This article was published in the October 2018 edition of Complete Nutrition (CN) Magazine.

Soya (or ‘soy’) is the protein which comes from soybeans. There are many different forms of soya products, such as: edamame beans, tofu, soya meat (i.e. textured vegetable protein or TVP), natto (fermented soybeans), miso, tempeh, bean curd, soy sauce, and soya-based alternatives to dairy like soya milk and soya yoghurts.

There are a lot of mixed messages about whether soya is a healthy food to include in our diet, or whether it is harmful.

This article examines the evidence related to the nutritional impact of soya. 

Nutritional Composition:

Soybeans are one of the only plant based sources of protein which contains all nine essential amino acids in significant amounts.

This can be useful for vegans, as other vegan sources of protein are often low in the essential amino acid lysine.

Soybeans are also low in fat and provide: fibre, calcium, iron, phosphorus, magnesium, potassium and manganese.

Vegetarian and vegan meat alternatives which are made from soya protein, such as vegetarian ‘chicken’ and ‘sausages’, are usually:

• Low in calories
• Low in fat
• Contain a good amount of protein
• Contain fibre

However, some of these products can be high in salt and they are not always fortified with iron, zinc and B vitamins.

Products which use the entire soybean such as tofu, tempeh and bean curd also tend to be low in calories and fat, as well as being good sources of protein and calcium.

Soya based dairy alternatives like soya milk and soya yoghurt contain similar amounts of protein to cow’s milk, whereas other plant based milks like almond milk, oat milk and coconut milk are often very low in protein. However it is important to check the labels of these products to make sure that they are fortified with calcium and vitamin B12.

Very few soya milks contain iodine, but fortunately this is beginning to change.

Where soya milk is not fortified with iodine, it is important to include other sources of iodine in the diet (such as fish, shellfish or eggs).

This is because in the UK dairy is our main source of iodine. Seaweed is also high in iodine, but there can be a risk of getting too much iodine from this (especially from brown seaweed like kelp). Therefore, it is advised not to eat seaweed more than once per week, especially for pregnant women¹. 

Iodine supplements, which contain no more than 150 μg of iodine per day, can be useful for some people who don’t eat fish or dairy. However, seaweed or kelp-based supplements are not recommended, and those with thyroid issues are advised to speak to their doctor before commencing iodine supplements¹.

Health Benefits of Soya:

Soya has been found to have LDL-cholesterol lowering properties, which in turn improves heart health and lowers the risk of heart disease².

For example, a meta-analysis by Anderson and Bush (2011) concluded that consuming an average of 15 – 30g of soya protein per day (which is equivalent to 1-2 servings of soya protein) was associated with a significant reduction in LDL cholesterol of roughly 5%³.

Soya may also help to keep our blood vessels healthy, which further reduces the risk of heart disease by preventing the development of thrombosis and atherosclerosis^2,4. But more research is needed to look into this.

Furthermore, products which include soya protein often replace other sources of protein in the diet which are higher in saturated fat, like red meat. This may further boost the heart healthy effects of soya.

Some studies have found that a regular intake of soya is also associated with good bone health, in terms of reduced bone loss in younger postmenopausal women and improved bone mineral density within Asian populations⁵. However, the evidence is mixed about this as some randomised controlled trials using soy isoflavones have not found much of an improvement in bone mineral density⁶. Therefore more research is needed to investigate the potential effects of soya on bone health.

Soya has also been found to reduce the frequency and severity of hot flushes.

For example, meta-analysis data has identified that consuming 2-3 servings of soya-based foods per day could reduce symptoms of hot flushes in menopausal women by roughly 20%⁷. However, it is important to note that hormone replacement therapy (HRT) is still the most effective and evidence-based way to manage menopause related hot flushes².

Soya and Cancer:

Soya contains phytochemicals called isoflavones which are thought to act in a similar way to the hormone oestrogen in our body; however the effects of isoflavones are milder than oestrogen. As high levels of oestrogen are linked with a higher risk of breast cancer, it was previously thought that consuming soya isoflavones may also increase this risk.

Some older studies found a link between consuming soya and increased breast cancer risk in animals. However, we now know that humans breakdown and absorb soya in a different way to animals.

Overall evidence from human studies shows that consuming soya doesn’t increase the risk of cancer^8-9.

