Is soy healthy based on science or does soy cause cancer?

Soy may be a controversial topic in the world of nutrition because it contains phytoestrogens and there are studies that link estrogen to the increased risk of diseases such as breast cancer.

Unlike many other plant proteins, soy protein is considered a complete protein, containing all nine essential amino acids that the body cannot make which must be obtained from the diet. That’s another reason to find out if soy is healthy for us. Click here to read my post and find out which protein is better based on science: animal or plant protein.

So, is soy healthy for us based on science? Does soy estrogen cause cancer? Let’s find out the answers to these questions in this article.

The problem with soy and estrogen

Estrogens are hormones that are important for sexual and reproductive development, mainly in women. So, usually estrogens aren’t dangerous for us. However, in some cases high amounts of estrogens have been associated with an increased risk of certain cancers. [1]

Soybeans naturally contain a class of phytoestrogens called isoflavones. That’s why some people assume that soy can cause cancer.

So, is soy good or bad for you? Let’s find out.

Is soy healthy?

A meta-analysis of 15 RCTs says that blood pressure is an important risk factor for cardiovascular disease and it looks like soy consumption significantly improves systolic blood pressure (SBP) and diastolic blood pressure (DBP). [2]

A systematic review of 8 studies shows that soy products and soy constituents (soy protein and soy isoflavones) may be associated with a lower risk of type 2 diabetes mellitus. [3]

Soy and cancer

The results of a meta-analysis of 16 RCTs show that soy isoflavones (ISFs) improve cognitive function in adults. [4]

A systematic review of 23 prospective studies says that soy and its isoflavones may favorably influence risk of mortality. In addition, soy protein intake was associated with a decreased risk in the mortality of breast cancer. [5]

A meta-analysis of 14 studies says that fermentation of soy foods may play a significant role. From the separate analysis of nonfermented soy foods and fermented soy foods, the researchers found that consumption of tofu and soy milk was associated with a reduction in prostate cancer risk of ≈30% when the highest reported category of intake was compared with the lowest reported category of intake, whereas intake of fermented soy foods was not associated with the risk. [6]

So, it looks like soy’s impact on our health may depend on a type of soy (whole soy foods such as soybeans, fermented or unfermented foods, soy supplements, etc.).

Miso, tempeh and soy sauce are examples of fermented soy foods. Tofu, soy milk, soybeans, edamame are examples of unfermented soy foods.

A systematic review of 131 articles (40 RCTs, 11 uncontrolled trials, and 80 observational studies) says that soy consumption may protect against the development of breast cancer, breast cancer recurrence and mortality, although this is based on observational data only. [7]

A meta-analysis of 8 studies shows that women with high soy and isoflavone consumption have a lower risk of being diagnosed with breast cancer when compared to women that do not have soy in their dietary plan. [8]

A meta-analysis of 30 studies shows that total soy food intake is associated with a reduced risk of prostate cancer. [9]

A systematic review of 17 studies also shows that soy isoflavone consumption is significantly associated with a reduced risk of colorectal cancer risk. [10]

How much soy is healthy for us?

A meta-analysis of 15 RCTs on soy intake and blood pressure says that one of the RCTs shows that 40 g of soy protein containing 68 mg of phytoestrogens, improved blood pressure abnormality and another RCT says administration of 40 g of soy protein containing 118 mg of isoflavones improved blood pressure in healthy men and women. [2]

A systematic review of 131 articles on soy and cancer says that protective effects on breast cancer mortality and recurrence were seen at a soy intake of approximately >35.3mg soy isoflavones or >15.78g soy protein. [7]

A meta-analysis of 16 RCTs on soy and cognitive function says that the dose of ISFs ranged from 60 to 160 mg/d. [4]

One serving of a traditional soyfood, such as 100 g of tofu or 250 mL soy milk, typically provides about 25 mg isoflavones. [11]

That means that 60 to 160 mg/d is approximately 240 to 640 g/d of tofu.


So, is soy healthy based on science or does soy estrogen cause cancer? Well, it looks like scientific research shows that soy is really healthy for us and it doesn’t cause cancer. Moreover, research shows that soy may be associated with a reduced risk of cancer development and recurrence. Also, soy consumption may be associated with a cognitive function improvement and reduced risk of some other diseases such as type 2 diabetes.

However, it’s important to note that some research shows that consumption of unfermented soy foods such as tofu, soy milk and soybeans may be associated with a reduction in cancer and consumption of fermented soy foods such as miso and tempeh may not be associated with the risk.

How much soy is healthy for us? Well, it looks like about 200-600 g/day of soy products may be beneficial for us.

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