Soya food and health – an overview

Introduction

Soya foods have traditionally been eaten in Eastern cultures for thousands of years. They are now a popular alternative to animal-based products such as meat and dairy for those who want to adopt a more plant-based way of eating for health, ethical or environmental reasons.(1)

Soya food and drinks are popular across the globe and several national food-based dietary guidelines recognise the important role played by plant foods, including soya, to a healthy and environmentally sustainable diet.

Soya beans have a beneficial nutrition profile:(1,2)

• They are a source of high-quality protein containing all essential amino acids (comparable to dairy and meat)

• They have a beneficial fat profile: low in saturated fat and a good source of unsaturated fat (omega 3 and 6)

• Provide minerals including calcium, potassium, iron, and magnesium

Soya beans can be consumed whole or in other forms such as soya drinks, soya alternative to yogurt, tofu or soya-based meat alternatives. Clinical research has consistently demonstrated that inclusion of 1-3 daily servings of soya foods, as part of a healthy balanced diet, is associated with improved health outcomes.

Soya protein

A commonly reported barrier to adopting plant-based eating centres on the protein adequacy. However, soya beans contain 36-46% of high-quality protein (dry mass basis).

The FAO classifies protein quality using the Protein Digestibility Corrected for Amino Acid Score (PDCAAS) and the revised Digestible Indispensable Amino Acid Score (DIAAS).(3-6) The PDCAAS for soya protein ranges from 0.9 to 1.0 (maximum score), demonstrating soya protein to be of the highest quality and comparable to that of beef, milk and eggs.(1,2)

The revised FAO index, the DIAAS, is considered to be more accurate for measuring the digestibility of individual amino acids in the small intestine and values are not truncated to a maximum value of 1. Based on this, the values for soya and whey protein, although not identical, are comparable for quality – 0.90 vs 1.09 respectively. The FAO permits protein claims to be made for any foods achieving a DIAAS of >0.75.

Although a lot of emphasis is often placed on the academic protein quality scores of individual foods, this has little value when assessing the protein quality of an individual’s diet overall which is composed of multiple protein sources each providing variable quantities of complementary amino acids. 

Plants do in fact contain all essential amino acids, be it some at levels below the daily threshold.(7-10) It has been demonstrated that it is the overall intake of amino acids over the course of a day that determines the ability of a diet to meet essential amino acid needs.(9-12) A number of metabolic studies have shown that nitrogen balance is achieved over a 24-hour period and irrespective of protein source as long as energy requirements are met.(10,13) 

Additionally, it is understood that the body holds a pool of essential amino acids which it can call upon to complement dietary intakes.(14,15) Therefore, diets based solely on plants which meet the daily energy requirements of an individual, will also meet all essential amino acid needs. The scientific evidence is clear that protein intakes persistently exceed requirements even in individuals who follow vegan and vegetarian diets.(16-19) 

Excellent fat composition

Soya beans have an exceptional fat profile providing polyunsaturated fat whilst being low in saturated fat.(1) The predominant fatty acid in soya oil is the essential omega-6 fatty acid, linoleic acid (~54%), but it also contains significant amounts of the omega-3 essential fatty acid, alpha-linolenic acid (1-9%). It is now well established that diets low in saturated fat with modest quantities of unsaturated fats result in significantly improved serum lipid profiles and are associated with better heart health outcomes.(20,21)

Nutrients in soya

Soya beans (cooked) also provide moderate quantities of essential minerals such as calcium, magnesium, potassium and copper as well as vitamins B1, B6, folate and E. The quantity of minerals and vitamins will of course vary depending on the end product consumed (2).

Soya provides a number of other plant nutrients such as isoflavones, phenols and phytosterols which may have health-promoting benefits via various proposed mechanisms such as their antioxidant properties. In particular, soya isoflavones have received a lot of research attention.

Soya isoflavones

Isoflavones are naturally found in many plants but soya beans and foods made from soya beans are uniquely rich sources (22,23). Other beans and legumes contain very low levels. Soya food and drink provide on average 3-4mg of isoflavones per 1g soya protein.

Soya isoflavone consumption – West vs East: In Japan, the average isoflavone intake is approximately 40 mg/d compared to Europe and US at less than <3mg/d.

Soya isoflavones have a chemical structure that is similar, but not identical to, the human hormone oestrogen. For this reason, it is sometimes classified as a phytoestrogen or plant oestrogen. Small differences in chemical structure can result in huge differences in physiological effects. A good illustration of this point is the differing effects of dietary cholesterol and phytosterols; phytosterols, which are almost identical in chemical structure to cholesterol, lower blood cholesterol levels, whilst dietary cholesterol increases blood cholesterol levels.

