Soy Protein And Estrogen: What's The Connection? (2025)

Soy contains isoflavones, a type of plant estrogen, which has a similar chemical structure to human estrogen. Soy protein isolate is a stripped-down version of soy that only contains protein. Epidemiological studies have shown that soy supplementation increases the ratio of 2:16 OH-E1 in women, but no studies have investigated the effects of soy supplementation on estrogen metabolism in men. In the context of premenopausal women, soy may act like an anti-oestrogen, but among postmenopausal women, soy may act more like an oestrogen.

Soy protein isolate supplements

The type of soy being studied is also important. Whole soy foods such as tofu and soybeans, processed versions like soy protein powders, or soy-based veggie burgers may have different effects on estrogen levels. Fermented or unfermented soy foods may also have varying effects. If supplements are used, it is important to consider whether they contain isoflavones or soy protein.

Whole soy foods

Soy contains isoflavones, which are plant estrogens. These are different from human estrogens and can act as either an anti-oestrogen or an oestrogen, depending on the context. For example, in premenopausal women, soy may act like an anti-oestrogen, whereas in postmenopausal women, it may act more like an oestrogen.

It is important to note that the effects of soy on estrogen levels can vary depending on individual factors, such as menopausal status and health conditions. For example, in the case of breast cancer, soy consumption after a diagnosis can be beneficial as it provides plant-based protein and may improve breast cancer outcomes. However, it is always recommended to consult with a healthcare professional for personalised advice.

Processed soy foods

Soy contains isoflavones, which are a type of plant estrogen (phytoestrogen). These have a similar chemical structure to human estrogen, but they bind to the estrogen receptors on breast tissue differently, potentially acting as a tumour suppressor.

Soy protein isolate is a stripped-down version of soy that only contains protein and none of the other benefits. It is commonly found in protein bars and supplemental powders. Epidemiological studies have shown that soy supplementation increases the ratio of 2:16 OH-E1 in women, but no studies have investigated the effects of soy supplementation on estrogen metabolism in men.

Whole soy foods such as tofu and soybeans, as well as processed versions like soy protein powders and soy-based veggie burgers, are also available. Fermented and unfermented soy foods are another option. If supplements are used, it is important to check if they contain isoflavones or soy protein.

It is recommended to avoid ultra-processed foods, including processed meat, and limit intake of red meat and high-fat dairy. Eating a wide variety of colourful vegetables and fruits, whole grains, legumes, nuts, and seeds can be helpful in preventing and surviving breast cancer.

Fermented or unfermented soy foods

Soy contains isoflavones, which are plant estrogens. These are different from human estrogens and can act as an anti-oestrogen in premenopausal women, but may act more like an oestrogen in postmenopausal women.

Soy protein isolate is a stripped-down version of soy that only contains protein and is commonly found in protein bars and supplemental powders. Epidemiological studies have shown that soy supplementation increases the ratio of 2:16 OH-E1 in women, but no studies have investigated the effects of soy supplementation on estrogen metabolism in men.

Unfermented soy foods are those that have not undergone this fermentation process and include products like soy milk, soy yoghurt, and edamame beans. These foods still contain the full complement of soy proteins and isoflavones, but the isoflavones may be less bioavailable due to the presence of anti-nutrients.

Soy and breast cancer

Soy contains isoflavones, which are plant estrogens (phytoestrogens) with a similar chemical structure to human estrogen. However, soy binds to the estrogen receptors on breast tissue differently, potentially acting as a tumour suppressor.

The effect of soy on the body depends on the consumer's menopausal status. Premenopausal women have much higher circulating levels of estradiol (the major form of estrogen in the human body) than postmenopausal women. In this context, soy may act like an anti-oestrogen, but among postmenopausal women, soy may act more like an estrogen.

In addition, women with breast cancer are classified into hormone types: either hormone-positive (ER+/PR+) or hormone-negative (ER-/PR-) breast cancer. These tumours respond differently to oestrogens.

Soy protein isolate is a stripped-down version of soy that only contains protein and none of the other benefits. It is commonly found in protein bars and supplemental powders. Epidemiological studies have shown that soy supplementation increases the ratio of 2:16 OH-E1 in women. However, no studies have investigated the effects of soy supplementation on estrogen metabolism in men.

"Soy consumption after a breast cancer diagnosis not only is a great way for you to get a plant-based protein, it also improves your breast cancer outcomes," says Dr Mussallem. It is important to avoid ultra-processed foods, including processed meat, and limit the intake of red meat and high-fat dairy. Eating a wide variety of colourful vegetables and fruits, whole grains, legumes, nuts, and seeds can be helpful in preventing and surviving breast cancer.

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