Soy Misconceptions

by Jordan Stachel MS, RDN

Soy protein seems to be a constant topic of debate within the nutrition and health community. Some people have become fearful of soy, thinking that it may dangerously boost estrogen levels, or feel that the plant-based protein is inferior to animal-based protein. The truth is: soy is a great source of protein that may provide more health benefits than many people realize. While it is important to be an educated consumer, soy, when included as part of a healthful, well-rounded diet, is absolutely nothing to fear.

Myth #1: Soy contains estrogen and therefore can increase the risk of cancers/raise estrogen levels too high

The first key to understanding soy protein is understanding the mechanisms by which soy acts on within the body. Soy contains isoflavones which are organic compounds related to flavonoids that act as phytoestrogens. Isoflavones are powerful antioxidants and soybeans are the most common source in the diet. Isoflavones are phytoestrogens, meaning that they are estrogen-like compounds sourced from plants. Much of the concern surrounding soy lies in its isoflavone/phytoestrogen content. However, research shows that isoflavones found in soy do not negatively affect one’s health or increase one’s risk for cancers (1). This is because the hormone estrogen that is produced in the body is not acted upon in the same way as the isoflavones in soy. While isoflavones found in soy have a similar chemical structure to the hormone estrogen, its action on these receptors is not harmful for the body, rather the chemical structure simply allows for isoflavones to bind to alpha and beta receptors.

It is notable that the preference for isoflavones is for the beta estrogen receptor. Isoflavones have been shown to have selective effects on some tissues and are thus classified as selective estrogen receptor modulators (SERMS) (1). While these SERMS exert estrogen-like effects when activated under certain circumstances, research also shows that some SERMS are used to treat breast cancer and osteoporosis, a common concern and misconception surrounding the dietary consumption of soy-based products. Because phytoestrogens don’t always mimic estrogen in the body, and in some cases, have the ability to block estrogen, soy consumption can actually be cancer and osteoporosis-protective in certain individuals (2).

Myth #2: Soy protein is not protective against diseases

Another benefit to soy protein is that it can actually be protective against some diseases. Research shows that soy protein can be protective of cardiovascular disease, as it reduces serum concentrations of total cholesterol, low-density lipoproteins (LDL) and triglycerides (3). When consuming around twenty-five grams or more of soy-based protein per day, individuals with high cholesterol saw beneficial results. While the exact mechanisms for the ways in which soy protein helps to lower serum cholesterol are unknown, it is hypothesized that this relationship exists due to the upregulation of the hepatic LDL receptor that occurs during the digestion of soy protein (1).

Due to soy’s ability to help decrease serum cholesterol and thus, reduce one’s risk for developing cardiovascular disease, dietary intake of soy foods also helps to decrease one’s risk for stroke and overall arterial damage. Due to the composition of soy-based protein products and that they are typically lower in saturated fats compared to animal-based protein products, increasing one’s intake of soy typically lowers overall saturated fat intake. Replacing saturated fat in the diet with polyunsaturated and monounsaturated fatty acids, the fatty acids found in soy protein, has been shown to be protective of heart disease, high blood pressure and stroke (1).

Lastly, research indicates that soy protein can be protective of bone health and osteoporosis, a common concern for the aging population, particularly aging women. This is due to the fact that soy protein provides skeletal benefits as it exerts a synthetic isoflavone within the body (1). Research shows that women who consumed more soy protein had less fractures and lower incidences of developing osteoporosis than women who consumed less soy protein (1). These studies show promising results for post-menopausal women, as an increase in soy has been positively linked to improved bone mineral density, increasing bone turnover rates and improving bone calcium content (1).

Myth #3: Soy protein is inferior to animal-based protein

Another common myth surrounding soy is that it is lacking when compared to animal-based protein. Soy contains all of the essential amino acids and is found in many plant-based products including soybeans, tofu, soy milk, tempeh, miso and soy sauce (4). The quality of soy protein measured by the protein digestibility corrected amino acid score (PDCAAS) range from 0.9 to 1.0, with 1.0 being the highest digestibility score achievable. Foods high in soy protein are also typically lower in carbohydrates and moderate in poly and monounsaturated fatty acids, making them generally heart-healthy and mindful dietary choices (1).

Next, dietary sources of soy protein are also good sources of vitamins and minerals, most notably, potassium. In addition, soybeans are also a good source of iron and calcium, with high levels of bioavailability accessible to the body for direct absorption. This is of note, as individuals who follow a plant-based diet can sometimes struggle with dietary intake and absorption of iron.

Are There Downsides to Soy?

After a review of the current literature, the main downside to conventional soy consumption is the possible exposure to cadmium. Cadmium is a heavy metal that is mainly ingested through smoking and via certain foods. High levels of cadmium have been linked to kidney and liver damage as well as unfavorable health conditions including osteoporosis, hypertension, peripheral artery disease, diabetes and some cancers (5). Soy-based products are high in cadmium due to the fact that soybeans can absorb cadmium from the soil and retain it in seeds. This suggests that when buying and consuming soy-based products, it is important to buy organic, as the soil composition directly affects the cadmium levels in the food.

In conclusion, there are several myths surrounding soy that do not prove to be exclusively factual. While it is important to be a mindful consumer and buy organic soy-based products, soy protein can serve as a healthful addition to a well-rounded diet. Soy is a good source of protein that can be beneficial when consumed in appropriate quantities. Just as with any food/food group, it is important to consume mindfully and in moderation.

References:

  1. Messina M. Soy and health update: Evaluation of the clinical and epidemiologic literature. Nutrients.2016;8(12):754. https://www.ncbi.nlm.nih.gov/pubmed/27886135. doi: 10.3390/nu8120754.
  2. Straight talk about soy, from: https://www.hsph.harvard.edu/nutritionsource/soy/.
  3. Montgomery K. Soy protein. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1595159/. Updated 2017.
  4. Michelfelder AJ, MD. Soy: A complete source of protein. American Family Physician. 2009;79(1):43-47. https://www.clinicalkey.es/playcontent/1-s2.0-S0002838X09600320.
  5. Adams SV, Newcomb PA, Shafer MM, et al. Sources of cadmium exposure among healthy premenopausal women. Science of the Total Environment. 2011;409(9):1632-1637. https://www.sciencedirect.com/science/article/pii/S004896971100074X. doi: 10.1016/j.scitotenv.2011.01.037.