Why are some people so afraid of soy?

This article is the first in a monthly series where UNH grad students answer common nutrition questions, topics and trends. This is a partnership with Nutrition Connections and the NUTR 860: Behavioral Nutrition and Counseling graduate course of the Department of Agriculture, Nutrition, and Food Systems in the College of Life Sciences and Agriculture.

People have been eating soy for thousands of years. With origins in Asia, it has since become popular all over the world in recent centuries and is used for a wide number of products.(1,2) Although it is common knowledge that soy is in familiar foods, such as soy milk and tofu, and soy is also an ingredient in retail food products, infant formula, and even in animal feed.(3) Soy is a member of the legume family, meaning it is related to plants such as beans, peas, and lentils, and is a frequently used plant-based protein source.(4) And, unlike other plant-based proteins, it is considered a complete protein, meaning it contains all nine essential amino acids (amino acids are the building blocks of protein). (5) Soy is often considered more comparable to animal protein due to its high protein, higher fat, and lower carbohydrate content, than other legumes like black beans, which have higher concentrations of starch.(6) However, despite its ancient role in society and its desirable nutrition content, some individuals still feel apprehension of soy’s hormonal effects.

Our bodies contain many different hormones to help us function. Estrogen is one of the primary female sex hormones and is responsible for reproduction and female physical features. Though it plays a valuable role in both sexes, it is less concentrated in men.(7) Throughout the lifespan, and for a variety of reasons, estrogen levels fluctuate, which can impact the body in a variety of ways. For example, females with high levels of estrogen might experience weight gain, whereas men might experience swelling of breast tissue.(7)

Some men are worried about soy’s potential “feminizing” hormonal effects9 because soy contains large amounts of isoflavones, a substance derived from the phytoestrogen family that can mimic estrogen. Though isoflavone concentration is highest in soy, it is also present in other plants, such as wheat, apples, and carrots. A large amount of research has been conducted on isoflavones, finding them to generally be beneficial for human health. It is easy to get confused between phytoestrogen and estrogen since they are similar in structure and name.(8)

The primary misunderstanding of soy is that isoflavones have a feminizing effect on the body. Despite the circulating rumors surrounding soy’s distorting of sex hormones in men, there is little evidence to support it. Isoflavones can influence hormones, but are much weaker than estrogen. A couple of cases have been reported describing erectile dysfunction and low testosterone and/or sperm levels, also known as hypogonadism, with the consumption of soy. However, these individuals were eating over 150 times the average dietary isoflavone intake in Western countries! This is about 2 mg/day, or 1/3 cup of soy milk.(5,10) Another individual who was reporting symptoms was drinking over 3 quarts of soy milk, or 12 cups a day.(11) These cases were extremely rare and occurred under unusual circumstances that likely would not apply to the general population. Despite these potentially concerning occurrences, there have been no harmful side effects reported in individuals reporting normal amounts of soy consumption.(12)

Not only is there no need to be afraid, but soy has been well-researched on its benefits in reducing the risk of several chronic diseases, including the reduction of heart disease, osteoporosis, and type 2 diabetes.(8) It also boasts an impressive vitamin and mineral content, including high calcium and iron concentration, two nutrients of concern in the vegetarian diet and in other special populations.(12) Soy is also high in fiber, B vitamins, potassium, and magnesium, so don’t be afraid to give it a try!(5)

Because soy is in such a wide diversity of products, there is no clear adequate intake. The general recommendation is that adults could benefit from 2-4 servings of soy a day, preferably from a variety of sources, and to replace less healthy choices, such as red meat.(12) Since it is present in so many different types of foods, serving sizes vary. For example, if consuming soybeans or edamame, it is recommended to follow the serving size of other legumes, ½ cup. Likewise, a serving of soy milk is 1 cup, which is the same serving for dairy milk.12

 Like many nutrition topics, the science around soy is constantly advancing. And like most foods, soy is a complex substance made of many different compounds and it should not be reduced to its isoflavone content. Thousands of years of use and its evolution into thousands of food products, as well as the current evidence, suggest that there is nothing wrong about reaching for that glass of soy milk, and in fact, it might serve you some good.