Page 862 - Veterinary Toxicology, Basic and Clinical Principles, 3rd Edition

820 SECTION | XIII Estrogenic Toxicants

VetBooks.ir TABLE 60.2 Typical Phytoestrogen Concentrations in Plants used for Livestock Forage and Soy Food

Concentration (mg/kg or ppm dry weight)
Plant
Phytoestrogen
Alfalfa Coumestrol 25 65 a
Red clover Formononetin and biochanin A 3,000 15,000 b
Genistein 300 1,500
Daidzein ,300
White clover Isoflavonoids 100 600
Coumestrol ,10
Soy foods Daidzein and genistein B1,000 3,500
a
Concentrations from 18 to .180 mg/kg coumestrol have been associated with infertility in cattle (Mostrom, 2010).
b
Concentrations .500 to 750 mg/kg have been associated with infertility in cattle (Mostrom, 2010).
From Saloniemi et al. (1995) and Franke et al. (1995).

Animal Biotransformation

Similar to any drug or toxin, the dose or intake of phytoes-
trogens is not equivalent to the dose at the active site or
receptor in tissue. Dietary phytoestrogens undergo the pro-
cesses of absorption in the gastrointestinal tract, biotransfor-
mation, distribution, and excretion in urine, bile, feces, and
milk. Effects of phytoestrogens may vary with the individual
phytoestrogen; species exposed; sex; the route, dose, and
duration of exposure; and the timing of exposure during
reproductive development and cycling. Most phytoestrogens
occur in plants as biologically inactive glycoside conjugates
with glucose or carbohydrate moieties. Plant glycosides can
be hydrolyzed by plant enzymes, or after consumption the
glycosides are hydrolyzed and further demethylated in the
acidic gut or rumen by microbes and the heterocyclic phe-
nols (aglycones) are free in the gastrointestinal tract. Gut
flora may become adapted over a matter of days to the diet
and expand their populations for enhanced metabolism.
Microbial metabolism of isoflavones can vary greatly
between individuals. Isoflavone absorption and bioavailabil-
ity in humans can vary with intestinal microbial population,
gut transit time, fecal digestion rates, and fiber content in
the diet (Neilsen and Williamson, 2007).

Absorption

In ruminants, a majority of the metabolic transformations of
phytoestrogens occur in the rumen by microbial action.
Complete metabolic pathways have not been defined
(Lundh, 1995). Basically, in ruminants, biochanin A
is demethylated to genistein and via ring cleavage to para-
ethyl phenol and organic acids (Fig. 60.2). Para-ethyl phe-
nol is considered a non-estrogenic compound. Formononetin FIGURE 60.2 Schematic of rumen metabolism of biochanin A to
is primarily demethylated to daidzein and further genistein (Cox and Davies, 1988).