The truth about soya

by Dr Laura Freeman

Soya is one of the healthiest foods to incorporate in a plant-based diet and in fact in any diet. Yet, there remains widespread confusion about its impact on health. This is in part due to some studies in non-human animals that have shown harmful effects and of course push back from the dairy industry who consider soya drinks and yogurts as direct competition. Some of the commonest misconceptions are around the role of soya in cancer prevention, women and men’s hormonal health, thyroid health and soya consumption in children. The only reason to avoid soya in the diet is if you are one of the rare individuals with an allergy to soya protein. This affects less than 0.5% of the population (1). So, let’s review what we know about the impact of soya on human health.

Nutritional properties

Soya foods such as tofu, tempeh and edamame beans are a great source of plant protein, containing good quantities of all 9 essential amino acids. In fact, soya protein has a biological value similar to meat protein (2). Soya is also a good source of fibre, calcium and polyunsaturated fatty acids, whilst being low in saturated fat and naturally cholesterol-free.

Cancer prevention

Soya contains plant oestrogens and early studies in animals suggested these compounds may promote the growth of cancer. However, studies in humans show quite the opposite because these plant oestrogens act in a different way to human oestrogens. Soya consumption has been associated with reduced rates of a number of cancers and especially hormone-driven cancers such as breast cancer in women and prostate cancer in men (3). This reduction in cancer is seen with both tofu and soya milk and for breast cancer, the impact is greatest when soya consumption starts in childhood (4–6). Soya consumption after a diagnosis of breast cancer can also improve the chances of staying in remission (7).

Heart health

Soya protein is able to lower blood cholesterol levels and thus reduces the risk of heart disease and stroke. The well-established, therapeutic diet for lowering blood cholesterol levels, The Portfolio Diet, contains soya as one of the active foods (8).  In fact, soya food producers are able to legitimately use a health claim on their products to inform consumers that soya consumption reduces the risk of heart disease. So, there is no doubt that soya is heart healthy.

Thyroid health

There have been some lingering concerns that soya can adversely affect thyroid health. Again, there are no scientific data to support this. The main nutrient required for adequate thyroid hormone production is iodine and this can readily be obtained from fortified foods, sea vegetables or as a supplement. The only association with thyroid health is if you have an underactive thyroid and are taking thyroxine as a medication. Soya can reduce the absorption of thyroxine and so it’s best to leave an hour’s gap before and after taking your tablet and consuming a meal with soya (9).

Men’s health

There is prevailing myth that the plant oestrogens in soya can have feminising effects in men. Again, there is no good evidence to support this notion. A huge study from 2021 has clearly demonstrated that soya consumption has no impact on the levels of oestrogen or testosterone in men (10). In fact, you should be more concerned about adverse effects on hormone levels if you are consuming cow’s milk, which may raise oestrogen and lower testosterone levels in men (11). Regular consumption of soya reduces the risk of the commonest cancer in men, prostate cancer. One study examining the impact of drinking soya milk at least once per day, showed a 70% reduction in the risk of prostate cancer (12).

Women’s health

In addition, to reducing the risk of breast cancer, soya milk and foods can reduce the frequency of hot flashes for those experiencing menopausal symptoms (13). Bone health can also be an issue for women in later life and soya actually has an oestrogen-like effect on bone, thus promoting bone health (14).

Soya consumption in children

Soya milk and foods are great for children (15). Fortified soya milk provides similar amounts of calcium and protein to cow’s milk without the high levels of saturated fat. Children can start eating soya foods from as early as 6 months and it can make a great finger food, or it can be pureed up with vegetables. Calcium-set tofu is a good way to ensure adequate calcium intake for children.

 

Top tips for incorporating soya into diet

• Choose an unsweetened, fortified soya milk for your morning porridge oats or smoothie
• Have some unsweetened soya yogurt with fruit as a snack or dessert
• Use firm tofu in curries and stir-fry’s
• Add silken tofu to make a creamy sauce

References

1. Messina M, Venter C. Recent Surveys on Food Allergy Prevalence. Nutr Today. 2020;55(1).
2. Young VR. Soy protein in relation to human protein and amino acid nutrition. Journal of the American Dietetic Association. 1991.
3. Li N, Wu X, Zhuang W, Xia L, Chen Y, Zhao R, et al. Soy and Isoflavone Consumption and Multiple Health Outcomes: Umbrella Review of Systematic Reviews and Meta-Analyses of Observational Studies and Randomized Trials in Humans. Molecular Nutrition and Food Research. 2020.
4. Korde LA, Wu AH, Fears T, Nomura AMY, West DW, Kolonel LN, et al. Childhood soy intake and breast cancer risk in Asian American women. Cancer Epidemiol Biomarkers Prev. 2009;
5. Fraser GE, Jaceldo-Siegl K, Orlich M, Mashchak A, Sirirat R, Knutsen S. Dairy, soy, and risk of breast cancer: those confounded milks. Int J Epidemiol. 2020;
6. Wang Q, Liu X, Ren S. Tofu intake is inversely associated with risk of breast cancer: A meta-analysis of observational studies. PLoS One. 2020;
7. Shu XO, Zheng Y, Cai H, Gu K, Chen Z, Zheng W, et al. Soy food intake and breast cancer survival. JAMA – J Am Med Assoc. 2009;302(22).
8. Chiavaroli L, Nishi SK, Khan TA, Braunstein CR, Glenn AJ, Mejia SB, et al. Portfolio Dietary Pattern and Cardiovascular Disease: A Systematic Review and Meta-analysis of Controlled Trials. Progress in Cardiovascular Diseases. 2018.
9. Messina M, Mejia SB, Cassidy A, Duncan A, Kurzer M, Nagato C, et al. Neither soyfoods nor isoflavones warrant classification as endocrine disruptors: a technical review of the observational and clinical data. Critical Reviews in Food Science and Nutrition. 2021.
10. Reed KE, Camargo J, Hamilton-Reeves J, Kurzer M, Messina M. Neither soy nor isoflavone intake affects male reproductive hormones: An expanded and updated meta-analysis of clinical studies. Vol. 100, Reproductive Toxicology. 2021.
11. Michels KB, Binder N, Courant F, Franke AA, Osterhues A. Urinary excretion of sex steroid hormone metabolites after consumption of cow milk: A randomized crossover intervention trial. Am J Clin Nutr. 2019;
12. Jacobsen BK, Knutsen SF, Fraser GE. Does high soy milk intake reduce prostate cancer incidence? The Adventist Health Study (United States). Cancer Causes Control. 1998;
13. Messina M. Soy foods, isoflavones, and the health of postmenopausal women. In: American Journal of Clinical Nutrition. 2014.
14. Akhlaghi M, Ghasemi Nasab M, Riasatian M, Sadeghi F. Soy isoflavones prevent bone resorption and loss, a systematic review and meta-analysis of randomized controlled trials. Critical Reviews in Food Science and Nutrition. 2020.
15. Messina M, Rogero MM, Fisberg M, Waitzberg D. Health impact of childhood and adolescent soy consumption. Nutr Rev. 2017;