Does Soy Protein Increase Estrogen Levels?

As explained by the Centers for Disease Control and Prevention (CDC), soy-based foods are the biggest dietary suppliers of isoflavones, which when consumed and metabolized have the potential to act as phytoestrogens, a class of compounds with weak estrogenic effects.

The CDC says soy estrogens, such as edamame or tofu estrogens, are much less potent than the hormone produced by the human body, but depending on your diet can be present in much greater quantities in the blood.

For example, infants who consume soy-based formula can have blood concentrations of isoflavones that are 13,000 to 22,000 times higher than the natural level of estrogen in babies not fed infant formula. This might seem like a worry, but followed through to adolescence and young adulthood, soy-fed babies show no adverse reproductive effects.

Neither does consuming soy, estrogens appear to have a negative impact in men. An August 2018 study in Scientific Reports found that when men combined soy protein supplementation with resistance exercise training, there were no changes to their levels of either male or female hormones.

Read more: Is Eating Soy Actually Bad for Your Health?

According to Harvard Health Publishing, the effects of soy on hormones in females can be to either increase or decrease the effects of estrogen, which may benefit both younger and older women.

Premenopausal women have much higher circulating levels of estradiol — the main type of estrogen hormone — than postmenopausal women, and in this younger age group soy isoflavones may compete with the actual hormone for receptor sites in the body, damping down the effect of excess estrogen. As too much estrogen can be linked with abdominal weight gain, fibroids, fatigue and anxiety, this may be beneficial for premenopausal women.

Around menopause, when estrogen levels dramatically fall, isoflavones can switch roles and are thought to help modestly supplement low levels of estrogen, which might have benefits for this group. Results are mixed, but a December 2014 review in the journal Climacteric found that plant estrogens, including soy estrogen, helped reduce the frequency of hot flashes in menopausal women, without harmful side-effects.

Read more: 13 Surprising Vegetarian Sources of Protein

The big question for most women wondering whether soy is safe is, can it contribute to breast cancer?

In fact, far from being a breast cancer risk, the Mayo Clinic says that soy is one of the foods that can reduce breast cancer risk. Those who eat plenty of soy products from an early age reduce their risk of breast cancer in later life. This protective effect is less dramatic for women who eat smaller amounts of soy or who start eating soy later in life.

Even if you are a breast cancer survivor, it's OK to carry on eating soy products and to do so may be beneficial. A June 2017 study in Cancer surveyed more than 6,200 American and Canadian women with breast cancer and found those women who ate the highest amounts of isoflavones had a lower risk of dying from any cause.

Expert advice from the American Cancer Society is that soy foods are healthy and safe. But soy supplements contain much higher isoflavone concentrations than food and aren't recommended until more research is carried out on them.