other than the β-carbon atom of the propyl side chain.
When a part of the human diet, some plant lignans are metabolized by intestinal bacteria to mammalian lignans enterodiol (1) and enterolactone (2).  Lignans that can be metabolized to mammalian lignans are pinoresinol, lariciresinol, secoisolariciresinol, matairesinol, hydroxymatairesinol, syringaresinol and sesamin. 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 isoflavones and coumestans.
Plant lignans are co-passengers of dietary fiber, and therefore fiber-rich food items are often good sources of lignans. Flax seed and sesame seed contain higher levels of lignans than most other foods. The principal lignan precursor found in flaxseed is secoisolariciresinol diglucoside. Other sources of lignans include cereals (rye, wheat, oat and barley - rye being the richest source), soybeans, cruciferous vegetables such as broccoli and cabbage, and some fruits, particularly apricots and strawberries.
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%. This distribution may change as the contributions of syringaresinol and hydroxymatairesinol have not properly been quantified in foods.
Sources of lignans:
|Source||Amount per 100 g|
|Flaxseed||300,000 µg (0.3 g)|
|Sesame seed||29,000 µg (29 mg)|
|Brassica vegetables||185 - 2321 µg|
|Grains||7 - 764 µg|
|Red wine||91 µg|
A recent study 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.
|Rye bran||1547||3540||not detected||1503||462||729||1017|
|Wheat bran||138||882||not detected||672||868||410||2787|
|Oat bran||567||297||not detected||766||90||440||712|
|Barley bran||71||140||not detected||133||42||42||541|
Lignans serve an antioxidant role in the plant's defenses against biotic and abiotic factors, and have shown anti-inflammatory and antioxidant activity in basic research models of human diseases.
Lignans may also have anticarcinogenic activities. Some epidemiological studies have shown that lignan exposure associates with lower risk of breast cancer.  Experimental studies have confirmed that enterolactone, the intestinal metabolite of many dietary plant lignans, has anti-carcinogenic activity in experimental models of breast cancer.   
- neolignane at en.wiktionary
- Heinonen, S; Nurmi, T; Liukkonen, K; Poutanen, K; Wähälä, K; Deyama, T; Nishibe, S; Adlercreutz, H (2001). "In vitro metabolism of plant lignans: New precursors of mammalian lignans enterolactone and enterodiol". Journal of agricultural and food chemistry 49 (7): 3178–86. PMID 11453749.
- Axelson, M; Sjövall, J; Gustafsson, B. E.; Setchell, K. D. (1982). "Origin of lignans in mammals and identification of a precursor from plants". Nature 298 (5875): 659–60. doi:10.1038/298659a0. PMID 6285206.
- Borriello, S. P.; Setchell, K. D.; Axelson, M; Lawson, A. M. (1985). "Production and metabolism of lignans by the human faecal flora". The Journal of applied bacteriology 58 (1): 37–43. PMID 2984153.
- Linus Pauling Institute at Oregon State University
- Milder IE, Arts IC, van de Putte B, Venema DP, Hollman PC (2005). "Lignan contents of Dutch plant foods: a database including lariciresinol, pinoresinol, secoisolariciresinol and matairesinol". Br. J. Nutr. 93 (3): 393–402. doi:10.1079/BJN20051371. PMID 15877880.
- Smeds AI; Eklund, Patrik C.; Sjöholm, Rainer E.; Willför, Stefan M.; Nishibe, Sansei; Deyama, Takeshi; Holmbom, Bjarne R. et al. (2007). "Quantification of a Broad Spectrum of Lignans in Cereals, Oilseeds, and Nuts". J. Agric. Food Chem. 55 (4): 1337–1346. doi:10.1021/jf0629134. PMID 17261017.
- Korkina, L; Kostyuk, V; De Luca, C; Pastore, S (2011). "Plant phenylpropanoids as emerging anti-inflammatory agents". Mini reviews in medicinal chemistry 11 (10): 823–35. doi:10.2174/138955711796575489. PMID 21762105.
- Korkina, L. G. (2007). "Phenylpropanoids as naturally occurring antioxidants: From plant defense to human health". Cellular and molecular biology (Noisy-le-Grand, France) 53 (1): 15–25. PMID 17519109.
- Boccardo, F; Puntoni, M; Guglielmini, P; Rubagotti, A (2006). "Enterolactone as a risk factor for breast cancer: A review of the published evidence". Clinica chimica acta; international journal of clinical chemistry 365 (1–2): 58–67. doi:10.1016/j.cca.2005.07.026. PMID 16168401.
- Adlercreutz, H (2007). "Lignans and human health". Critical reviews in clinical laboratory sciences 44 (5–6): 483–525. doi:10.1080/10408360701612942. PMID 17943494.
- Saarinen, N. M.; Huovinen, R; Wärri, A; Mäkelä, S. I.; Valentín-Blasini, L; Sjöholm, R; Ammälä, J; Lehtilä, R; Eckerman, C; Collan, Y. U.; Santti, R. S. (2002). "Enterolactone inhibits the growth of 7,12-dimethylbenz(a)anthracene-induced mammary carcinomas in the rat". Molecular cancer therapeutics 1 (10): 869–76. PMID 12492120.
- Bergman Jungeström, M; Thompson, L. U.; Dabrosin, C (2007). "Flaxseed and its lignans inhibit estradiol-induced growth, angiogenesis, and secretion of vascular endothelial growth factor in human breast cancer xenografts in vivo". Clinical cancer research : an official journal of the American Association for Cancer Research 13 (3): 1061–7. doi:10.1158/1078-0432.CCR-06-1651. PMID 17289903.
- Lindahl, G; Saarinen, N; Abrahamsson, A; Dabrosin, C (2011). "Tamoxifen, flaxseed, and the lignan enterolactone increase stroma- and cancer cell-derived IL-1Ra and decrease tumor angiogenesis in estrogen-dependent breast cancer". Cancer research 71 (1): 51–60. doi:10.1158/0008-5472.CAN-10-2289. PMID 21097717.