Remember when soy or soya was the wonder food of the 90s? In the form of tofu or bean curd, it was purported to be the perfect protein source without the bad health effects of meat, plus low in fat and calories. And for those who were lactose intolerant, especially little babies, soy milk was a god-sent milk substitute that was both protein-rich and healthy. For vegetarians, soy and soy products were the perfect substitute for meat and dairy products. In addition to its health benefits, soy and soy products were cheap and easily available.

Even government health agencies such as the US FDA went so far as to allow soy food labeling with health claims on the association between soy protein and the reduced risk of coronary heart disease.

Sounds like the perfect health food, right? Aside from being more than 77% water, tofu yoghurt is rich in vitamins and minerals, low in fat, carbs and sugar. And soy is said to contain omega-3-fatty acids, also known as "the heart-friendly fats", isoflavones, antioxidants which are thought to have some some anti-cancer properties, and lastly even cholesterol-lowering compounds.

So what could go wrong? In recent years, scientific evidence has questioned the health benefits of soy, and more importantly, has brought to light possible adverse effects involving primarily the human reproductive and neurologic systems.

(1) Soy and the reproductive system. The main isoflavones in soy, genistein and daidzein, have antioxidant properties but also are phytoestrogens or endocrine-disrupting substances which can create havoc with hormonal balance. Scientists believe that the soy plant uses phytoestrogens as a defense mechanism from predators. Estrogen-mimicking substances interfere with mammalian hormones, thus regulating the proliferation and growth of mammalian predators - including humans. In women, there are contradicting studies as to whether these phytoestrogens prevent or promote the development of breast cancer. Stronger evidence of the hormone disrupting action of phytoestrogens in soy has been observed in men. There have been anecdotal reports of adult men developing abnormal breasts (gynecomastia) as described in an article in Men's Health.

Some of the peer-reviewed studies on this topic are summarized below:

In 2005, Canadian researchers reported an inverse relationship between soy intake and testosterone levels in men. This may have the effect of reducing the risk for prostate cancer but reducing male fertility as well. In animals, a 2008 Chinese study observed that exposure of juvenile rats to the phytoestrogen daidzein resulted in erectile dysfunction that persisted in adulthood. The same group of researchers observed abnormal histological changes in the penile tissues of daidzein-exposed rats. Thus, the phytoestrogen daidzein seems to induce changes in the structure and function of the reproductive organs of rats. Harvard researchers reported in 2008 that the isoflavones in soy may lead to decreased fertility in human males and produced data which suggest that higher intake of soy foods and soy isoflavones is associated with lower sperm concentration.

(2) Soy and the brain. Soy protein is said to slow down aging by maintaining bone mineral density and cognitive function. Again, the current state of evidence is contradictory. In 2004, Dutch researchers investigated using soy isoflavones as an alternative to estrogen therapy in postmenopausal women. The reported results "do not support the hypothesis that the use of soy protein supplement containing isoflavones improves cognitive function, bone mineral density, or plasma lipids in healthy postmenopausal women when started at the age of 60 years or later." A 2007 study reported that supplementation with soy isoflavones do not significantly affect cognitive functioning in postmenopausal women. Most research studies did not find any significant positive effects of soy consumption on aging but no negative effects were reported either. However, one study by researchers at the National Institute on Aging suggests that high soy consumption in midlife is linked to cognitive decline and atrophy of the brain later in life.

(3) Other adverse effects of soy. As early as 1991, Japanese researchers reported that excessive soy consumption can suppress thyroid function in elderly patients and cause goiter. A recent clinical trial reported that soy protein and isoflavones are not effective in improving bone mineral density in postmenopausal women. This contradicts the claims that soy supplements may prevent osteoporosis. Researchers at the Osteoporosis Research Center in Omaha, NE reported in 2000 that even calcium-fortified soy milk is not comparable to cow milk as a calcium source. Calcium from the soy milk is absorbed only at 75% the efficiency of absorption from dairy milk.

The Weston A. Price Foundation has issued the following summary of the dangers of soy:

Soy is rich in phytic acid that hinders the uptake of calcium, magnesium, copper, iron and zinc. High phytate diets have caused growth problems in children. Soy contains trypsin inhibitors which interfere with protein digestion and may cause pancreatic disorders. The phytoestrogens in soy disrupt hormonal balance and may cause infertility and breast cancer. Phytoestrogens also interfere with the functioning of the thyroid gland. In infants, consumption of soy formula has been linked to autoimmune thyroid disease. Soy contains vitamin B12 analogs that are not easily absorbable and may actually increase the body's requirement for vitamin B12 and vitamin D. Processing of soy protein results in the formation of lysinoalanine which is toxic and nitrosamines which are highly carcinogenic. Free glutamic acid or MSG, a potent neurotoxin, is formed during soy food processing and additional amounts are added to many soy foods. Some soy foods were found to contain high levels of aluminum which is toxic to the nervous system and the kidneys.

If soy has some dubious health effects on adults, how about children exposed to soy at an early age? Approximately 25% of infant formula sold in the US is made from soy. In 1998, toxicologist Daniel Sheehan of the National Center for Toxicological Research of the US FDA wrote that "among human exposures, infant soy formula exposure appears to provide the highest of all phytoestrogen doses, and this occurs during development, often the most sensitive life-stage for induction of toxicity. Large, carefully controlled studies in this exposed infant population are a high priority."

In 1999, advocacy groups led by the Weston A. Price Foundation called for the removal of all soy-based formula from the marketplace. They based their call on reports that a high number of boys consuming soy are experiencing delayed physical and sexual development whereas girls are going into premature puberty. They attribute these changes to high intake of phytoestrogens. According to Mary Enig, a nutritionist and then president of the Maryland Nutritionists Association, "the amount of phytoestrogens that are in a day's worth of soy infant formula equals five birth control pills."

Ten years later, the issue of soy milk still hasn't been clarified or resolved. In a very recent report (February 2009), researchers from the National Institute of Environmental Health Sciences detected high amounts of the isoflavones genistein and daidzein in the urine, saliva, and blood of babies fed with soy-based formula. Compared to non-soy feeding children, concentrations of these compounds were about 500 times higher in soy-fed children. The results, however, could not determine whether the said phytoestrogens are biologically active in babies or are simply excreted into the urine. The authors conclude that further studies "focusing on physical and developmental findings reflecting the effects of estrogen exposure" are needed.

According to a review by Italian researchers, "available data does not appear to unequivocally support beneficial effects of soy isoflavones, and warn against their wide use, in the absence of satisfactory clinical findings."

And in 2008, the Weston A. Price Foundation submitted a petition to the US FDA to withdraw the privilege of soy products to carry a heart disease health claim. The petition submitted a body of data "which establishes a lack of consensus among experts, qualified by scientific training and experience, about claims that soy protein prevents heart disease or even lowers cholesterol".

Soy: friend or foe? Unfortunately, this is a question that is not easily answered. However, there is a general belief among experts that high intake of soy, especially as a sole source of protein is not advisable, unless there is no other alternative (e.g. in lactose-intolerant babies).