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
- Antiviral, antibacterial, insecticidal or fungistatic
(Adlercreutz and Mazur
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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