LIGNANS

Phytoestrogens: New Health Effects Revealed?

Rye grains contain relatively high concentrations of plant lignans, which are converted to weakly oestrogenic mammalian lignans in the colon. The composition of the intestinal microflora and the availability of substrate in the diet influence the efficiency of this process.

It has been found that a diet high in dietary fibre also contains certain amounts of biologically active compounds that may have significant physiological effects. These compounds are found in the same parts of the plant material as dietary fibre and they may be the cause of some of the health effects associated with fibre.

The lignans belong to the group of phytoestrogens, that display weak oestrogenic activities. Other phytoestrogens are isoflavones, mainly found in soy, and coumestrol, occurring in alfalfa. The isoflavones are more estrogenic than lignans but are still 1000 times less active than oestradiol. The lignans in plants probably have no oestrogenic activity, but plant lignans (pinoresinol, lariciresinol, syringaresinol, secoisolariciresinol and matairesinol) in the gut are converted to enterodiol and enterolactone. These s.c. mammalian lignans bind very weakly to the oestrogen receptor and exert oestrogenic effects of about 1/10000 to 1/100000 that of oestradiol. It is not possible to experience any oestrogen-related adverse effects by consuming a diet containing lignans. However, anti-oestrogenic effects have been described.

Phytoestrogens affect sex hormone metabolism and function by influencing some sex hormone metabolizing enzymes at the cellular level. They also stimulate the production of sex hormone binding globulin (SHBG) in the liver and in this way they favourably influence sex hormone levels in the body. In addition, they have a great number of other interesting biological activities, which appear to make them potential anti-cancer compounds. Many experimental and epidemiological studies support the theory that a diet high in phytoestrogens may inhibit the development and growth of hormone-dependent cancers, such as prostate and breast cancer, but no definite evidence has been found to date (Magee and Rowland 2004, Arts and Hollman 2005). Some evidence suggests that the inhibiting effect of enterolactone on breast cell proliferation may be due to growth factor action. Isoflavones inhibit vitamin D metabolism in the prostate and reduce androgen action at the receptor level. It is also possible that other components of such a diet may contribute to or even cause, the observed effects (Adlercreutz 2002).

Phytoestrogens and Western Diseases: Potential Health Benefits

  • Hormone-altering
  • Estrogenic/antiestrogenic
  • Antioxidative
  • Cancerprotective
  • Anticarcinogenic
  • Antiproliferative
  • Cardioprotective
  • Antiatherogenic
  • Hypocholesterolemic
  • Bone-maintaining
  • Antiviral, antibacterial, insecticidal or fungistatic

(Adlercreutz and Mazur 1997)

In recent years scientists have been able to assess the phytoestrogen content in food and diet samples (Mazur et al. 1996, Mazur 1998; Peņalvo et al. 2005).  Among grains rye has the highest content of lignans, while soy and a few other legumes are rich in isoflavones.

Rye contains the plant lignans
syringaresinol (SYR), pinoresinol (PINO), lariciresinol (LAR), medioresinol (MED), isolariciresinol (ISOLAR), secoisolariciresinol (SECO) and matairesinol (MAT) (Heinonen et al. 2001, Peņalvo et al. 2005). The plant lignans undergo metabolic conversions in the gut. The intestinal microbes convert the plant lignans into the mammalian lignans enterodiol (END) and enterolactone (ENL). After the conversion, END is oxidized to ENL (Borriello et al. 1985). END and ENL are hormone-like compounds that have the ability to bind to oestrogen receptors with low affinity and with weak estrogen activity (Adlercreutz and Mazur 1997).

The biotransformation of plant lignans to mammalian lignans has been studied in both human and animal experiments and in the laboratory. Experiments with ileostomy patients have shown that the entire enzymatic and fermentative activity of the intestine is needed for a complete bioconversion of rye lignans SECO and MAT to mammalian lignans END and ENL. Therefore, the
ileostomy subjects were not able to complete the bioconversion (Petterson et al. 1996). In healthy humans, clearly higher plasma and urinary enterolactone levels were observed during the intake of whole grain rye bread than in a diet containing white wheat bread (Juntunen et al. 2000).

The biotransformation of plant lignans to mammalian lignans requires a suitable bacterial microflora in the gut. Some people are not able to convert the plant lignan precursors to enterolactone and enterodiol. This seems to be a consequence of frequent treatment with antibiotics, which destroys the microorganisms that perform the conversion from plant to mammalian lignans. After a course of antibiotics it may take more than a year before the microflora is again producing enterolactone normally (Kilkkinen et al. 2002).

The ENL and END that are formed in the large intestine are absorbed by passive diffusion similar to what is seen for short-chain fatty acids. The studies of Bach Knudsen et al. (2001) and McIntosh et al. (2003) suggest that rye fibre not only stimulates the formation of butyrate but also the formation of ENL resulting in higher plasma or faecal levels of both metabolites (Bach Knudsen et al. 2001).

Biotransformation of Lignans in the Large Intestine



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