Soya – Good or bad for us?

This article looks at some of the evidence in favour of adding soya products to our diet, as well as considering whether soya supplements might be harmful to us. 

• What is soya?
• Types of soya products available in the UK
• Evidence to suggest that soya might be good for our health
• Evidence to suggest that soya supplements might be harmful for our health
• Conclusions 

 

What is soya?

 

Mature soya beans are a light cream colour while edamame beans are soya beans that are picked when they are still young, soft and green.

 

 

Soya-based products are made from the soya bean, originally found in parts of East Asia. This bean is high in protein and is the main source of protein for many communities. Soya products can be good alternatives to meat and dairy products.

As the demand for meat and dairy replacements increases, soya products can be found in many local supermarkets around the world.

 

Types of soya products available in the UK

Soya products include:

 

Soya beans contain a number of chemical components which may be beneficial to our health, including chemicals called isoflavones which may help menopausal and post-menopausal women.

Preparations containing isoflavones are marketed as food supplements and so-called 'dietary food for special medical purposes' to alleviate health complaints of peri- and postmenopausal women. However, isoflavones may cause unintended effects on the female breast both in healthy women and in breast cancer patients, and on the thyroid hormone system.

 

The picture therefore is not totally clear, with some publications suggesting that soya products might not be as beneficial to our health as was once thought. We compare the evidence below.

 

Evidence to suggest that soya might be good for our health

Soya contains a number of beneficial components such as:

• Omega-3 fatty acids
• Omega-6 fatty acids
• Isoflavones
• Vitamins and minerals such as vitamin K, Folate and Thiamin

Soya also contains saturated fats, but only at low levels.

Menopause

Overall, studies have suggested that soya may have beneficial health effects, for post-menopausal women. For example:

• High soy protein intake appears to increase HDL levels (‘good’ cholesterol) and reduce LDL cholesterol (‘bad’ cholesterol), triacylglycerol (the main constituent of body fat in humans as well as of vegetable fat).
• Soy protein, compared with soy supplements (for instance, isoflavone formulations), also reduces total cholesterol levels.
• Soy protein appears to decrease the risk of ischemic heart disease.
• Soy protein appears to decreases the risk of atherosclerosis and maintain blood vessel health, particularly if taken soon after the menopause. This is in line with the oestrogen-timing hypothesis, which suggests that soon after the menopause is the optimum time for health benefits from hormone therapy.
• Soy protein appears to reduce the risk of colorectal cancer risk in women by about 21%, although this effect is not seen in men.

Breast cancer

Soya appears to reduce the risk of reoccurrence of breast cancer, although there is some evidence to the contrary. In particular, an analysis of 9,514 breast cancer survivors who were followed for 7.4 years found that higher post-diagnosis soy intake was associated with a significant 25% reduction in tumour recurrence. This is a significantly larger, longer-term study than those that suggest an increased risk. It focused on soya-based food consumption rather than on soy supplements, and it focused on actual tumour growth rather than on any gene changes that might possibly lead to tumour growth. As such, this evidence appears more robust and more reliable than the more negative studies.

In a recent meta analysis of eight studies, the authors concluded that soy consumption – and therefore isoflavone intake – is beneficial for breast cancer prognosis and lowers breast cancer manifestation. They conclude that women with high soy and isoflavone consumption have a lower risk of being diagnosed with breast cancer when compared to women who had no soy in their dietary plan.

The largest study to date (300,000 Chinese women) found that each 10 mg per day increment in soy isoflavone intake was associated with a 3% reduced risk of breast cancer.

 

Bone mass

Several studies provide evidence that dietary soy isoflavones decrease menopause-induced bone loss caused by osteoporosis both by decreasing bone resorption and by stimulating bone formation. 

Losing bone mass is a normal part of ageing and can lead to osteoporosis and an increased risk of broken bones. Women lose bone mass rapidly in the first few years after the menopause.

 

Prostate cancer

In a meta-analysis of 30 studies from the U.S., Europe, Japan, and China, intakes of total soy foods, isoflavones genistein and daidzein, and unfermented soy foods were significantly associated with a lower risk of prostate cancer (PCa). Fermented soy foods were not associated with a lower risk of PCa.

Examples of unfermented and fermented soy foods : 

Unfermented soy-based products (associated with a lower risk of PCa)	Fermented soy-based products
Raw soy beans Tofu Soya flour Soya milk	Miso Natto Tempeh Natural brewed soy sauce

 

Evidence to suggest that soya supplements might be harmful for our health

While the evidence for soy-based foods tends to be positive, as indicated above, a number of studies have suggested possible health risks when it comes to the use of soy supplements by susceptible risk groups.

For instance, a comprehensive review showed that soy supplementation may pose a risk to people with:

• an iodine deficiency (especially during pregnancy)
• subclinical hypothyroidism
• congenital thyroid dysfunction
• breast cancer or with a history of breast cancer.

As we have noted in our review of vitamins and minerals, 
taking supplements can sometimes increase health risks, whereas consuming food which contains the same vitamins and minerals is usually beneficial for health. A similar situation may apply to soy.

 

Conclusions

More research on soya is needed, for instance:

• larger scale, longer-term studies which focus on soya-based food (rather than soya supplements) and the effects at different stages in life for Western women. These studies would complement the greater number of studies among Asian women, many of whom may have been consuming soya-based food from an early age.
• studies that include men, including research into the possible initial prevention of PCa. These studies would complement existing research into preventing the reoccurrence of the cancer. 

Overall though, the evidence to date suggests that:

• Soya-based foods (like soya milk and tofu) have greater health benefits and fewer risks than soy supplements (for instance, isoflavones).
• Consumption of soya products from early in life appears to have a more protective health effect, in particular for Asian women.
• Soya-based food appears to have health benefits for post-menopausal women, if taken soon after the menopause. These benefits include healthier cholesterol levels; healthier blood vessels; reduced risk of heart disease; reduced bone loss; reduced hot flushes; reduced risk of colorectal cancer, and a reduced risk of the recurrence of breast cancer.
• There are limited beneficial effects for men (other than possibly for prostate cancer), although more research is needed here.
• There is a place for soy-based foods as part of a healthy balanced diet, because it is a good source of protein and nutrients, and they can be good alternatives to meat and dairy products.

Reviewed and updated by Elisa Knebel, March 2023. Next review date, February 2027.