Because of their weak estrogenic activity, soy isoflavones
have been hypothesized to improve several estrogendependent
conditions, including perimenopausal vasomotor
symptoms (hot flashes) and postmenopausal bone loss. A
recent review examined 11 clinical trials of soy protein or
isoflavones67 for treating hot flashes. Only 3 of 8 studies with
treatment lasting 6 weeks found modest improvement in
hot flashes, and most benefits disappeared after 6 weeks. Five
additional studies68–72 not included in that review showed no
benefit for hot flashes of soy isoflavones. Longer studies
showed no benefit of isoflavones at 24 weeks73 or 2 years.71
Substantial reduction in hot flashes, often 40% to 60%,
occurred in the placebo or control group in these studies,
similar to the reduction in the soy group. In contrast, estrogen
replacement markedly reduces hot flashes, more so than
placebo. Thus, it seems unlikely that soy isoflavones have
enough estrogenic activity to have an important impact on
vasomotor symptoms of estrogen deficiency in perimenopausal
women.