Soy and cancer



Many herbalists and naturopaths feel strongly about the use of soy as a food and medicine. Many others sit in a confused middle ground or off field awaiting a calming of the arguments! Authors, who take a strong stand in the popular literature, frequently cite concerns over ‘endocrine disruption’, breast cancer, infant exposure, brain aging, GMO, soy isolates and ‘anti-nutrients’. On examining the scientific literature, it is evident that these arguments are also bipolar, with in vitro studies on one end and epidemiological studies on the other. In response to the proposal of the United States FDA to promote labeling of all foods containing soy protein as beneficial for cardiovascular health, Sheehan and Doerge, researchers from the National Centre for Toxicological Research, wrote a letter of protest including specific concern over soy protein isolate and high doses.


They stated that, “… it is inappropriate to allow health claims for soy protein isolate………While isoflavones may have beneficial effects at some ages or circumstances… Isoflavones are like other oestrogens in that they are two-edged swords, conferring both benefits and risk. The health labeling of soy protein isolate for foods needs to be considered…” They may well have reason for concern as some studies utilised doses of isoflavones in excess of 3 times a normal Asian dietary intake. In the following table I have highlighted some of the main points of controversy from available scientific literature.


Point of interest Possible/probable negative effects Possible/probable positive effects
GMO May change nutritional profile, increase risk of allergy, increase pesticide residue & resistant organisms In regards to health – none
Soy protein isolate (SPI), soy oil, textured vegetable protein (TVP) Highly processed at high temperatures, denaturing some proteins and reducing nutrition; contain flavourings, emulsifiers, preservatives, sweeteners, MSG & aluminium; nitrites formed during spray-drying; SPI increases vitamin requirements and reduces mineral absorption none – any food is compromised if highly processed and refined.
Aluminium Soybeans processed into SPI and isoflavone isolates contain aluminium leached from acid washing in aluminium tanks none
Phytic acid Reduces absorption of iron, magnesium, calcium and zinc; diets high in phytates may contribute to deficiencies found in the Third World Phytates in soy appears to improve calcium balance; grains, seeds & other legumes also contain phytates; other ingredients may ameliorate effects
Protease inhibitors (PI) and trypsin inhibitors (TI) Inhibition of protein digestion; possible growth retardation; possible pancreatic toxicity Growth inhibition shown in animals at excessive doses; PI & TI have anticancer effects; PI & TI deactivated by heating; toasted soy flour loses 85-94% of the TI
Phyto-oestrogens May cause ‘endocrine disruption’; lengthen menstrual cycle and have oestrogenic effects; promote cancer cell proliferation in in vitro & animal studies – though the relevance of this is yet to be determined; excess in pregnancy may affect endocrine system in foetus Soy shown to reduce breast cancer risk in humans; overwhelming evidence that soy inhibits cancer cell growth (especially those hormonally responsive – breast, prostate, colon, uterus); may act as ‘Selective Oestrogen Receptor Modifiers’; animals fed soy diets (20% by weight) do not have affected fertility; human studies show no adverse endocrine effects
Goitrogens; Inhibition of thyroid function; link to infant autoimmune thyroid disease and adult hypothyroidism; more likely with SPI effect depends on dosage & susceptibility; effect countered by iodine; found in Brassicacea vegetables in higher amounts, less likely with traditional soy products; soy reduces risk of thyroid cancer
Paediatric use May affect growth and reproductive development; may cause allergy; may cause autoimmune thyroid disease Soy formula does not lead to different adulthood outcomes compared with cow formula; cancer protection of soy may be conferred before puberty
Brain function affected Japanese American men living in Hawaii who ate at least 2 serves of tofu/week more likely to develop mental impairment and lower brain weight in old age. Soy isoflavones may protect the brain and lower chances of Alzheimer’s disease; lecithin and choline in whole soy improve memory
Cholesterol claims Most effective only for those with elevated cholesterol over 259 mg/dL; requires 25g/day of soy protein or more; interindividual variations in response, therefore not predictable Numerous trials show soy reduces high cholesterol, LDL and triglycerides; improves arterial compliance & reduces BP; whole soy more effective than SPI & isolated isoflavones
Menopausal claims Not all women benefit for hot flushes; soy cannot replace HRT in strength and predictability; many studies show no difference from placebo Soy protein intake prevents bone loss compared with whey protein; soy increases bone mineral content & prevents urinary loss; reduces hot flushes additionally over placebo; decreases FSH & LH in peri-menopausal women


A recent review of soy isoflavones by the North American Menopause Society concluded that “It is preferable to obtain isoflavones from whole foods.” Whole soy foods…”may provide other phytochemicals that enhance the effect of the isoflavones; these effects could be lost when isoflavones are given as additives alone.”… “In addition, the matrix in which the isoflavones are delivered will have an impact on the release of the agents for absorption and, therefore, will produce differential effects on intestinal metabolism and microflora.” This highlights some basic holistic principles


  • If a little is good, more is not necessarily better
  • Any food is compromised when highly processed, refined and tampered with.
  • Observe and learn from traditional methods of processing, preparation and eating habits. (eg. Fermentation, precipitation, soaking & slow cooking or roasting all improve soy nutrition)
  • Place soyfood consumption into the whole dietary influence perspective, rather than relying on a wonderfood-bullet.
  • Assess individual needs and health status (eg. gastrointestinal health)

To put it simply, whole soy foods are much safer and produce better clinical results.


Melanie Koeman
Her combination of medical science and naturopathic qualifications places Melanie in a unique position within the natural and complementary medicine field. As a practitioner, Melanie specialises in male and female reproductive health at the Jocelyn Centre for Natural Fertility Management in Sydney. Melanie has been lecturing since 1993 to students and practitioners at both complementary and orthodox medicine conferences. Melanie’s work as a researcher and technical writer since 1992 provide her with a contemporary scientific background which supports her clinical practice.

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