Ah, soy. Such a divisive little bean! In one corner, we have the hardcore Paleo diet advocates who claim that soy is toxic; in the other, vegans who regularly consume soy milk, tofu, tempeh and other soy products.
Firstly, soy products include soy milk, tempeh, tofu, natto, edamame, soy yoghurt, texture vegetable protein (TVP), isolated soy protein powders (available as protein supplements or in processed foods), miso paste, miso soup, soy sauce and tamari.
So many clients I see in my clinical practice avoid soy, with the most common reasons for doing so being:
‘Soy contains estrogen and I don’t want to grow breasts’ (from the men)
‘Soy contains estrogen and I don’t want breast cancer’ (from the women)
‘I have a thyroid condition and soy is bad for the thyroid’
‘Soy is toxic’
‘Soy is all genetically-modified (GM)’
‘Soy contains anti-nutrients/enzyme inhibitors’
In order to thoroughly dispel all these myths, I’ve waded through a ridiculous quantity of scientific literature and will address each of these concerns in turn.
Does soy contain anti-nutrients and enzyme inhibitors?
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. Interestingly, the anti-nutrients in soy (and in other legumes, grains, vegetables and fruits) 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 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 calcium, zinc and iron. However, it is only thought that this inhibitory effect can lead to mineral deficiencies under ‘nonvaried and nonbalanced 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 (e.g. tomatoes, red capsicum or oranges), adding a side dish of fermented food such as miso, kimchi or sauerkraut (or consuming fermented soy in the form of tempeh), and adding a good source of beta-carotene to your meal (e.g. carrot, pumpkin, sweet potato or kale)(4).
Schlemmer et al recently reviewed the available scientific evidence for the role of dietary phytates 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 (the Bowman-Kirk inhibitor and the Kunitz soybean inhibitor), 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, for example, by soaking soybeans overnight and heat-treating the soybeans (as per the production of soy milk or tofu), or by fermentation (as per tempeh, natto or miso production) and we don’t eat soybeans raw, they are of little consequence 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 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 female reproductive hormone), but this doesn’t mean that soy products contain estrogen. Isoflavones can bind to estrogen receptors in the body, but they are much weaker than human estrogen. For a hormone to have biological activity, it needs to be able to bind to a receptor (much like a lock and key), which initiates the effects of the hormone within the cell. Isoflavones can either act like estrogen, by binding and activating the receptor, or have anti-estrogen activity, by binding to the receptor and hence preventing the more potent human estrogen from binding and having its effect. 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 enhance and block the action of estrogen. 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 have been isolated incidences reported of adverse effects of soy consumption in men, however, these men were consuming very large quantities of soy foods (in excess of 12 servings per day).
Soy Protein Quality
Protein quality is determined by two factors: amino acid content and digestibility. Proteins are made up of 20 different amino acids, of which 9 are essential, which means they can’t be made by the body and must be taken in through the diet. We don’t have a requirement for protein per se, rather a requirement for amino acids and nitrogen. Proteins in food are classed as either ‘complete’ or ‘incomplete’ proteins, based on the amounts of the 9 essential amino acids they contain. All animal products (meat, dairy, eggs and fish) are complete proteins, as they have similar levels of the essential amino acids to human tissues. Most plant proteins are incomplete proteins, but soy, buckwheat and quinoa are known to be complete proteins, so just comparing the amino acid content of soybeans, they are on par with meat, dairy, eggs and fish.
The most widely-accepted measure of protein quality is the Protein Digestibility Corrected Amino Acid Score (PDCAAS). Casein and whey from milk, egg, milk and soy protein all have a rating of 100%, which is the highest possible rating. Whole food soy products do have a lower digestibility compared to its isolated protein, and as such, the PDCAAS for soybean is slightly lower (91%), however, is on par with beef (92%)(11).
Soy & Thyroid Function
Soy is often referred to as goitrogenic or containing goitrogens, but what does this mean? Goitrogens are substances (chemicals found naturally in certain foods) that can suppress thyroid function and potentially cause a goiter, which is an enlargement of the thyroid gland. Many other foods contain also these chemicals, such as Brassica vegetables (cabbage, kale, broccoli, cauliflower, Brussels sprouts, kohlrabi), sweet potatoes, millet and linseeds, but luckily, they are inactivated by heating these foods, so they pose no threat to thyroid function.
With regard to soy products, the isoflavones in soy can also inhibit one of the enzymes involved in the production of thyroid hormones (thyroid peroxidase or TPO), but this only occurs in people who are deficient in iodine. The Recommended Daily Intake (RDI) for iodine is 150mcg. If you’re on a plant-based diet, the best sources are vegan multivitamins (which generally contain 100% of the RDI), iodised salt and sea vegetables (such as nori, wakame and dulse flakes). As a public health initiative to reduce the incidence of iodine deficiency, bread is required by law to contain iodised salt, however, 2 slices only contains around 30mcg (20% of the RDI), and organic breads are exempt from having to use iodised salt. Fresh food should be a good source of iodine, but unfortunately in Australia our soil is very low in iodine, which means that our fruits and vegetables aren’t very good sources of iodine.
If you have been diagnosed with borderline low thyroid function (subclinical hypothyroidism), there is some evidence that soy products can increase the progression of subclinical hypothyroidism to true hypothyroidism (12) so if you know your thyroid function is low, it may be wise to limit your intake of soy foods to 1 serve per day (1 cup soymilk, 100g firm tofu or tempeh or ½ cup edamame or cooked soybeans), and ensure you’re getting enough iodine.
If you have overt hypothyroidism and take thyroid hormone medication (thyroxine), soy products can interfere with the absorption of the medication, so it’s wise to separate any soy foods from taking these medications by 3 hours or so.
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 is equal to:
· 100g firm tofu or tempeh
· 150g silken tofu
· ½ cup (120) shelled edamame
· 1 cup soy milk
· 1 tablespoon of miso paste
Healthy soy consumption
- When choosing a soy milk, read the label to make sure it’s made from ‘whole soybean’ rather than from ‘soy protein’ or ‘soy protein isolate’
- Choose non-GM (genetically-modified) soy if that is important to your values. We’re lucky in Australia as we have strict food labelling laws which require GM foods to be declared on the label
- Go organic if you can afford it
- Avoid isolated soy protein (also known as soy protein isolate) in protein powders and bars, mock meats and textured vegetable protein (TVP). If you want a vegan protein powder, I’d choose one made from brown rice or pea protein instead
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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.
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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/