A comparison of oral micronized estradiol with soy phytoestrogen effects on tail skin temperatures of ovariectomized rats.

OBJECTIVE: Whether phytoestrogen-containing soy supplements have beneficial effects on hot flashes of postmenopausal women and how those effects, if any, compare to estrogen replacement therapy has been uncertain. It is possible that the uncertainty is due to the low doses of soy isoflavones (30-60 mg per day) used in the studies. We used ovariectomized retired breeder rats and a higher dose of soy phytoestrogens to approach these uncertainties experimentally.
DESIGN: The treatment groups were as follows: (1) Control group fed a casein/lactalbumin-based diet; (2) Soy(-) group fed alcohol-washed soy protein isolate with the phytoestrogens extracted; (3) Soy(+) group fed phytoestrogen-containing soy protein (equivalent to a woman's dose of 144 mg isoflavones per day)--a dose two to three times higher than that in most studies with women; and (4) E2 group fed oral micronized estradiol (E2) at a dose equivalent to a woman's dose of 1 mg per day. A temperature-transponder was taped to the surface of the tail to measure temperature. Tail skin temperature was significantly increased within a week after ovariectomy. The animals were pair-fed during the last 21 days of treatment for daily temperature measurement.
RESULTS: Soy(-) had no effect on skin temperature. E2 had a large effect on skin temperature (about 1.4 degrees C reduction from Control). Soy(+) was intermediate between the E2 treatment and no treatment (about 0.8 degrees C reduction from Control).
CONCLUSIONS: Soy phytoestrogens have a modest effect on average skin temperatures, being about half that of E2, even at high doses in the rat model.