Soy Protein Side Effects

Many people have allergies or intolerances, or do not wish to consume dairy based proteins, or proteins of animal origin. For those people, soy protein is an excellent alternative.

Health Benefits of Soy Protein

It may come as a surprise to know that soy is one of the best quality proteins available. Biological value is a measure of digestibility and amino acid quality, and along with whey, widely known as the protein gold standard, soy ranks at the very top of this scale with a score of 100.

Apart from its nutritional and muscle building benefits as a protein, soy is known for its positive influence on heart health. Whole soybeans are known for their fibre content, while soy isoflavones are known for their ability to reduce serum levels of LDL cholesterol and triglycerides (1).

Soy Protein Side Effects and Oestrogen

The role of soy isoflavones is controversial – while these compounds have cardiovascular benefits, this is thought due to properties that mimic the female hormone oestrogen, and soy isoflavones have even found use as a natural alternative to hormone replacement therapy for women.

The two isoflavones in soy, genistein and daizein, are known for their ability to reduce testosterone levels in men, particularly at high intake levels. This ability varies amongst the population. It is estimated that 30-50% of people in western countries harbour a type of gut bacteria which has the ability to convert soy isoflavone daizein into a more potent oestrogen-mimic known as equol. These people will see a much greater oestrogenic effect from the consumption of soy isoflavones than people lacking this bacteria (2). This is beneficial to carriers of this bacteria if soy is being used for its cardiovascular or hormonal benefits.

Soy protein side effects for Men

The good news for people consuming soy protein and most processed soy-based foods as a source of high quality protein to supplement their nutritional regime, is that processing removes the vast majority of the isoflavone content in soy. It is estimated that commercially produced soy protein concentrate and isolate, which form the basis of soy protein supplements contain approximately 2% of the isoflavone content in raw soybean (1).

A number of studies have shown that men using soy products, even those containing isoflavones, in normal quantities, were highly unlikely to experience any decrease in testosterone levels or fertility, and did not show any evidence of feminising traits (3).

In spite of this, there have been isolated reports of testosterone decreases, particularly amongst men who were eating unbalanced diets containing unusually high levels of soy based foods, such at tofu and textured vegetable protein, which typically contain 20-45% of the isoflavones in raw soy, a ten to twenty-fold increase on the amounts present in protein powders (4). These cases are isolated and unusual, and the average man can consume normal amounts of soy based food products with no ill effect.

Reversing the effects of Soy

Men who have experienced feminising effects from soy isoflavones are typically treated for this rare side effect under medical supervision. This typically includes exclusion of all soy products from the diet, and results in the re-establishment of normal testosterone levels.

Soy Protein Side Effects – Thyroid

There is some evidence that extremely high levels of soy isoflavones can cause the inhbition of thyroid peroxidase, an enzyme that contributes to normal thyroid function. Research has shown that reduced thyroid peroxidase activity does not lead to any change in thyroid function in normal, healthy people, but when combined with iodine deficiency, or other factors implicated in thyroid disease, the combined effect can be detrimental to proper thyroid function. These effects were observed when high levels of soy isoflavones were ingested, and, like the effect on testosterone, a soy protein powder is extremely unlikely to have any detrimental effect on thyroid function or metabolism (5).

Soy Protein Side Effects – Digestive

As with many protein powders, some people report that soy protein can be difficult to digest, which may lead to a variety of uncomfortable gastric side effects like stomach cramps, diarrhoea, constipation or flatulence. These can be reduced in a number of ways. Splitting soy protein into a number of small doses rather than a large single dose, taking soy protein with a meal, or abstaining from using using soy protein directly before a workout may help relieve these symptoms. If these symptoms persist, there are a number of other vegetarian proteins on the market, like pea, rice and egg, that may be more agreeable.

While soy protein, like any other protein, has its fair share of side effects, there are many myths around the use of soy which have turned people away from this high quality, nutritious and excellent quality protein. Hopefully with the debunking of these myths, people will feel more confident supplementing their protein intake with soy.

(1) Dewell A, Hollenbeck PL, Hollenbeck CB. Clinical review: a critical evaluation of the role of soy protein and isoflavone supplementation in the control of plasma cholesterol concentrations. J Clin Endocrinol Metab. (2006)
(2) Lori. Coward , Neil C. Barnes , Kenneth D. R. Setchell , Stephen. Barnes. Genistein, daidzein, and their .beta.-glycoside conjugates: antitumor isoflavones in soybean foods from American and Asian diets. J. Agric. Food Chem., 1993, 41 (11), pp 1961–1967
(3) Messina M. Soybean isoflavone exposure does not have feminizing effects on men: a critical examination of the clinical evidence. Fertil Steril. (2010)
(4) Siepmann T, et al. Hypogonadism and erectile dysfunction associated with soy product consumption. Nutrition. (2011)
(5) Messina M, Redmond G. Effects of soy protein and soybean isoflavones on thyroid function in healthy adults and hypothyroid patients: a review of the relevant literature. Thyroid. 2006 Mar;16(3):249-58.