In fact the American Institute for Cancer Research (AICR) report that “evidence is suggestive, but too inconsistent to conclude, that soy reduces risk of breast cancer”⁸. The AICR also report that there is limited evidence which has found an association between soy or isoflavone intake and a lower risk of prostate and stomach cancer⁸. Furthermore, a small amount of studies have found an association between soya or isoflavone intake and a lower risk of colon and lung cancer⁸. However, our genes, gut bacteria and how different people metabolize soya may also play a role in how this affects cancer risk for different people, and research is ongoing in this area⁸.

Recent good quality research has found that consuming 1-2 portions of soya per day does not cause any harm for breast cancer survivors⁹.

This may even be protective for those using tamoxifen (breast cancer prevention medication)⁹. In terms of prostate cancer survivors, there has been no harm related to consuming soya identified and soya consumption may be protective^8,10. However, further research is needed in both instances. 

Overall, there appears to be no increased risk of cancer from having 1-2 servings or soya per day. Consuming soya may even reduce the risk of certain types of cancer, but we need more human studies to look into this.

Soya and Male Fertility:

Some weak studies have found that the oestrogen-like effect of isoflavones may reduce testosterone levels in men and cause: reduce fertility, erectile dysfunction and increased breast tissue to form. But larger, better quality studies have not found this to be the case^11-12.

For example, a meta-analysis from 2010 found that “neither soy foods nor isoflavone supplements alter measures of bioavailable testosterone concentrations in men”¹².

Therefore, soya is considered to be safe for men to consume.

Soya and Thyroid Health:

The latest research shows that consuming soya is not harmful for people who have a healthy thyroid gland¹³.

However, having a low intake of iodine along with a high intake of soya may increase the risk of developing hypothyroidism¹³.

People with an under-active thyroid gland (hypothyrodism) can usually continue to consume soya containing foods. But as soya isoflavones can reduce the absorption of thyroid hormone medication, they should discuss this with their doctor and have their thyroid levels closely monitored in case their dose of thyroid hormone needs to be increased^2,13.

Soya and Pregnancy:

Soya containing foods are considered safe to consume during pregnancy¹⁴.

But, it may be wise for those who are pregnant or planning a pregnancy to keep soya intake to a moderate level (i.e. 1-2 potions per day) and to avoid using soya protein or isoflavone supplements, as these often contain a higher levels of isoflavones¹⁵.

Soya and Infant Health:

Most babies under six months old are not advised to consume soya.

This is because the possible oestrogen-like effects could affect their reproductive development. This is more of a concern with infants as their lower body weight means they would be consuming much higher levels of isoflavones per kg, when compared to other children or adults who consume soya as part of a varied diet^16-17.

Soya infant formula also contains glucose which can be damaging for infants’ teeth¹⁶. Furthermore, soya protein is slightly lower in the essential amino acid methionine and is less digestible for infants than protein derived from cow’s milk¹⁷.

Soya formulas are usually only advised for babies under 6 months of age if they have certain medical conditions, such as: galactosemia or primary lactase deficiency¹⁷. Soya infant formulas can also be used in some cases when the family chooses to follow a vegan diet¹⁷.

There is no evidence to support the use of formula derived from soya protein for preterm infants¹⁷.

Soya is considered safe for babies over 6 months old to consume¹⁷.

Families of infants with an allergy to soya should receive support from a health professional about the initial avoidance, and potential future reintroduction of soya containing foods.

Conclusion:

Soya is a nutritious food and there is no good evidence of harm to the general public related to having 1-2 portions of soya based foods or drinks per day, as part of a balanced diet.

The only exception to this is for those with a soya allergy or those on hypothyroid medication (as soya can reduce the absorption of this medication).

Similarly, most babies under 6 months of age are not advised to consume soya.

Those who use soya based dairy replacements would be mindful of choosing versions which are fortified with vitamin B12 and calcium, as well as ensuring that they consume an adequate intake of iodine.

There is quite good evidence that having a regular intake of soya is good for heart health and may also reduce menopausal hot flushes.

A moderate daily intake of soya (1 -2 portions per day) is not associated with increasing the risk of cancer, which also applies to cancer survivors.

Evidence is suggestive that soya may promote bone health and may reduced the risk of certain types of cancer; however, further, high quality research is needed in these areas.

For more information about the health impact of soya, check out this podcast episode I recorded with Bahee from UK Kids Nutrition.

