Updated 12 April 2013

Soy - healthy or harmful?

Soy has been touted the miracle food of the 20th century, but new research suggests it may be doing more harm than good. Are you confused? Then read these comments by DietDoc.

Soy, or soya, is currently arousing controversy.

On the one hand, some research studies are finding that soy, or the so-called biological compounds found in soy such as isoflavones and phyto-oestrogens, have beneficial effects on health. On the other hand, studies looking at certain aspects of phyto-oestrogens seem to indicate that these compounds may be harmful under certain circumstances.

This type of nutritional controversy leaves the general public confused and unsure. So let's have a look at both the negative and positive findings that are accumulating about soy and the biological compounds it contains.

Positive aspects
Isoflavones are also called bioflavonoids and recent research indicates that such phytochemicals can have many positive health effects.

The following positive effects have been documented for soy isoflavones:

  • Reduction of raised blood cholesterol levels and protection against heart disease. Studies have found that an intake of 25 to 50 g of soy protein per day can lower LDL, or 'bad', blood cholesterol levels significantly by up to 10%.
  • Protection of prostate health and prevention of prostate cancer by acting as oestrogen-like agonists to prevent testosterone from accelerating prostate tumour growth.
  • Possible reduction in the incidence of breast cancer. Epidemiological or population studies indicate that populations where the intake of phyto-oestrogens is high, for example in China and other Eastern countries, have a decreased incidence of hormone-related cancers such as breast cancer. Soya intake has also been found to increase the length of the menstrual cycle, a factor that protects against breast cancer, and decreases the levels of FSH (follicle stimulating hormone) and LH (luteinising hormone) in premenopausal women. These are all factors that point towards soy foods exerting a protective effect against breast cancer. There is also some experimental evidence obtained with animal studies that exposure to dietary isoflavones prior to birth or before puberty decreases carcinogen-induced breast cancer by increasing the proportion of differentiated cells in the mammary gland. It is thus been suggested by some researchers that exposure to a diet rich in isoflavones early in life confers a protective effect.
  • Improvement of menopausal symptoms preventing hot flushes and other negative symptoms of the climacteric.
  • Improved expansion of the blood vessels (vasodilation) which can prevent heart failure, hypertension and narrowing of the arteries in diabetes and heart disease.
  • Protection against osteoporosis by increasing bone density. Once again, it has been observed that Asian women who consume diets rich in soy do not have the same high incidence of osteoporosis as western women.

The list of positive effects attributed to the ingestion of soy and the phytochemicals these legumes contain, provides good reason why western populations should increase their intake of soy and other legumes.

Negative aspects
Increased consumption of soy and exposure to the phytochemicals it contains have, however, also been linked to negative health consequences.

The following potentially negative aspects of increased soy consumption are at present receiving attention:

  • Researchers have found that infants fed soy-based formulas are exposed to very high levels of circulating phyto-oestrogens (13 000 to 22 000 times higher than would normally be the case) and have suggested that this overexposure to phyto-oestrogens may lead to hormone imbalances and competition with enzymes that metabolise steroids, drugs and other biological compounds and/or have a negative effect on the future reproductive function of the individual. Because infants weigh much less than adults, they are exposed to much higher levels of phyto-oestrogens than adults or older children who eat some soy foods as part of a mixed diet. In experimental animals it was found that feeding the infants large quantities of phyto-oestrogens had a negative effect on the development of their reproductive organs and future fertility. Few studies have been conducted with human infants to date and we don't yet know if high level phyto-oestrogen exposure during infancy will affect human reproduction or not.
  • A working group of the Independent Scientific Committee on Toxicity in the UK is presently investigating if the phyto-oestrogens in soy foods can have a negative effect on hypothyroidism.
  • The phyto-oestrogens in soy are known to act both as oestrogen agonists and antagonists. This means that one and the same chemical may either stimulate cells in the human body that are sensitive to oestrogen or block the effect of oestrogen. In view of this complex action, we cannot as yet be sure if the use of soy and phyto-oestrogens is completely safe in women who already suffer from breast cancer, particularly those that have oestrogen-sensitive tumours.

Moderation and caution
So what are we supposed to believe about soy and what should we do?

At the moment, experts advise consumers to approach the controversy surrounding soy and its biologically active compounds with caution.

You can certainly include soy and soy products such as tempe and bean curd, textured vegetable protein and soy protein foods in your diet on a regular basis, but don't go overboard.

If you suffer from heart disease or vasoconstriction, you can consider including 25 g of soy protein in your diet per day. Menopausal women who do not have a family or personal history of breast cancer can probably use phyto-oestrogens to combat menopausal symptoms.

However, the use of infant formulas based on soy should be discussed with a paediatrician. If you suffer from hypothyroidism or oestrogen-dependent breast cancer, it is advisable not to use soy in large quantities and not to use phyto-oestrogens at all.

Scientific research is being conducted to clear up these problems associated with soy. Until we have definitive answers, the best approach is moderate intake soy foods or phyto-oestrogens.

Read more about The pros and cons of soy.

(References: Ingram et al (1997) The Lancet, 350, 990-994; Setchell et al (1997). The Lancet, 350, 23-27; Krause's Food, Nutrition & Diet Therapy (2000). Edited by Mahan & Escott-Stump, Chapter 12, pp 275-276. WB Saunders Co, Philadelphia, USA; Arbor Clinical Nutrition Updates (2001), 102 Phytoestrogens & Heart Disease; Arbor Clinical Nutrition Updates (2003), 165, Phytoestrogens &Vasodilation; asksam/agesandstages/childrenandbabies; newsarchive/ 2003/may/ phytoreport0403news)

Dr Ingrid van Heerden is a registered dietician and holds a doctoral degree in Nutrition and Biochemistry. She believes that "we are what we eat" and offers free nutrition and weight loss advice via her DietDoc service on Read more of her articles.


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