Soy – properties, nutritional value, application, and potential adverse effects

The most extensively cultivated soybean variety is vegetable soybean, scientifically designated as *Glycine max* (L.) Merr., an annual leguminous plant originating from Southeast Asia. Its cultivation history in China spans **four millennia**, underscoring its enduring agricultural significance. Presently, the **United States** and **Brazil** dominate global production, with vast monocultures dedicated to this crop. The surge in its popularity over recent decades is largely attributable to the expanding adoption of plant-based diets—both vegan and vegetarian—as soy represents one of the **few complete plant-based protein sources**, furnishing all essential amino acids in balanced proportions. With regard to protein quality, the **Protein Digestibility-Corrected Amino Acid Score (PDCAAS)** for soy protein isolate attains the **maximum value of 1.00**—equivalent to that of chicken egg protein—whereas whole soybeans achieve a score of **0.91**. Meanwhile, the **biological value (BV)** of soy protein stands at **74**, while that of whole soybeans reaches **96** (compared to **97** for chicken eggs). Beyond its protein content, soy is a rich source of **polyunsaturated fatty acids**, including linoleic acid (omega-6) and α-linolenic acid (omega-3), as well as B-complex vitamins such as thiamine (B₁), riboflavin (B₂), and folic acid (B₉). A nutritional breakdown reveals that **100 grams of dried soybeans** fulfill daily requirements for the following minerals: **potassium** at **45%**, **calcium** at **24%**, **phosphorus** at an exceptional **106%**, **magnesium** at **54%** (for men) or **70%** (for women), and **iron** at **89%**, positioning it among the most nutrient-dense legumes available commercially.

The nutritional composition of 100 grams of dried soybean seeds is characterized by the following parameters: an energy yield of 412 kilocalories, a protein content of 34.3 grams, and a total lipid content of 19.6 grams—comprising 2.82 grams of saturated fatty acids, 4.07 grams of monounsaturated fatty acids, and 11.29 grams of polyunsaturated fatty acids. Carbohydrates account for 32.7 grams in total, with 15.7 grams attributed to dietary fiber. Soybeans are particularly notable for their potassium content, amounting to 2132 milligrams, and further provide 240 milligrams of calcium, 743 milligrams of phosphorus, 216 milligrams of magnesium, and 8.9 milligrams of iron. In terms of vitamins, the seeds contain 0.78 milligrams of alpha-tocopherol (vitamin E), 0.69 milligrams of thiamine (vitamin B₁), 0.189 milligrams of riboflavin (vitamin B₂), and 1.18 milligrams of niacin (vitamin B₃).

The systematic incorporation of soy-based products into daily nutritional regimens may yield multiple health benefits. Even partial substitution of meat-containing meals with soy alternatives leads to a substantial reduction in saturated fat and cholesterol intake while simultaneously increasing dietary fiber consumption. These dietary modifications contribute to lowering low-density lipoprotein (LDL) cholesterol levels—commonly referred to as "bad" cholesterol—which in turn diminishes the risk of developing cardiovascular diseases. The U.S. Food and Drug Administration (FDA) has authorized the inclusion of a health claim on packaging for products containing a minimum of 6.26 grams of soy protein, stating that "25 grams of soy protein per day, as part of a diet low in saturated fat and cholesterol, may reduce the risk of heart disease." Furthermore, scientific evidence suggests that soy may help lower blood glucose levels in individuals with diabetes, although additional clinical research is necessary to confirm these findings. The phytoestrogens present in soy exhibit a mitigating effect on menopausal symptoms, particularly hot flashes, due to their structural similarity to endogenous estrogens. Additionally, research indicates a potential positive impact of soy on bone mineral density (BMD) in perimenopausal and postmenopausal women, which could decelerate the progression of osteoporosis.

Soybeans serve as a fundamental agricultural commodity, primarily utilized for soybean oil extraction and as a high-protein feedstock for livestock. Within Asian culinary traditions, fermented soybeans are transformed into staple products such as tofu (a curdled soy milk derivative), soy sauce (fermented soybean-based condiment), and tempeh (another fermented soybean product). Soy flour, comprising approximately 50% protein and 5% dietary fiber, surpasses wheat flour in nutritional density—offering elevated levels of thiamine, riboflavin, phosphorus, calcium, and iron while remaining gluten-free. Its functional properties include acting as a thickening agent in sauces, extending the shelf life of baked goods, and minimizing oil absorption during frying. Soy-based meat substitutes (e.g., soy patties) provide plant-based alternatives, whereas soy milk serves as a replacement for individuals with lactose intolerance or cow’s milk protein allergies. However, research published in the *Proceedings of the National Academy of Science* indicates that soy isoflavones (genistein and daidzein) may compromise thymus function, leading to reduced immune cell counts. Furthermore, a study in the *Journal of the American Medical Association* involving 800 adults revealed an increased prevalence of asthma and allergies among those fed soy milk in infancy compared to cow’s milk. Consequently, soy milk consumption in children is advised only in cases of cow’s milk allergy—and then in minimal quantities—with complete avoidance in infants. Soy derivatives are also incorporated into processed meats (e.g., pâtés, sausages, deli meats), food concentrates, sauces, baked goods, confectionery fillings, desserts, and ice cream, primarily leveraging soy protein isolate for its cost-effectiveness and nutritional benefits relative to meat.

Excessive consumption of soy-based products may be associated with adverse health effects, including hormonal disruptions such as gynecomastia in men. Soy contains phytoestrogens—specifically genistein and daidzein—whose chemical structure closely resembles that of the female sex hormone estradiol. While estradiol occurs naturally in men in trace amounts and plays a vital role in proper endocrine function, an excess—potentially induced by high soy intake—can lead to undesirable outcomes, such as the development of breast tissue. Furthermore, emerging evidence suggests that soy may adversely affect sperm quality. Animal studies in which rats were administered daidzein demonstrated reduced testosterone secretion and decreased elasticity of muscle tissue. Clinical observations from 2008 also indicated that men who regularly consumed large quantities of soy products exhibited lower sperm concentrations compared to those who avoided soy. Additionally, soy protein—though more affordable than animal-derived protein—has been shown to possess lower nutritional value and a diminished capacity to stimulate protein synthesis in the body, as confirmed by research published in the *Journal of Nutrition*. Soy also contains goitrogens, compounds that impair iodine absorption—a critical element for neurological development and thyroid function, making it particularly unsuitable for children. The high phytic acid content in soy further inhibits the absorption of essential minerals, including calcium, magnesium, iron, and zinc. Scientific opinions on soy’s health impact remain divided: some studies suggest a potential protective effect against breast cancer, while others indicate that soy may accelerate the proliferation of cancerous cells. Given these contradictions, it is challenging to definitively determine whether soy is harmful or beneficial. However, the principle of moderation applies: current dietary guidelines recommend limiting legume consumption, including soy, to no more than two cups per week. It is also worthwhile to explore nattokinase—a fermented soy-derived enzyme that may offer distinct health benefits.