Ah, soy. Such a divisive little bean!
Many clients I see in my clinical practice deliberately avoid soy, with the most common reasons for doing so being:
‘I have heard soy causes breast cancer’
'Soy contains anti-nutrients'
‘I have a thyroid condition and heard soy is bad for the thyroid’
‘Soy is all genetically-modified’
In order to thoroughly dispel all these myths, I’ve waded through the scientific literature on the subject and will address each of these concerns in turn.
Firstly, soy products include foods made from soybeans, such as soy milk, tempeh, tofu, edamame, soy yoghurt, texture vegetable protein (TVP), isolated soy protein powders (available as protein supplements or in processed foods such as vegetarian sausages), miso paste, soy sauce and tamari.
Does soy contain anti-nutrients?
Anti-nutrients are defined as dietary components that can reduce or inhibit the utilisation of specific nutrients in food. They are naturally-occurring in almost all plant foods, as a defense mechanism against predators. The anti-nutrients in soy including protease (enzyme) inhibitors, saponins, phytates and lectins are increasingly being considered as having numerous health benefits (1), including being protective against cancer (2).
Saponins are a broad range of compounds found in a number of plant foods, including soybeans. The name is derived from their ability to form a soap-like foam in liquids (if you’ve ever rinsed quinoa you’ll now know what the foam is!). As some of the saponins have been shown to be highly-toxic, the entire class of compounds known as saponins have all been ‘mistakenly labeled' as toxic (3).
Lucky for soybeans, the saponins found in soybeans have no reported adverse effects in humans, and have are being extensively researched for their antioxidant properties, cholesterol-lowering effect, attenuating (blunting) the blood glucose response to a meal and anti-cancer potential (3).
Phytic acid (and its salt, phytate) is a storage form of phosphorous and minerals for the growing grain or seed kernel. Phytate is found in cereals, legumes, oilseeds (including soybeans) and nuts, and can bind to and inhibit the absorption of certain minerals including zinc and iron. However, it is only thought that this inhibitory effect can lead to mineral deficiencies under ‘non-varied and non-balanced dietary conditions’ (4).
Furthermore, the inhibitory effect of phytate on iron and zinc absorption can be reduced by adding a good source of vitamin C to your meal (such as tomatoes or red capsicum), and adding a good source of beta-carotene to your meal (such as carrot, pumpkin, sweet potato or kale) (4).
Schlemmer et al recently reviewed the available scientific evidence on dietary phytates and human health and found evidence for a blood glucose lowering effect, antioxidant capacity and anticancer activity, and in light of these findings, noted that terms for phytate such as ‘antinutrient’ should ‘belong to the past’ (4). Another author (Shamsuddin, 2002) went so far as to propose that phytic acid should be considered an ‘essential nutrient (perhaps a vitamin)’ (5).
In summary, in well-balanced diets, the inhibitory effect is relatively low and phytate is increasingly being considered a beneficial component of food.
Many foods contain enzyme inhibitors, including chicken eggs, potatoes and soybeans. The enzyme inhibitors in question in soy are trypsin inhibitors, which block the action of human the digestive enzymes responsible for breaking down proteins into smaller peptides and amino acids (such as trypsin and chymotrypsin). However, as the majority of these enzyme inhibitors are inactivated by soaking soybeans overnight and heat-treating the soybeans (as per the production of soy milk or tofu), or by fermentation (as per tempeh or miso production). As we don’t eat soybeans raw, they are of no significance to human health .
Lectins (also known as haemagglutinins) are widely-distributed in the plant foods, including soybeans (known as soybean agglutinin or SBA). These proteins have been extensively studied since early experiments showed that lab animals (mainly rats) experienced growth problems when fed raw soybeans (6). Further animal studies have shown that lectins from raw soybean withstand digestion in the gastrointestinal tract and can bind to the intestinal epithelium (the cells lining the gastrointestinal tract) and cause a number of adverse effects including increased intestinal permeability (‘leaky gut’) (6). However, studies in animals using raw soy products are of little relevance to human health as the lectins in soy are easily deactivated by 'wet' heat, and hence pose no threat to human health (6)(7).
Does soy contain estrogens?
Soy is rich in a class of phytonutrients known as isoflavones, which are classed as phytoestrogens (‘phyto’ meaning plant-derived). Other plant foods also contain isoflavones, including flaxseeds and whole grains. The isoflavones in soy are structurally similar to human estrogen (a reproductive hormone), but this doesn’t mean that soy products contain estrogen. Isoflavones can bind to estrogen receptors in the body, but they generally exert a much weaker effect than human estrogen, and can have pro- or anti-estrogenic effects in the body.
There are also two types of estrogen receptors (ER), named ER-alpha and ER-beta, which are found in different locations in the body. Isoflavones have been shown to be ‘selective’ in their binding to these receptors, choosing to bind to ER-beta over ER-alpha, and hence are known as selective estrogen receptor modulators (SERMs). This helps to explain how they can both enhance and block the action of estrogen, depending on the tissue type. They can also reduce levels of circulating estrogen by increasing the level of sex hormone binding globulin (SHBG), which binds and inactivates estrogen.
