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It's a common belief that foods rich in estrogen (like Soya milk) increases breast size.

MD-health.com claims that it works:

Eat Enough Estrogenic Foods. Estrogen is a potent female sex hormone and proper levels can lead to enlargement of breast tissue. Testosterone is the male hormone counterpart and diminishes breast growth. Avoid carb rich foods such as chips, crackers, white rice and baked goods to help minimize testosterone production. Excellent sources of natural estrogen include: pumpkin, garlic, red beans and split peas, soy foods eggplant, flax seeds, pumpkin and squash.

[...]

Try Soy. Soy products are high in isoflavones that can help the body increase its own estrogen levels and promote breast growth. Soy products are also rich in protein and can help build and repair damaged tissues in the body. Soy milk is available in a variety of flavors.

But the Straight Dope claims that this isn't true:

For women who do produce enough estrogen of their own, phytoestrogens actually decrease overall estrogen activity by competing with the homegrown estrogen for positions on estrogen receptor sites; when phytoestrogens latch onto these sites, they push aside the real estrogen and provide only a weaker version. In fact, that could be their real benefit, some experts think--by lowering the body's effective estrogen level, phytoestrogens may reduce a woman's chance of developing breast cancer. But here's the thing: if so, they'd likely make breasts smaller, not larger.

In short, whatever uses phytoestrogens may have, increasing breast size isn't one of them. Many breast-enlargement products contain only small amounts of phytoestrogens anyway, and none has been proven to work in double-blind laboratory tests.

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    Note that the two claims are not necessarily contradictory. It may be that phytoestrogens do not increase (or decrease) breast size in healthy, mature women, but that they increase breast size in pre-pubescent women or in men--two variations of the anti-Soy claim I have heard before. – Flimzy May 13 '14 at 21:44
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Phytoestrogens have both anti-inflammatory and anti-oxidant abilities independent of their Estrogen Receptor (ER) binding actions. The latter action brings them under the class of endrocrine disruptors:

Perhaps the most well characterized mode of phytoestrogen action is estrogen receptor (ER) binding. There are two major ER subtypes in mammals, ERα and ERβ (also referred to as ESR1 and ESR2, respectively). As such, phytoestrogens, particularly the isoflavones, fit the Environmental Protection Agency’s definition of an endocrine disruptor which characterizes these compounds as those which, “alter the structure or function(s) of the endocrine system and cause adverse effects.” This definition includes disruption of lactation, the timing of puberty, the ability to produce viable, fertile offspring, sex specific behavior, premature reproductive senescence and compromised fertility .... [1]

The effect on breast tissue is unclear. Infants don't have breasts as such but [2]

The hypothesis that exposure to soy phytoestrogens early in life can alter the timing and character of breast development is supported by a 2008 cross-sectional study of 694 girls in Israel, which found increased prevalence of breast buds in 2-year old girls fed soy formula as infants

The situation is less clear in adults.

Because they bind ERs with relatively high affinity, some researchers and clinicians are concerned that high phytoestrogen intake may increase the risk of carcinogenesis and put breast cancer survivors at risk for reoccurrence. Others have proposed that the opposite is true, citing traditionally low cancer rates in Asia as evidence [1]

The effect on breast cancer cells appears to be related to the dose as well as other things

For example, in vitro, genistein can inhibit proliferation of ER-positive and ER-negative breast cancer cells at high doses (>10 M), but, paradoxically, promote tumor growth at lower, more physiological doses [1]

which is similar to Tamoxifen, a selective estrogen receptor modulator (SERMs) used for breast cancer therapy

There are no studies looking at the effect on breast size in adults, and the measurement itself is somewhat difficult to do reproducibly.


[1] Patisaul HB, Jefferson W. The pros and cons of phytoestrogens. Front Neuroendocrinol 2010 Oct;31(4):400-19. doi: 10.1016/j.yfrne.2010.03.003. PubMed PMID: 20347861+.

[2] Zung A, Glaser T, Kerem Z, Zadik Z. Breast development in the first 2 years of life: an association with soy-based infant formulas. J. Pediatr. Gastroenterol. Nutr. 2008 Feb;46(2):191-5. doi: 10.1097/MPG.0b013e318159e6ae. PubMed PMID: 18223379.

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