Whether or not soy is healthy or even safe to consume has been a source of continual controversy over the past few decades. In fact, it’s one of the most researched foods in the world. In the centre of this controversy are it’s effects on hormones and nutrient absorption – and all of the voices for and against it. Mixed messages make for confused consumers, and in such situations it might seem best to avoid soy to be on the safe side. Using research is always important when looking at the nutritional qualities of a food, so I’ve trawled through the latest evidence to come up with the current facts about soy.
Soy in our Food Supply
Initially, soy was mainly grown and eaten in Asia, where it has been a dietary staple for thousands of years. It was eaten in the form of whole soybeans, tofu and fermented foods like miso, soy sauce and tempeh (soy milk and many other products came about more recently). Now, soy is grown more widely and mostly used for animal feed. Due to advancements in food processing, different extracts from the soybean are used to bulk, stabilise, thicken, flavour and bind foods (amongst other uses), and soybean oil is one of the most abundantly used oils worldwide. So, even if you don’t intentionally consume soy products, if you eat bread, deli meats, muesli bars, margarine or really anything from a packet, you are probably ingesting some form of soy.
Soy and Safety
Let’s start off with the most common argument against soy: ‘soy has oestrogen, which causes cancer’.
The short story:
Soy contains ‘phytoestrogens’ (or ‘isoflavones’) which are structurally alike to the hormone oestrogen. Oestrogen causes some breast cancers to grow. However, phytoestrogens interact with breast cells differently and do not cause cancer. Instead, they block oestrogen from the cancer cells. Vegans regularly eating excessive amounts of soy can have increased levels of the cancer-promoting hormone ‘IGF-1’, however levels and cancer risk are similar to that of meat and dairy consumers.
The long story:
This claim originated from a few studies published around 20 years ago, showing that rodents with breast cancer who were fed soy ended up with faster-growing tumours than those not fed soy. The type of soy used in these studies was a purified form extremely high in isoflavones, a class of phytoestrogens or ‘plant oestrogens’ (found largely in soy but also in beans, nuts, seeds and wholegrains). Structurally, isoflavones are similar to the hormone oestrogen in our bodies. Oestrogen plays an important role in reproductive and sexual development, but it can also cause negative effects like encouraging the growth of certain hormone-sensitive cancers (like most breast and prostate cancers). The idea is that because of this, phytoestrogens will act in the same way as oestrogen and also promote cancer growth as seen in the rodents. Makes sense, right? But there’s something missing from this explanation. A key difference between phytoestrogen and oestrogen is that they bind (connect) differently to cells in order to bring about changes in the cells (and possibly cause growth of a cancerous tumour)*. This difference in binding means that phytoestrogen either amplifies or blocks the action of oestrogen in different cells. In breast cells, phytoestrogen blocks oestrogen so it cannot cause a breast tumour to grow. What about the studies in rodents, though? It turns out that phytoestrogens are used in a rodent’s body in very different ways to humans, so the results aren’t applicable to us.
Interestingly, what can amplify oestrogen in cancer cells, possibly leading to tumour growth in certain cancers, is actual oestrogen in the food supply. Oestrogen has been found in milk from pregnant cows and beef from cows injected with growth hormones.
The only potential cancer-related risk with soy consumption arises when large amounts are consumed regularly. Studies show that this can lead to an increase in IGF-1 (insulin-like growth factor 1), a hormone that promotes cancer growth. However, the level to which soy has been shown to increase IGF-1 is not unusually high – in fact, it’s about as high as most meat and dairy consumer’s IGF-1 levels, as animal protein causes IGF-1 to rise, too (vegans have much lower levels). It’s thought that this is because of the complete protein content of both animal protein and soy. It’s important to note that studies have not shown those with excessive soy intake to actually have an increase in incidence of cancer (like studies have shown with animal products), only an increase in a marker of the disease.
2. 'Feminising' Effects
One reason males tend to avoid soy is because of the belief that it causes disruption of hormonal levels which lead to various feminising effects and infertility. These claims mostly originated again from rodent studies, or small, low-quality human studies. A large majority of the evidence shows that soy consumption does not increase men’s oestrogen levels or reduce their testosterone levels and hence lead to feminisation, nor has it been found to lead to infertility (in either gender).
3. Thyroid Health
Another common reason for avoiding soy is due to it’s anti-nutrients (compounds in foods that can prevent our body from absorbing nutrients) and the related argument, ‘soy causes thyroid problems’. The mineral iodine is important for regulating thyroid hormones in the body, and a deficiency can lead to various symptoms including lethargy, hair loss and goitre, a condition where the thyroid gland in the throat swells up. Soy is a significant source of ‘goitrogens’, anti-nutrients that inhibit iodine absorption (other sources of goitrogens include cruciferous vegetables like kale, cabbage and broccoli). Cooking and possibly fermenting food greatly reduces the effects of goitrogens, so as almost all soy foods are consumed cooked (except for Nama Shoyu, a raw soy sauce alternative), goitrogens do not pose much of a health risk. Soy isoflavones can also interfere with thyroid hormone production (by inactivating the enzyme TPO), however studies show that consuming high amounts of isoflavones does not reduce thyroid function.
The only circumstances in which goitrogens pose a threat to human health are when someone already has low iodine intake or is consuming an excessive amount of raw cruciferous vegetables (more than about 1kg on a daily basis).
