Lignans are counted among the secondary plant compounds and thus, unlike fats, proteins, and carbohydrates, among the substances without nutritional value – “anutritive ingredients.”
Since lignans have estrogen-like effects, they, like isoflavones, are referred to as phytoestrogens (plant constituents with hormone-like effects). Lignans also have a sleep-promoting effect (“sleep glignans”). For example, lignans are found in valerian root.
Among the best-known lignans are:
- Matairesinol
- Secoisolariciresinol
- Sesamin
- Sesaminol
Lignans serve the plant as structural components as well as defense substances. In addition, they control plant growth.
Humans absorb lignans mainly in the context of a diet rich in cereals and fiber. Flaxseed contains particularly high levels of lignans, but so do other oleaginous fruits such as sunflowers (sunflower seeds), sesame seeds, peanuts and olives. Other good sources of lignans are wheat bran, rye flour, buckwheat flour, soybean flour and oat flour. Fresh vegetables such as carrots, broccoli, fennel, onions and garlic also have relatively high levels of lignans. Furthermore, fruits such as cherries, apples, pears, peaches and plums contain lignans.
After ingestion, lignans are broken down by intestinal bacteria and their structure is thus altered. The metabolites (intermediate stage or degradation product) are bioactive. Thus, the precursors matairesinol and secoisolariciresinol, contained in flaxseed, are transformed into the estrogen-active enterolignans enterodiol and enterolactone. After absorption (uptake), they are conjugated with glucuronic acid in the liver, making them water-soluble and excreted in the urine.
Due to their estrogen-like structure, lignans, like isoflavones, bind to estrogen receptors (alpha and beta receptors) and therefore act as SERMs (estrogen receptor modulators). However, they have only 0.1-0.01% of the effect of endogenous estrogens. A diet rich in phytoestrogens is recommended for the prevention and supportive therapy of mammary carcinoma (breast cancer).Furthermore, lignans are potent antioxidants. They have beneficial effects on cholesterol metabolism and cardioprotective effects. They are also thought to have beneficial effects on prostate health.
