Soy foods have been touted as being both a wonder food and a health risk. Numerous studies and clinical trials have been completed to determine the effects of soy products on health. Organizations, medical professionals and even governments have made health claims about the benefits of soy. However, during recent years, many individuals and groups have expressed concerns about soy products, warning about the 'dangers of soy.'
Soy proponents point to studies that have shown that soy protein reduces the risk of heart disease, lowers the levels of LDL cholesterol, is beneficial for treating hot flashes, reduces bone loss after menopause and decreases the risk of certain types of cancers. Soy opponents cite studies where soy was found to be either harmful or neutral; they argue that soy may promote the development of cancer and thyroid disease, and does not have a beneficial effect for reducing cholesterol, reducing bone loss or decreasing the incidence of hot flashes. So, what is it: do soy foods promote health and protect against disease, cause illnesses or are they neutral?
A review of the studies on soy shows that soy may not be a ‘wonder food’, but it is likely not as harmful as the soy opponents suggest, if eaten in moderation. Overall, the clinical tests do not provide conclusive proof that soy is harmful or extremely beneficial to one’s health.
The initial studies on soy found that consuming soy protein daily had a beneficial effect on reducing cholesterol. Researchers completed a meta-analysis of the studies on this topic and found that soy protein was most effective at reducing cholesterol levels for individuals with very elevated cholesterol levels, and that it was not necessarily linked to the amount of soy protein consumed. Consuming soy protein was found to be somewhat effective for those with moderately elevated cholesterol levels but it was found to have no effect on individuals who had only mildly elevated or normal cholesterol levels. There was also a correlation between diet and the effectiveness of soy protein; soy protein was found to be less effective for individuals who ate a lower-fat and low-cholesterol diet than for those eating a higher-fat and higher-cholesterol diet. Therefore, if soy protein replaces dairy protein and/or animal proteins in a person’s diet, it is more likely to be beneficial for reducing cholesterol levels.
Further research found that it might not be the soy protein itself that has a beneficial effect, but that there is some other compound in soy that reduces cholesterol levels. This was based on a study in which two different soy compounds were used, one which was made up of 50% soy protein, and the other which was made up of 80% soy protein. The higher protein compound was found to be no more effective in reducing cholesterol levels than the lower protein compound.
Soy isoflavones were initially thought to be able to reduce the incidence of certain types of cancers and to have beneficial effects for menopausal symptoms, particularly hot flashes. Isoflavones are organic compounds with a chemical structure similar to estrogen, which bind to estrogen receptors and exhibit weak estrogen-like effects. Researchers postulated that soy isoflavones could be used in the same manner as estrogen-replacement therapy to treat menopausal symptoms. However, the studies have not been able to demonstrate that soy isoflavones are effective at reducing bone loss after menopause or in reducing hot flashes. While direct estrogen replacement therapy has been demonstrated to be effective in increasing both bone mineral content and density and in markedly reducing hot flashes, soy has not been found to reduce the incidence of hot flashes any more than in the placebo group. Nor has soy been demonstrated to have a positive effect on reducing bone loss. It may be that soy isoflavones do not have enough estrogenic effect to provide a huge benefit.
Consuming soy isoflavones has also not been conclusively shown to have a beneficial effect on reducing the incidence of cancer. The studies that have been completed on cancer and soy isoflavones have produced variable results: some have shown protective associations against breast cancer, others have shown no association in reducing the incidence of breast, prostrate or endometrial cancers. Some studies have even shown potential adverse effects, although there has not been a definite or strong link between consuming soy isoflavones and developing cancer.
Various factors could affect the initiation and progression of cancer: variations in isoflavone metabolism, differences in the composition of isoflavones, and the processing of soy foods. Essentially, what the literature showed was that more studies are needed with larger sample sizes and more specific doses. Based on the inconclusiveness of the studies, supplementing with soy isoflavones is not recommended. However, consuming whole soy products, such as tofu, tempeh, soy flour, soy milk could be a beneficial and healthy source of protein, as it contains little saturated fat, high content of polyunsaturated fats, fiber, vitamins, and minerals.
Another warning about soy is that soybeans contain various substances that can block the absorption of certain minerals, including magnesim, calcium, iron and zinc, and that they contain inhibitors that block the uptake of certain enzymes, needed for protein digestion. However, fermenting soybeans (eg: such as in tempeh and miso) reduces or de-activates these inhibitors, and cooking also reduces the amount of these substances. As such, cooked products such as tofu and soy milk, will have reduced levels of these substances.
Soy processors have found another way to eliminate these anti-nutrient compounds from soy foods, which may be potentially harmful. To isolate the soy protein and create soy protein isolate (a key ingredient in most processed soy foods, and used to create textured vegetable protein [TVP]), soy beans are mixed in an alkaline solution to remove fiber, and separated in an acid wash usually made up of hexane, methanol, ethanol or isopropyl alcohol. Hexane is potentially toxic petrochemical solvent made by refining crude oil. While hexane does evaporate, not all of the hexane evaporates before the products are consumed. Tests have found that soy oils contained less than 10 ppm hexane residue, soy grits contained 14 ppm hexane residue, and soy meal contained 21 ppm hexane. The effects of consuming foods containing hexane-extracted ingredients are unknown, but over-consuming products with hexane residues could be harmful. To avoid consuming products that have used hexane in the extraction process, it is best to consume USDA organic soy products, as they prohibit the use of hexane.
The lesson from the literature on soy foods illustrates that consuming soy foods can provide a good source of protein, iron and fiber for one’s diet, but that it is best to eat in moderation. It is also best to eat whole soy foods such as tempeh, tofu and soy milk, as these have been minimally processed, and do not include additional potentially harmful substances. Processed soy products (such as tofu dogs, soy meats and soy burgers) do not provide the benefits of the whole food, are processed in high temperate conditions, which may reduce the healthy nutrients in soy, likely contain residues of potentially harmful substances and may also be laden with unhealthy oils to create a good texture. Eliminating meat products and exchanging them directly for soy products does not promote a balanced diet. Individuals wanting to follow a vegetarian or vegan diet should be careful to ensure that they obtain protein from a variety of sources, and not simply rely on one product. As with all things, eaten in moderation, soy can be a fine addition to one’s diet, but over-consuming soy may not be beneficial to one’s health.