References:

1. British Dietetic Association (2016). Food Fact Sheet ‘Iodine’. Accessed online: https://www.bda.uk.com/foodfacts/Iodine.pdf (April 2018).
2. British Dietetic Association (2014). Food Fact Sheet ‘Soya and Health’. Accessed online: https://www.bda.uk.com/foodfacts/soya_and_health.pdf (April 2018).
3. Anderson JW and Bush HM (2011). Soy protein effects on serum lipoproteins: a quality assessment and meta-analysis of randomized, controlled studies. J Am Coll Nutr; 30(2): 79-91. Accessed online: https://www.ncbi.nlm.nih.gov/pubmed/21730216 (April 2018).
4. Zhang B, et al. (2008). Greater habitual soyfood consumption is associated with decreased carotid intima media thickness and better plasma lipids in Chinese middle-aged adults. Atherosclerosis; 198(2): 403-11. Accessed online: https://www.ncbi.nlm.nih.gov/pubmed/18022626 (April 2018).
5. Messina M, Ho S and Alekel DL (2004). Skeletal benefits of soy isoflavones: a review of the clinical trial and epidemiologic data. Curr Opin Clin Nutr Metab Care; 7(6):649-58. Accessed online: https://www.ncbi.nlm.nih.gov/pubmed/15534433 (April 2018).
6. Alekel DL, et al. (2010). The soy isoflavones for reducing bone loss (SIRBL) study: a 3-y randomized controlled trial in postmenopausal women. Am J Clin Nutr; 91(1): 218-30. Accessed online: https://www.ncbi.nlm.nih.gov/pubmed/19906801 (April 2018).
7. Taku K, et al. (2012). Extracted or synthesized soybean isoflavones reduce menopausal hot flash frequency and severity: systematic review and meta-analysis of randomized controlled trials. Menopause; 19 (7): 1-15. Accessed online: http://menopau.com/wp-content/uploads/2013/02/Isoflavon-reduce-memopause-significant.pdf (April 2018).
8. American Institute for Cancer Research (2010). Soy Research. Accessed online: http://www.aicr.org/foods-that-fight-cancer/tab-content/soy-research-1.html (April 2018).
9. American Institute for Cancer Research (2012). Soy is Safe for Breast Cancer Survivors. Accessed online: http://www.aicr.org/cancer-research-update/2012/november_21_2012/cru-soy-safe.html (April 2018).
10. Cancer Research UK (2016). Food controversies: Soy. Accessed online: http://www.cancerresearchuk.org/about-cancer/causes-of-cancer/diet-and-cancer/food-controversies#food_controversies5 (April 2018 via).
11. Balk E, et al. (2012) “Effects of soy on health outcomes”. Evid Rep Technol Assess (Summ); 126: 1-8. Accessed online: https://www.ncbi.nlm.nih.gov/pubmed/16194125. (April 2018).
12. Hamilton-Reeves JM, et al. (2010). Clinical studies show no effects of soy protein or isoflavones on reproductive hormones in men: results of a meta-analysis. Fertil Steril; 94(3): 997-1007. Accessed online: https://www.ncbi.nlm.nih.gov/pubmed/19524224 (April 2018).
13. Messina M and Redmond G (2006). Effects of soy protein and soybean isoflavones on thyroid function in healthy adults and hypothyroid patients: a review of the relevant literature. Thyroid;16(3): 249-58. Accessed online: https://www.ncbi.nlm.nih.gov/pubmed/16571087 (April 2018).
14. NHS Choices (2018). Can I eat soya products during pregnancy? Accessed online: https://www.nhs.uk/chq/Pages/can-I-eat-soya-products-during-pregnancy.aspx?CategoryID=54&SubCategoryID=216 (April 2018).
15. West M (2007). The impact of dietary oestrogens on male and female fertility. Current Opinion in Obstetrics and Gynecology: 19(3): 215–221. Accessed online: https://journals.lww.com/co-obgyn/Abstract/2007/06000/The_impact_of_dietary_oestrogens_on_male_and.3.aspx (April 2018).
16. NHS Choices (2016). Your Pregnancy and Baby Guide. Accessed online: https://www.nhs.uk/conditions/pregnancy-and-baby/types-of-infant-formula/ (April 2018).
17. Agostoni C, et al. (2006). Soy protein infant formulae and follow-on formulae: a commentary by the ESPGHAN Committee on Nutrition. J Pediatr Gastroenterol Nutr; 42(4): 352-61.Accessed online: https://www.ncbi.nlm.nih.gov/pubmed/16641572 (April 2018).