Is Soy Healthy?
By Carlijn Wagenaar, MD and Wendy Walrabenstein, RD from PAN The Netherlands.
Whether you do a google search, pick up a book, watch the news, or scroll through your Instagram feed, you will find polarizing facts about soy. So, what should we believe? Are soy lattes the reason the precious Amazon Rainforest is being cut down? Is soy going to cause so-called “man boobs”? Is soy a genetically modified crop? Read this article for evidence-based answers to these questions and more.
The truth about soy
76% of all soy produced worldwide is used to feed livestock. While only 7% of the world’s soy production ends up in products we recognize as “soy products” such as dairy substitutes (like soy milk and yoghurt), soy sauce, and meat substitutes, including tofu and tempeh (1). Another 13% is used as vegetable oil in, for example, processed foods like margarine, sauces, cookies, chocolate, and ice cream. About half of all products in the supermarket contain soy in one way or another.
Image: Hannah Ritchie and Max Roser (2021) – “Soy”. Published online at OurWorldInData.org. Retrieved from: ‘https://ourworldindata.org/soy‘ [Online Resource]
Using animals to convert plant protein to animal protein in meat and dairy products is inefficient. Eggs have the highest protein conversion efficiency, meaning 25% of the protein in animal feed is efficiently converted to protein in the animal product (2). The remaining 75% of protein is lost conversion. Poultry, pork, and red meat are less efficient with protein conversion efficiencies of roughly 20, 8.5, and 4% respectively (2). With this inefficient conversion other health promoting components of plant proteins are also lost, such as fiber, and instead animal proteins are packaged with saturated fat which have negative health effects.
Those who eat animal products are most likely also consuming soy, albeit indirectly. For example, 1 kg of chicken requires more than 600 grams of soy chicken feed. Furthemore, the average Dutch person eats about 23 kg of chicken per year, which translates to an indirect yearly intake of more than 13 kg of soy chicken feed (yum!) (3). If you also add the average consumption of soy from pigs and beef, you arrive at a total of more than 30 kg of soy that ends up on the plate of the average omnivore indirectly via the feed of chickens, pigs, and cows. And let us not forget the soy that’s required to feed the cows producing milk, yoghurt, and cheese, or the chickens laying eggs.
Image: Hidden soy: amount of soy feed required for animal products. Adapted from the Soy Barometer 2014 (4)
In comparison, it is estimated that the average Western vegan consumes about 10-12 grams of soy protein per day, which is approximately 350 ml of soy milk (5). If we translate this back to ‘animal feed’, this compares to eating 11 kg per year. An average omnivore consumes about 3 times as much soy as the average vegan via meat.
Soy production and its environmental impact
Nearly 70% of all soy used today comes from the US or Brazil (6). Over the last 60 years, the demand for soy has increased dramatically. Most of this growth has come from increased demand for processed soy for animal feed (70% of global demand), biofuels and vegetable oil. By 2013, the demand for processed soy increased from 88 million to 227 million tons. Yet, during this period, demand for soy products such as tofu and soy milk increased by only 3 million tons (6). This is no surprise. Global meat production has more than tripled over the past 50 years. This increase is strongest in poultry – the largest consumer of soy feed.
Although the area used to grow soy has more than quadrupled, several studies come to a similar conclusion: the direct causes of deforestation in the Brazilian Amazon rainforest are largely due to grasslands for cattle ranching, not soy production (7-9). Yet, soy still plays a significant role when we take its indirect impacts into account. Because most deforestation is driven by expanding pastures for beef, or soy to feed poultry and pigs, for consumers reducing meat consumption is a key way to make an impact (6).
Kid with Raspberries
Genetically modified soy
More than half of the world’s soybean production is genetically modified. Assuming 30 kg of animal feed is needed per year per person for meat production, it is likely that the average omnivorous Dutch person indirectly ingests about 15 kg of genetically modified soy via meat and poultry.
Most soy products made for human consumption, for example from brands such as Alpro, are not made from genetically modified soy (10). Any product containing genetically modified soy must state it clearly on its packaging. European legislation requires that if a product contains more than 0.9% genetically modified ingredients, it must be stated very clearly on the packaging (11). This rule does not apply to animal products derived from animals that have eaten genetically modified feed. In organic livestock farming, animals are not fed genetically modified soy. So, if you wish to eat meat, drink a glass of milk, or scramble an egg from animals that have not been fed genetically modified food, choose organic. If you don’t care, you will most likely be ingesting food produced using genetically modified soy. Whether or not genetically modified soy is bad for your health is not discussed here. The reality is: we don’t know.
Soy and breast cancer
Soy contains phytoestrogens. These are plant substances (phyto) that are similar to female hormones (estrogens) in the body. This is the reason why some people think soy increases the risk of breast cancer or causes so-called “man boobs”!
However, recent research shows that soy consumption does not increase the risk of breast cancer, and instead may decrease it (12). In the figure below you can see why this may be the case, explained using genistein, a phytoestrogen found in soy. Because estrogen and phytoestrogens look alike, phytoestrogens can attach to the body’s estrogen receptors. But, instead of activating these receptors (and thus stimulating tissue growth), in some cases, they occupy them and block the real estrogen from being able to bind (thus reducing the stimulation of tissue growth) (13).
