Does Soy Milk Affect Estrogen Levels?

soy beans and soy milk
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Rumors about soy milk and estrogen may cause you to question the safety of this beverage. But don’t worry—you don’t have to skip your favorite soy latte. While soy milk does contain compounds that are structurally similar to estrogen, they do not function exactly the same as this hormone. For many people, soy milk can be part of a nutritious, balanced diet—and may actually have a few beneficial effects.

Does Soy Milk Contain Estrogen?

Soy milk does not contain estrogen, but it does contain phytoestrogens. These are primarily present in the form of three different isoflavones: 

  • Daidzein
  • Genistein
  • Glycitein

The amount of isoflavones in soy milk (and other soy products) can be affected by agricultural conditions, the soybean cultivar, and processing. Regardless of the specific concentrations of isoflavones in your glass of soy milk, rest assured that phytoestrogens are not the same as estrogen. 

The Difference Between Estrogens and Phytoestrogens

Estrogens are a group of steroid hormones, usually classified as female sex hormones. The three primary estrogens that are produced in the female body are:

  • Estrone (E1)
  • Estradiol (E2)
  • Estriol (E3)

The most prominent is estradiol, also known scientifically as 17β-estradiol.

Phytoestrogens—the isoflavone compounds found in soy—are nonsteroidal plant hormones that are structurally similar to estrogen.

They can bind to the two types of estrogen receptors in the body, known as ERα and ERβ, and typically prefer the later of those two receptors.

When phytoestrogens bind to these receptors, your body may react similarly or differently than when estrogen binds to them. For example, when estradiol binds to the ERβ receptor it is thought to promote the growth of certain cancer cells.

Yet when certain isoflavones bind to the receptor, it is hypothesized that they may prevent the growth of those cancer cells.

Do Phytoestrogens in Soy Milk Affect Estrogen Levels in the Body?

Most research suggests that moderate consumption of soy products, including soy milk, does not affect estrogen levels. This has been shown true among premenopausal women in a large meta-analysis.

In this same analysis, there were also no statistically significant changes among post-menopausal women. There was a slight non-significant increase in estradiol among post-menopausal women that may warrant additional research. Even so, the authors note that the lack of other hormonal changes likely “argues against a physiologically important estrogenic effect."

What About Soy Milk and Men?

If there's one fear that tends to be pervasive, it's the unsubstantiated claim that soy milk will cause high estrogen levels and feminizing side effects in men.  

Here are a few key facts to clear up these misconceptions:

  • Despite the perception that estrogen hormones are limited to women, men naturally produce estrogen too—just at levels far lower than those in the female body. 
  • Men actually need these small levels of estrogen to support proper bone health. High levels of estrogen in men, though, can increase the risk of cardiovascular disease, stroke, and sexual dysfunction.
  • Moderate soy consumption does not cause high estrogen levels in men. A literature review in Fertility and Sterility found no association between soy intake in men and altered estrogen levels or altered testosterone levels.

There are rare case studies that have noted hormonal changes at extreme levels of soy consumption. For example, a case study published in Endocrine Practice found increased estrogen levels and breast tenderness in a 60-year old man, thought to be associated with his soy milk intake.

However, he was polishing off three quarts – or 12 cups – of soy milk a day. Even nutritious foods can be detrimental in excessive amounts (akin to how water is essential to life but can cause deleterious effects with excess).

Overall, moderate consumption of soy milk has been shown to be safe among men without risk of feminization or other negative health effects.

Phytoestrogens in Soy Milk and Cancer Risk

Much of the debate surrounding soy milk and estrogen stemmed from early concerns that phytoestrogens would mimic estrogen and increase the risk of hormone-dependent cancers. Certain cancers of the breast, for example, are associated with high estrogen levels.

Take comfort in the fact that the phytoestrogens in soy milk have not been associated with increased breast cancer risk. In fact, those isoflavones can bind to the estrogen receptors, potentially blocking the activity of the more potent estrogen hormone. According to the American Cancer Society, this may actually reduce the risk of breast cancer and other cancers.

