Soy contains 26% protein. It has the highest protein content amongst the plants. It is also the highest quality protein amongst the legumes.
Grains are typically low in lysine, beans are typically low in sulfur, methionine and cysteine. However the level of sulfur amino acids is higher in soy than in other beans.
Soybeans are high in fats, too. Most legumes (except peanut) contains 2-14% of fat, soy contains 31% of fat. Most fats in soy is unsaturated.
A serving of soy provides about 8 gr of dietary fibre. However some soy foods are processed in a way that decreases the fibre content, like tofu or soy milk. Processing soy at a high temperature can denature some of the proteins and reduce their quality.
Soy provides important nutrients: calcium, vitamin B12, iron, magnesium and selenium.

Interestingly, whole soybeans are rarely consumed in Western countries. The majority of soy in the diet comes from the refined products that are processed from the soybeans.

The fatty acids in soybeans are mostly Omega-6 polyunsaturated fats. This can be problematic because too many Omega-6s in the diet can lead to inflammation and all sorts of health issues. For this reason, it is very important to avoid soybean oil (and other vegetable oils high in Omega-6) and processed foods that contain it.

The nutrition composition of soy products depends on the type of soy food. Refined soy products, like soy protein and soy bean oil might not be that nutritious at all.

There is some evidence that soy can lower cholesterol levels, although studies show conflicting results. Men who consume soy are at a lower risk of developing prostate cancer in old age.

Soy contains large amounts of biologically active compounds called isoflavones, which function as phytoestrogens… that is, plant-based compounds that can activate estrogen receptors in the human body. These isoflavones are classified as endocrine disruptors, chemicals that interfere with the normal function of hormones in the body. The key isoflavones in soy are genistein, daidzein and glycitein. This can cause reduced estrogen activity due to the isoflavones blocking the actual, more potent estrogen from binding, or it can lead to an increased estrogen activity due to the isoflavones activating the receptors.

Animal studies show that soy isoflavones can cause breast cancer. There are also human studies showing that soy isoflavones can stimulate the proliferation and activity of cells in the breasts.
This may indicate an increased risk of breast cancer, which is the most common cancer in women.

Even though men have some amount of estrogen, having significantly elevated levels is not normal. Therefore, it seems logical that increased estrogen activity from soy isoflavones could have some effects on men. Many believe that soy can reduce testosterone levels, but the effect appears to be weak and inconsistent.

The isoflavones in soy also function as goitrogens, which are substances that interfere with thyroid function. They can inhibit function of the enzyme thyroid peroxidase, which is essential for production of thyroid hormones.

It is important for women who are pregnant, plan on becoming pregnant, or are breastfeeding, to avoid soy and other sources of endocrine disrupting compounds.