Lignan

Not to be confused with Lignin.

For the wine and table grape, see Lignan (grape).

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.

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

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.

File:Enterodiol and Enterlactone.svg

Food sources

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 fruit, 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.

Foodstuff	Pinoresinol	Syringaresinol	Sesamin	Lariciresinol	Secoisolariciresinol	Matairesinol	Hydroxymatairesinol
Flaxseed	871	48	not detected	1780	165759	529	35
Sesame seed	47136	205	62724	13060	240	1137	7209
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

Pharmacology

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.

See also

• Herbalism
• Pharmacognosy

• Nutrition
• Phytonutrients

References

General references

• IUPAC lignan nomenclature

External links

• Lignan content in food

1. REDIRECT Template:Phenylpropanoid