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Soy and Other Plants: Overview

Soy Often we read about the possible protective effects of different foods, especially vegetables, against different diseases including breast cancer. Ironically, many vegetables, especially soy, contain a type of natural plant estrogen that when consumed as part of normal lifetime diet, may decrease risk of breast cancer.

Why might we think that soy and other vegetables could affect breast cancer risk?

Breast cancer rates historically have been substantially higher in the United States than in China or Japan. One common explanation of this finding has been the difference in traditional diets, with consumption of a low-fat, high-fiber, high-soy diet among Asian populations rather than a high fat, meat-based, low-fiber diet consumed by Western populations [Tham et al., 1998].

Plant estrogens: What are they?

Plant estrogens, also called phytoestrogens, are natural compounds found in many foods. There are two main groups: the isoflavones and the lignans. Isoflavones, which include genistein and daidzein are found in soy beans and are the most widely studied of the phytoestrogens. Lignans are found in flaxseed, cereals, fruits, and berries. Phytoestrogens are strikingly similar in chemical structure to the common estrogen estradiol and bind to estrogen receptors.

Although chemicals like genistein act like estradiol in many ways, they may also block or diminish the effects of a woman’s own estrogens. Therefore both protective effects and detrimental effects on breast cell development and proliferation are possible. The data from laboratory studies are very controversial and sometimes contradictory [Bouker & Hilakivi-Clarke, 2000].

What do the studies show: a quick summary

  • Data are controversial, but many studies suggest regular intake of soy and other sources of plant estrogens as part of a woman’s regular diet may be slightly protective against breast cancer. Other studies show no effect.
  • On the other hand, studies that look at dietary intake of soy during critical periods including just following birth and during adolescence suggest protective effects against later breast cancer development.
  • Some studies examining the effects of taking high levels of soy supplements (e.g., genistein pills) suggest that this may lead to changes that increase the risk of breast cancer.
  • Laboratory studies suggest that there may be important differences in effects, depending on dose and timing of phytoestrogen intake.