4. Nutrient Deficiencies
Soy also contains large amounts of phytates (as all legumes and grains do), which are anti-nutrients that can inhibit calcium, iron, magnesium and zinc absorption. While this is another reason people avoid soy (and in some cases, like the paleo diet, cut out all legumes and grains), it’s important to understand that phytates only limit some, not all, mineral absorption. While calcium and iron absorption from soy appear to be sufficient (and similar to absorption rates from milk and meat respectively), studies indicate that zinc absorption from soy food appears to be low. Fortunately, zinc can be sourced from other lower-phytate foods like nuts and seeds, as well as oats (which have a higher amount of zinc compared to phytates). To date, there are no studies looking at magnesium absorption rates from soy.
More recently research has indicated that phytates may not be so bad for us, and in fact have various health benefits, including anti-cancer effects (I plan on writing a blog on this in future). However, for those worried about phytates or dealing with nutrient deficiencies, there are a number of ways to improve nutrient absorption from soy foods:
Pairing them with vitamin C-rich foods (e.g. capsicum, tomatoes, berries, citrus fruits) to increase iron absorption
Pairing them with allium vegetables (e.g. onion, garlic, leek) and lysine-rich foods (e.g. all legumes, quinoa) to increase iron and zinc absorption
Avoiding high-oxalate foods (e.g. spinach, rhubarb, silverbeet) to prevent reduced calcium absorption
Choosing fermented soy products (e.g. tempeh, miso, natto), as fermentation reduces the phytate content
What about GMOs (Genetically Modified Organisms) and soy? A majority of the soy grown today is genetically modified in order to be resistant to pesticides used to improve crop yield. At this stage, there isn’t any evidence (i.e. long-term studies following humans eating GMO soy) to suggest that GMO soy causes any negative health effects.
Now that we know soy is safe to consume, is it worth eating anyway? According to a multitude of studies looking at soy and human health, yes. Although it was thought to promote breast cancer, the latest evidence shows that people who include soy as a part of their diet have either no risk or a reduced risk of developing breast cancer (with a particularly reduced risk when consuming soy during adolescent years). On top of this, studies show that consuming soy also appears to:
Reduce risk of prostate cancer
Have benefits for menopausal women by reducing hot flashes
Have mild benefits in heart disease by lowering LDL (‘bad’) cholesterol to some degree
Possibly prevent post-menopausal osteoporosis by increasing bone mineral density
How much soy?
To access the health benefits of soy, you don’t need to be eating a lot of it – around 1 serve each day is enough. Based on studies looking at soy intake amongst Asian populations, it appears that eating up to five serves of soy foods per day is healthy. To avoid potentially increasing IGF-1 (and risk of cancer), it’s best to limit soy intake to no more than 5 serves.
One serve of soy has roughly 25mg isoflavones and is equivalent to about 1 cup of soy milk, ½ cup (40g) of soy beans, 100g of tofu or 50g of tempeh.
The Bottom Line
Unfortunately soy has been the receiver of much criticism due to poorly designed and concluded studies. Soy is not only safe to eat, but it provides us with important nutrients that can provide health benefits. The phytoestrogens in soy provide antioxidants, and soy is a great source of fibre, resistant starch, complete protein, iron and calcium if fortified (most soy milk and tofu products are). Including a few servings of soy foods in our daily diet is healthy, particularly if those serves come from minimally processed soy products such as soy milk, tofu and tempeh. However, like with almost any food, don’t overdo it! For non-vegans, soy is a healthier option than meat and dairy products, and soy products are a great replacement for these.
*For more detailed scientific information on soy and health, I’d highly recommend reading dietitian Jack Norris’ information about it on his website.
Klein M, Nahin R, Messina M, et al. 2010. Guidance from an NIH workshop on designing, implementing, and reporting clinical studies of soy interventions. J. Nutr, 140(6), pp 1192S-1204S.
Ma D, Qin L, Wang P & Katoh R, 2008. Soy isoflavones intake increases bone mineral density in the spine of menopausal women: meta-analysis of randomised controlled trials. Clin Nutr, 27: pp 57-64
Messina M, 2010. Insights Gained from 20 Years of Soy Research. J. Nutr, 140, pp 2289S-2295S
Messina M, 2010. Soybean isoflavone exposure does not have feminising effects on men: a critical examination of the clinical evidence. Fertil Steril, 93(7): pp 2095-2104
Messina M, Nagata C & Wu A, 2006. Estimated Asian Adult Soy Protein and Isoflavone Intakes. Nutrition and Cancer, 55(1), pp 1-12
Murray-Kolb L, Welch R, Theil E & Beard J, 2003. Women with low iron stores absorb iron from soybeans. Am J Clin Nutr, 77(1): pp 180-184
Nelson H, Vesco K, Haney E, et al. 2006. Non-hormonal therapies for menopausal hot flashes: systematic review and meta-analysis. Jama, 295: pp 2057-2071
Oseni T, Patel R, Pyle J & Jordan V, 2008. Selective estrogen receptor modulators and phytoestrogens. Planta Med, 74(1): pp 1656-1665
Tang AL, Walker KZ, Wilcox G, et al. 2010. Calcium absorption in Australian osteopenic post-menopausal women: an acute comparative study of fortified soymilk to cows’ milk. Asia Pac J Clin Nutr, 19(2): pp 243-249