Studies evaluating the effects of soy and phytoestrogens on breast cancer tumor incidence and growth in mice and rat models have shown variable results (14). Yet, it has become apparent that mice and rats metabolize phytoestrogens differently than humans and thus the translation of these studies to humans should be done with caution (15,16). Although more research needs to be done, recent studies conclude that soy is likely to have a protective effect against cancer in general, especially lung and prostate cancer (17). The possible protective effects of soy are based on studies with soy-containing foods, not soy supplements.
It is also interesting to look at the difference in breast cancer cases between Europe and Asia. Countries with the highest number of breast cancer cases, yet relatively low soy consumption, include Belgium, Denmark, France, and the Netherlands (18). In these countries, 315-335 out of 100,000 people (age-standardized) develop breast cancer yearly. Contrarily, in countries like Japan, China, Thailand, and Indonesia, known for relatively high soy consumption, breast cancer cases remain between 140 and 280 per 100,000 people (19). Although this may also be due to other factors, various review studies conclude that soy appears to have a protective effect against breast cancer, especially in women who have had breast cancer before (20, 21). On the other hand, dairy from cows has been linked to a higher risk of death from breast cancer and a higher risk of prostate cancer in men (22, 23). Also, red meat has also been associated with an increased risk of cancer in general and in particular breast cancer (24).
Should we be worried about phytic acids?
Phytic acids are mainly found in nuts, grains, and legumes, including soy. Phytic acids are said to hinder the absorption of minerals such as zinc, iron, and calcium as they can bind to minerals in our body and are then excreted through the intestines. As a result, some may refer to them as anti-nutrients and therefore vegans who consume a relatively large amount of phytic acids could theoretically suffer significant deficiencies. However, this does not appear to be the case (25).
Studies show that consuming too many phytic acids in a deficient diet (think of developing countries or one-sided diets) can indeed aggravate the situation. Yet, this does not appear to be the case in those eating a well-balanced diet. Given the diseases that we frequently see in Western countries, it is much more interesting to look at the benefits of phytic acids. For example, phytic acids appear to be an antioxidant and have protective effects against cancer and atherosclerosis (26). Also, fermented soy products such as tempeh, miso, and soy yoghurt contain fewer phytic acids than non-fermented products, such as tofu, because they are broken down during the fermentation process.
Soy and thyroid health
Research and experience show that soy can have some impact on the thyroid gland (27, 28). Although research shows that soy does not cause a slow thyroid gland (hypothyroidism), soy can impair the absorption of iodine, and iodine is needed to make thyroid hormones. It is therefore important to ensure that the intake of iodine is also sufficient. Iodine can be found in iodized salt and bread, or seaweed. In general, if you use thyroid medicine, it is good to consult a doctor or dietician, because soy (supplements) and fibre-rich foods can reduce the absorption of these medicines (29).
Other health benefits of soy
Inflammation or chronic, low-grade inflammation in the body is a root cause of many diseases of affluence. Type 2 diabetes, cardiovascular disease, and cancer are associated with inflammation. Soy consumption appears to protect against inflammation, lowers LDL (‘bad’ cholesterol), reduces hot flashes, promotes glucose metabolism, relieves depressive symptoms and improves skin health (30-32). To unlock the LDL-lowering potential of soy, aim for an intake of 25 g soy protein per day as its effects have been shown at this level (31).
So, is soy healthy?
Soy can be a healthy choice as part of a balanced diet. Not only does it appear to be protective against certain types of cancer, as well as breast cancer, but it can also help to reduce cholesterol, inflammation, and hot flashes, and improve blood sugar levels, depressive symptoms, and skin health. Soy products are also a great replacement for dairy foods as they are higher in protein than other plant-based milk and yoghurts (250 ml soy milk = 8.25 g protein v.s. 250 dairy milk = 9 g protein). Additionally, when choosing soy products, especially as a replacement for dairy products, look for plant milk or yoghurt fortified with calcium.
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Carlijn Wagenaar, MD is the Chairperson for the branch of PAN in The Netherlands. She’s an MD and PhD researcher at Reade Rheumatology and Amsterdam University Medical Center. She investigates the influence of a plant-based lifestyle program on patients with rheumatoid arthritis and osteoarthritis, especially the microbiome and implementation in healthcare.
Wendy Walrabenstein, RD is the Treasurer for the branch of PAN in The Netherlands. She’s a dietician and PhD researcher at Reade Rheumatology in Amsterdam and Amsterdam University Medical Center. She investigates the effect of a plant-based lifestyle program on people with rheumatoid arthritis and osteoarthritis. Author of the book “Food Body Mind” which addresses the effects of lifestyle on inflammation and ageing, lecturer in Nutrition & Dietetics at the Amsterdam University of Applied Sciences, but also an economist with 15 years of experience in the international financial world.
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