Soy & Breast Cancer
Many people believe that the phytoestrogens in soy products may increase the risk of breast cancer, however, the majority of the evidence suggests that soy consumption is either neutral or protective against breast cancer. A review in 2010 in the Journal of Nutrition found that women who consumed moderate amounts of soy (1-2 servings per day) throughout their life have lower risk of developing breast cancer risk than women who do not consume soy; however, this protective effect may originate from soy intake early in life (in childhood and adolescence). The authors also concluded that soy consumption does not increase the risk of breast cancer recurrence in women who have previously survived breast cancer (8).
Soy & Men’s Health
Soy products may help to prevent prostate cancer in men. A recent meta-analysis (which pools a number of studies) found that men with the highest intake of soy had a statistically-significant reduction in prostate cancer risk, compared to the men who consumed the least soy (9).
A recent review on the topic found that neither isoflavone supplements nor soy products affected total or free testosterone levels, and there was no evidence to suggest that isoflavone exposure affects estrogen levels (10).
There are a handful of published case reports of men suffering from adverse hormonal effects relating to excessive soy consumption, but these men were all consuming very large quantities of soy (in excess of 12 servings per day). The main effects reported were growth of breast tissue (gynecomastia) and reduced libido.
How much is safe to have each day?
The use of soy protein for lowering LDL cholesterol levels has been extensively studied, which has resulted in the recommendation to have 25g of soy protein each day (the equivalent of 2-3 servings). This represents a minimum level of intake to reap the cholesterol-lowering benefits, and could be used as a conservative upper limit of intake if you are still concerned soy.
Dr. Michael Greger (of NutritionFacts.org) recommends an intake of 3-5 servings per day to maximise the health benefits and minimse any risk of harm from excessive consumption (13).
1 serve of a soy product contains around 7g of protein and 25mg of isoflavones, and is equal to:
- 100g firm tofu
- 50g tempeh
- 1 cup soy milk
My Tips for Healthy Soy Consumption
Choose whole food soy products such as tofu, tempeh, and soy milk made from whole soybeans (rather than from soy protein or soy protein isolate)
Avoid highly-refined soy products such as Textured Vegetable Protein (also known as TVP, which is a product made from defatted soy flour). Choose whole legumes (beans, peas, chickpeas and lentils) as a protein-rich option with main meals instead
Avoid soy protein bars, powders and other products in the interests of eating a whole food, plant-based diet
If you have elevated cholesterol levels, include three servings of soy products daily - this is a well-studied intervention for reducing cholesterol levels
If you'd prefer to be conservative with your soy product intake, don't exceed 1-2 servings per day (known as a 'moderate' intake, and in line with typical intakes in Asian countries such as Japan)
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2. Mathers JC. Pulses and carcinogenesis: potential for the prevention of colon, breast and other cancers. Br J Nutr. 2002 Dec;88(S3):273.
3. Shi J, Arunasalam K, Yeung D, Kakuda Y, Mittal G, Jiang Y. Saponins from edible legumes: chemistry, processing, and health benefits. J Med Food. 2004;7(1):67–78.
4. Schlemmer U, Frølich W, Prieto RM, Grases F. Phytate in foods and significance for humans: Food sources, intake, processing, bioavailability, protective role and analysis. Mol Nutr Food Res. 2009 Sep;53(S2):S330–75.
5. Shamsuddin AM. Anti-cancer function of phytic acid. Int J Food Sci Technol. 2002 Oct 1;37(7):769–82.
6. Liener IE. Implications of antinutritional components in soybean foods. Crit Rev Food Sci Nutr. 1994 Jan;34(1):31–67.
7. Ma Y, Wang T. Deactivation of Soybean Agglutinin by Enzymatic and Other Physical Treatments. J Agric Food Chem. 2010 Nov 10;58(21):11413–9.
8. Hilakivi-Clarke L, Andrade JE, Helferich W. Is Soy Consumption Good or Bad for the Breast?123. J Nutr. 2010 Dec;140(12):2326S – 2334S.
9. Yan L, Spitznagel EL. Soy consumption and prostate cancer risk in men: a revisit of a meta-analysis. Am J Clin Nutr. 2009 Apr 1;89(4):1155–63.
10. Messina M. Soybean isoflavone exposure does not have feminizing effects on men: a critical examination of the clinical evidence. Fertil Steril. 2010 May;93(7):2095–104.
11. Schaafsma G. The protein digestibility–corrected amino acid score. J Nutr. 2000;130(7):1865S – 1867S.
12. Sathyapalan T, Manuchehri AM, Thatcher NJ, Rigby AS, Chapman T, Kilpatrick ES, et al. The Effect of Soy Phytoestrogen Supplementation on Thyroid Status and Cardiovascular Risk Markers in Patients with Subclinical Hypothyroidism: A Randomized, Double-Blind, Crossover Study. J Clin Endocrinol Metab. 2011 Feb 16;96(5):1442–9.
13. Michael Greger. How much soy is too much? [Internet]. NutritionFacts.org. [cited 2014 Nov 12]. Available from: www.nutritionfacts.org/2013/02/19/how-much-soy-is-too-much/