Phytoestrogens have been shown in some clinical trials to reduce hot flushes significantly, although many of the trials were undertaken over short periods.

The active ingredients in most dietary supplements for menopause are phytoestrogens — chemicals found in plants that may act like the estrogen produced naturally in the body.
“These plant estrogens are thousands of times weaker than natural estrogen,” says nutritionist Mindy Kurzer of the University of Minnesota. “But they also circulate in the blood at levels thousands of times higher than natural estrogen.” That’s why researchers want to know if plant estrogens work like natural estrogen.

   The food that is richest by far in phytoestrogens is soybeans. A typical three-ounce serving of tofu, for example, contains about 23 milligrams of isoflavones (the major group of phytoestrogens). About a half-cup of shelled peanuts, on the other hand, has less than a tenth of a milligram. Menopausal supplements made from herbs like black cohosh, red clover, and dong quai may contain soy-like levels of plant estrogens.

 The interest in Phytoestrogens has developed because of the epidemiological evidence that diets rich in these compounds have led women in Japan and Asia to appear to have a much lower incidence of "Western diseases" such as heart disease, osteoporosis, and cancers of breast, colon, and womb. Women in these countries do not appear to suffer the same way with hot flushes and sweats as we do in the western world.

Phytoestrogens or plant estrogens in our diets

CEREALS: oats, barley rye, brown rice, couscous and bulgar wheat.

SEEDS: sunflower, sesame, pumpkin, poppy, linseeds

PULSES: soya beans and all soya based products (except soya sauce which does not contain any!)

BEANS: chickpeas, kidney beans, haricot beans, broad beans, green split peas

VEGETABLES: red onions, green beans, celery, sweet peppers, sage, garlic, broccoli, tomatoes and bean sprouts.

SOYA, LINSEEDS and RED CLOVER are the richest sources

References:

https://en.wikipedia.org/wiki/Phytoestrogens

http://www.menopausematters.co.uk/phytoestrogens.php

http://www.cspinet.org/nah/soy/phytoestrogen.html