Soya - healthy or toxic?

Soya bean consumption has grown exponentially over the last 50 years. Soy bean oil and other soy bean products can be found all over the place. We are now consuming much, much more than ever before. Is this a good thing?

There are good and bad reports relating to the effects of soy on health. If the bad reports are true, then many of us should be seriously considering cutting down on our intake.

What is soya?

Soya (or soy in the U.S.) comes from soya beans, which are legumes, originally from East Asia. They are now produced in the US, Brazil and Argentina, which together account for 81% of world production(1).

Soya is found in many processed foods in the form of soya milk, soya flour, soya oil, soya protein and soy lecithin. This is where most of the soy in the Western diet comes from. More concentrated forms of soya include soya milk derivatives such as tofu and many fermented soya products such as tempeh, natto, miso (as in miso soup), soy sauce and tamari.

Soya comprises 40% protein, 35% carbohydrate and 20% fat.

Pictured left is tofu.

The protein is considered a complete protein because it contains good amounts of all the essential amino acids. It is considered equal to animal protein in this regard. However, soya does contain substances that block protein digestion in the gut (trypsin inhibitors), meaning that it may not actually be as good a source in practice.

The carbohydrate in soya, like most legumes comprises sugars as well as oligosaccharides which are not digestible, and may produce digestive disturbance such as flatulence. At the same time these oligosaccharides may stimulate good bacteria to develop in our guts.

The fat portion of soya contains phytosterols, that have been associated with lower cholesterol levels. Other fat soluble substances called saponins may also help reduce cancer risk.

Soya also contains other components that can have a significant effect on health. They include the isoflavones, genistein and diadzein, phytate, trypsin inhibitors and linolenic acid.

Soya and health

Soya has generally had a good press over the last 3 decades. This is in part due to the efforts of the large soyabean industry, which produces around 300 million tonnes of soyabeans a year(1). At a price of £400 per tonne, this equates to an annual revenue of £120 billion(2). That's not far short of the size of the total economy of Ireland.

This huge industry has funded a marketing campaign over the years to use up all the by products of soya oil production. Little wonder that soya can be found in such a wide range of processed foods nowadays.

Pictured right is natto.

Health concerns

Male infertility 

The isoflavones in soya have oestrogenic (feminising) like effects. Many studies have associated increased intakes of soya with lower sperm counts. In a group of subfertile and overweight men a drop of 40% in sperm numbers was found between those who consumed the most soya compared with those who consumed none(3). Results were most pronounced in the most overweight and those who had the higher sperm counts. If you are trying for a baby it makes sense for the man to limit soya intake.

Thyroid problems

The isoflavones in soya, posibly along with other soya components appear to be goitrogenic, that is they have a bad effect on our thyroid(4). They inhibit an enzyme, thyroid peroxidase that helps us produce thyroid hormone. If we consume enough iodine (found in sea food, especially seaweed), this partly offsets the effect. It does not matter if the soya is fermented or not, it will have the same effect on your thyroid. That means that miso, natto, soy sauce and tempeh also have negative effects.

If you have low levels of thyroid hormones you will probably benefit from reducing the amount of soya you consume. It is also worth being aware that if you are on thyroid hormone such as levothyroxine, then reducing soya intake could reduce the amount of drug you require.

If you have high levels of thyroid, as in Grave's disease, then I don't advise increasing intake of soya. In fact soya consumption in childhood is associated with increased incidence of autoimmune diseases such as Grave's. However, it should be OK to consume a modest amount, but make sure you are getting some iodine at the same time.

Infants and formula feed

Infant formula that contains soya milk can be problematic. The problems concern both the oestrogenic efffects of soya and the goitrogenic effects. Because infants are so small, some infant formulas can provide the equivalent of four birth control pills of oestrogenic activity per day. Clearly not a good thing, especially for young boys. The effects on the thyroid are also of concern, which is why iodine is now added to most infant formula.


While it has been widely reported that soya can reduce risk of breast and prostate cancer it is not so well known that soya protein can produce carcinogenic substances such as lysinoalanine and nitrosamines.

Soya can also interfere with the anti-cancer effects of tamoxifen, a common drug used to treat breast cancer.

Reported health benefits

Anti-cancer effects

The isoflavones in soya have been associated with reduction in risk of prostate and breast cancer. However the evidence has been epidemiological in nature. Specifically it has associated the lower incidence of breast cancer in Asia with higher intakes of soya in those areas. However there are a large number of other factors that could be responsible, so the evidence is far from compelling.

Heart health

Soya has been associated with increased heart health and some soya products carry health claims that it lowers cholesterol levels. However, most recent studies seem to show very modest effects with an average 3% drop in total cholesterol levels after substituting 50g of soy protein for animal protein(5).

Other studies had found that only those with significantly raised cholesterol levels would see any meaningful reduction. It is also true that reducing cholesterol levels is not neccessarily a good move anyway as I have mentioned before here. The typical advice to reduce levels from 6 to 4mmol/L is not warrented, and soya is unlikely to reduce cholesterol levels if your level is below 6mmol/L.

Athletic considerations

Athletes come across soya protein in protein shakes and other sports products. You are better off with whey protein. A 2011 study found that whey protein supplementation had weight loss effects and reduced waist size while soy protein did not(6). I suspect that you will become stronger and potentially leaner with whey protein than with soy protein, which could potentially reduce your strength. 

DrDobbin says

I would advise most people to limit consumption of soya. As an occasional food to add variety it is fine, but if you consume it daily you may be affecting your health adversely. 

To limit soya intake reduce the amount of processed foods you eat, both vegetarian and non-vegetarian. Look for words such as soya milk, soya flour, soya oil, soya protein and soy lecithin on the ingredients lists. If you are consuming a lot of soya milk or tofu then consider substituting them with alternatives.