An extensive technical review of over 400 clinical studies, observational studies, and systematic reviews and meta-analyses exploring the relationship between soya intake and hormone-related outcomes concluded that the consumption of soya foods as part of a varied and balanced diet is safe and do not increase levels of oestrogen in either men or women, nor have adverse effects on fertility or thyroid health.(22)

Soya and cardiometabolic health

More than 60% of deaths from diabetes and cardiovascular disease are associated with cardiometabolic risk factors such as hypercholesterolemia, hypertension, inflammation and obesity.(24) Soya food consumption, has been shown to have beneficial effects on cardiometabolic health, including:

• Lowering LDL-cholesterol between 4-6%(25)

• Modest reduction in blood pressure(1,2)

• Improving impaired endothelial function(1,2)

• Lowering inflammatory markers(2,24,26,27)

• Reducing waist circumference(26)

The cholesterol lowering benefits of soya foods are attributed to:

1. Indirect effects - As mentioned earlier, soya’s fat profile is predominantly made up of unsaturated fat with very little saturated fat. Soya foods and drinks often displace higher saturated fat containing animal foods in the diet. It has now been well established that reducing intakes of saturated fat and replacing with unsaturated fat is one of the most effective ways to lower LDL-cholesterol and improve heart health outcomes(21,22,29)

2. Direct effects - Soya protein per se, has been shown to directly lower serum cholesterol (a claim that has been approved by the FDA).(30) Although the exact mechanism has yet to be identified, it is proposed that soya protein has the capacity to upregulate hepatic LDL receptors and thus reduce intracellular cholesterol synthesis(28,31)

Read more about soya and cholesterol

A recent (2019) systematic review and meta-analysis of prospective studies concluded that increased soya foods consumption is associated with lower CVD mortality in women. Notably, pooled effect sizes for death from heart disease and stroke, in relation to soya intake, were 0.79 and 0.87 respectively which represents a large effect.(32) Additionally, soya isoflavones have been associated with improved arterial function and reduced stiffness.(33)

Soya and diabetes

Soya food consumption may also be useful for diabetes. Data from a large-scale cohort study (>13,000 participants) highlighted that women who reported consuming the most soya were 55% less likely to develop diabetes during a 10-year follow-up period compared to women who consumed the least amount of soya.(34)

A 2021 systematic review and meta-analysis of randomised control trials (RCT) concluded that soya consumption was associated with lowered fasting glucose, insulin, HbA1c, and HOMA-IR in diabetes patients.(35) Another review of clinical trials based on dietary interventions using soya foods observed reductions in blood glucose levels in participants with elevated levels at baseline.(27)

Read more about plant-based eating and diabetes

Soya and breast cancer

There is a significant body of research exploring the link between soya isoflavones and breast cancer risk and prognosis. Consuming soya foods has been found to be safe for breast cancer risk and survivors of breast cancer. According to the World Cancer Research fund, limited data indicates a potential protection against mortality and recurrence in breast cancer survivors.(36,37)

As well as numerous epidemiological studies demonstrating the safety of soya foods, a number of large cohort studies also indicate a modest (10-20%) reduction in risk with the consumption of 1-2 servings of soya foods daily.(22) Large population studies have investigated the impact of soya consumption at different stages of life on breast cancer risk. The studies compared the impact of consuming soya during adolescence only versus in adulthood only versus consumption starting in adolescence and continuing through adulthood. The data suggest that early life soya consumption has the highest protective benefit.(38)

However, those at high risk or survivors of breast cancer, may also benefit from soya consumption. A meta-analysis of 11,000 Asian breast cancer patients in China and the US showed a 26% reduction in recurrence and reductions in mortality of 16% in the patients that consumed the most soya compared with those consuming the least.(39) Additionally, this study demonstrated that soya consumption does not interfere with treatment of breast cancer e.g., tamoxifen.

In summary, 1-2 servings of soya foods as part of a nutritious balanced diet can be considered safe and potentially beneficial when it comes to breast cancer risk.

Read more about soya foods and breast cancer

Soya and prostate cancer

A meta-analysis of 14 studies published by Yan et al in 2009, found that consumption of more than one serving of soya foods daily is associated with a 26% decrease in prostate cancer risk, compared to low soya intake (less than two servings per week).(40)

Several studies in Asian populations consistently show that soya food and isoflavone intake is associated with a reduced prostate cancer risk.(40-45) However studies in Western men do not usually find an association. For example, results from the European Prospective Investigation into Cancer and Nutrition (EPIC) study did not find an association between isoflavone intake and prostate cancer risk in the European population.

An explanation for this lack of association is the very low intakes of isoflavones in Europeans making it difficult to compare intakes.(46) Average intakes intake in the EPIC study were on average just 0.9 – 1 mg per day, far too low to be able to identify a protective effect. For this reason intakes comparable to the daily intake of a traditional Japanese diet are recommended to reduce prostate cancer incidence.

The Seventh-Day Adventist study of 12,395 US men showed that frequent consumption of soya drink (at least daily) was associated with a 70% reduced risk of developing prostate cancer.(47)

Conclusion

Most soya foods and drinks have an excellent nutrition profile, rich in plant protein, low in saturated fats, provide unsaturated fats and, fibre. Thus, soya foods fit within a healthy, balanced eating pattern. Additionally, 1-3 daily servings of soya may contribute towards improved cardiometabolic health.