Updated: Jan 30
An unfortunate event has unfolded over the past few years. Soy has been discriminated and stereotyped. Once consumed without question, soy foods are now often scrutinised. But like most conspiracies, their discrimination lacks evidence. And contrast to popular belief, they actually deliver a frig load of health benefits.
What is soy?
Soybeans are a type of legume that are consumed in a variety of forms, including edamame (immature soybeans), soymilk and tofu. Fermented soy foods include tempeh, tamari, miso, natto and soy sauce. Processed forms of soy include yoghurt, cheeses, TVP (texturized vegetable protein) and protein powders.
To note, Australian soybeans are not genetically modified.
Soybeans nutrition profile (per 100g)
· Protein: 40g
· Healthy fats: 11g
· Fibre: 9g
· Iron: 16mg
· Calcium: 280mg
· Zinc: 5mg
· Isoflavones: 120mg
The health damaging myths of soy
Majority of the health concerns raised around soy are based on their isoflavone content. Isoflavones are a type of phytoestrogen. These are compounds that have similar characteristics (structures and functions) to the oestrogen produced in humans. Basically, they can weakly mimic the activities of human oestrogen; blocking or binding to oestrogen hormone receptors. And to echo my last line, this affect is very weak. It’s also important to note that concerns raised about isoflavone safety on human health are based almost exclusively in vitro (i.e. performed in test tubes or dishes) and animal studies, meaning these studies are not always applicable to human practice.
Soy foods Isoflavone content (per 100g)
· Soy protein isolate: 91-200mg
· Natto: 20-124mg
· Miso: 23-126mg
· Tofu: 5-64mg
· Tempeh: 7-63mg
· Soymilk: 15-30mg
· Edamame: 18mg
On average, Westerners consume about 1 to 2 mg of isoflavones per day. In Asian countries, the average intake is 25 to 50 mg per day.
Soy & Cancer risk
The European Prospective Investigation into Cancer and Nutrition Study cohort, which included more than 300,000 women, found no increase in cancer risk caused by soy isoflavone intake (11,12).
A meta-analysis of case-control and cohort studies in Western and Asian countries found a protective role of soy foods in breast cancer (13).
Rates of breast and prostate cancers are much lower in Asian countries, where soy intake is much higher than in Western countries (1).
A recent systematic review and meta-analysis showed a significant lower relative risk for colorectal cancer when soy foods are consumed (14).
The anticancer effects of soy have been largely ascribed to isoflavones. Isoflavones have been shown to modulate cell cycle, apoptosis, differentiation, proliferation and cell signalling (1).
Soy & male health (testosterone, sperm & semen)
A meta-analysis, including 32 reports, found no conclusive interaction between soy or isoflavones and testosterone concentrations (19).
Research on the exposure to isoflavones, including at levels higher than average Asian county intakes, has not been shown to affect testosterone or oestrogen concentrations. These studies also did not find an affect on the quality of sperm and semen (16-18).
Soy & thyroid health
A recent systematic review found that soy supplementation has no effect on thyroid hormones in healthy individuals (1, 20).
Studies in cell cultures and animals have shown that isoflavones can interfere with thyroid function, however this has not been demonstrated in healthy humans meeting their iodine requirements (1,20)
Those with clinical thyroid conditions are recommended to avoid consuming soy foods for 3-4 hours before and after taking thyroid medications. This is to maximise efficacy (21).
Soy’s health benefits
Soy foods are one of the richest plant-based sources of protein, calcium, iron and zinc. Furthermore, studies show that the consistent consumption of soy foods protects against heart disease, osteoporosis and certain cancers. Eating soy foods will also help to lower your total and LDL cholesterol, blood pressure and improve your blood vessel health.
An analysis of 38 controlled clinical trials showed that when ~50 g of soy protein was consumed daily (7)
- total cholesterol reduced by 9.3%
- LDL cholesterol reduced by 12.9%
- Triglycerides reduced by 10.5%
Aim to consume 30 to 100mg of soy-isoflavones per day. This will allow you to achieve the health benefits from soy, plus stay within the recommended safety upper limit. For example, ~30mg of isoflavones is found in 100g tofu or one cup of soymilk (250ml) or 85g of cooked soybeans.
Just released 2021: Plant-Based Guidebook