Estrone, estradiol, and estriol are the three major estrogens that are excreted into the bile. These estrogens are conjugated to glucuronic acid and/or sulfate. Upon excretion of these conjugated estrogens from the bile into the small intestine the conjugates are available substrates for bacterial metabolism. The bacteria of the lower small intestine and colon can hydrolyze the estrogen conjugate releasing free estrogens (45). The nonconjugated estrogens are then subject to additional bacterial action. A major reaction involves oxidoreduction of the C17 position. Bacteria can convert estrone to estradiol, and the fecal flora can also convert 16 alpha hydroxyestrone to estriol (46).
The intestinal bacteria can also modify androgens. The intestinal bacteria can reversibly oxidize and reduce the 3-hydroxy group, and reduce steroid nuclear double bonds at the one and four positions. The latter reactions can result in several interconversions of androgens (47).
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