Stick with getting your soy fix from foods though, rather that isoflavone supplements. High dose isoflavone supplements may cause different effects in your body—as the old adage goes, there may be "too much of a good thing."

Other Health Concerns With Soy Milk 

There are two other concerns related to soy milk and phytoestrogens that are worth exploring.

Soy Milk and Thyroid Health

Controversy exists over soy’s impact on thyroid health. The right recommendation likely depends on your current thyroid health:

  • If you have normal thyroid function and you get enough iodine each day, there is likely no harm in moderate soy consumption.
  • If you have hypothyroidism and are treated with synthetic thyroid hormone, you should avoid drinking soy milk or eating other soy food products within 4 hours of taking your medication.
  • If you have borderline hypothyroidism, some experts suggest minimizing soy intake. There are concerns about soy interacting with the thyroid in a way that it can push a person past the threshold into full hypothyroidism. This has not been proven but may be a strategy used by certain doctors or nutritionists.

Soy Formula and Babies

Experts have raised concerns about the potential hormonal effects of the phytoestrogens in soy protein formula. These formulas are often used as alternatives to cow’s milk based formulas, and estimates are between 20-25 percent of formula-fed infants are given a soy-based baby formula.

The phytoestrogens in soy formulas are absorbed by infants, and may lead to proportionally high levels in their relatively small bodies. Questions exist over whether this may affect reproductive development during this critical time, due to their ability to bind to estrogen receptors.

Some studies have indicated that soy formula during infancy led to heavier or longer duration menstrual bleeding during young adulthood in females.

On the flip side, early exposure to soy protein may lead to some of the beneficial protective effects against hormone-dependent cancers. 

The current position statement from the American Academy of Pediatrics states that “no conclusive evidence from animal, adult human, or infant populations that dietary soy isoflavones may adversely affect human development, reproduction, or endocrine function.” Nevertheless, more long-term research in this particular area is warranted.

It is important to note that only soy formulas—not commercial soy milk beverages—should be used as infant feeding options.  

Other Benefits of Soy Milk

Despite the controversies described above, for most people moderate soy milk consumption can be a nutritious addition or substitution to the diet. Consider these additional benefits:

  • Compared to other milk alternatives, it has the most similar nutritional profile to cow’s milk.  A cup contains a comparable amount of calories as well as 7 to 8 grams of protein (a nutrient which most other milk alternatives, like rice milk or almond milk, are lacking).
  • Soy protein lowers LDL cholesterol.  Though the amount is modest, experts believe it is in the range of a 3-5 percent reduction when eating 25 grams of soy protein per day.  You can find this protein in foods like soy milk, tofu, and edamame.
  • Some research suggests soy foods may be beneficial for people with PCOS.
  • Soy protein may reduce the severity and frequency of hot flashes during menopause.

Sources:

Barrett JR. The Science of Soy: What Do We Really Know? Environmental Health Perspectives. 2006;114(6):A352-A358.

Bhatia, J and Greer, F. Use of Soy Protein-Based Formulas in Infant FeedingPediatrics May 2008, 121 (5) 1062-1068.

Hooper L, Ryder JJ, Kurzer MS, et al. Effects of soy protein and isoflavones on circulating hormone concentrations in pre- and post-menopausal women: a systematic review and meta-analysis. Human Reproduction Update. 2009;15(4):423-440.

Jorge Martinez and Jack Lewi (2008) An Unusual Case of Gynecomastia Associated with Soy Product Consumption. Endocrine Practice: May 2008, Vol. 14, No. 4, pp. 415-418.  

Messina M. Soybean isoflavone exposure does not have feminizing effects on men: A critical examination of the clinical evidence. Fertility and Sterility. 2010; 93 (7) , pp. 2095-2104.

Rizzo G, Baroni L. Soy, Soy Foods and Their Role in Vegetarian Diets. Nutrients. 2018;10(1):43.