Soya and heart health

The burden of cardiometabolic disease

Cardiometabolic disease includes several common but often preventable conditions such as heart disease, type 2 diabetes and non-alcoholic fatty liver disease. Cardiovascular disease (CVD) continues to be the leading cause of death contributing to 32% of all global deaths(1) and the leading cause of diet-related deaths.(2)

Important behavioural factors associated with CVD risk include: unhealthy diet, physical inactivity, tobacco use, and excessive alcohol use.(3) The majority of premature heart attacks and strokes are preventable and about 49% of all premature deaths from CVD can be attributed to poor diet.(4)

In this scientific review, we discuss the role of soya food consumption as part of a varied plant-based diet to support cardiovascular health.

Population groups with higher soya intakes benefit from improved CVD outcomes

Evidence from observational studies supports the role of soya foods consumption in maintaining a healthy heart and reducing risk of CVD.

A 2016 meta-analysis of three case-control studies suggested that Asian women with the highest intakes of soya foods have a 46% lower risk of developing coronary heart disease (CHD), compared to those with the lowest intakes. In the same study, meta-analysis of data from cohort studies did not show the same relationship. However, a single dietary assessment was employed which means that the studies could not account for dietary changes during the follow up period.(5)

A more 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.(6)

Soya and cardiometabolic markers

Soya foods consumption, has been shown to have beneficial effects on cardiometabolic health, including:

• Lowering low-density lipoprotein (LDL) cholesterol between 4-6%(7)

• Modest reduction in blood pressure(8,9)

• Improving impaired endothelial function(8,9)

• Lowering inflammatory markers (8,10-12)

• Reducing waist circumference(11)

Additionally, soya isoflavones have been associated with improved arterial function and reduced arterial stiffness.(13)

Soya’s heart healthy nutrition profile

Soya beans and foods and drink made from soya are in the main low in saturated fat and can be good sources of the essential unsaturated fatty acids; linolenic acid (LA) and alpha-linoleic acid (ALA).(8) This is an important factor as saturated fat has been identified as the dietary factor that has the greatest impact on raising LDL cholesterol levels and CVD risk.(14-16)

Reducing intakes of saturated fat has been consistently shown to reduce total cholesterol and LDL cholesterol and lower saturated fat intakes have been associated with a 7-17% reduced incidence of CVD and CHD.(16)

Additional cardioprotective benefits are gained when saturated fats are replaced with unsaturated fats;(14,15) with greater reductions in LDL cholesterol and reductions in risk of CVD events by 27%.(15)

Soya’s beneficial fat profile and the fact that they are often used in place of higher saturated fat animal products, makes them an ideal addition to any heart healthy dietary eating pattern.

Soya and cholesterol

The fundamental importance of lowering serum cholesterol levels for improved CVD outcomes.

The evidence is significant for the direct correlation between changes in serum levels of LDL cholesterol and the risk of CVD. This was a key emphasis in the latest guidelines from the European Society of Cardiology (ESC) and European Atherosclerosis Society (EAS) for dyslipidaemia management and management of atherosclerotic CVD.(14)

The guidelines highlight the mounting evidence from both observational and high-quality human studies, including Mendelian and randomised controlled studies, which consistently demonstrate that lowering elevated LDL cholesterol reduces the risk of CVD proportionally to the absolute reduction achieved in LDL cholesterol.

Additionally, Mendelian randomisation studies clearly show that the duration of time one is exposed to lower LDL cholesterol is also important to overall CVD outcomes. Longer durations of exposure to lower LDL cholesterol proving more protective against CVD events compared to shorter periods of time.

Soya lowers cholesterol

One of the key-ways in which soya food consumption supports heart health is via its cholesterol-lowering effects. Soya protein has long been associated with heart-health benefits. In 1999, the FDA approved the health claim which has recently been revised:(17)

“25 grams of soy protein a day, as part of a diet low in saturated fat and cholesterol diet, may reduce the risk of heart disease."

A 2019 meta-analysis of 46 peer-reviewed studies (total 2,607 participants) has confirmed that a daily consumption of approximately 25g soya protein over a median follow-up period of 6-weeks, significantly decreased LDL cholesterol by 4.76 mg/dL (3.2%) and total cholesterol by 6.41 mg/dL (2.8%) compared to the control group.(18)

The analysis confirmed this modest effect to be intrinsic and further reductions could be gained if the soya food displaced higher saturated fat foods in the diet. This is in agreement with an earlier systematic review and meta-analysis demonstrating a 7.9-10.3% reduction in LDL cholesterol when 13-58g animal-based foods high in saturated fat and cholesterol in the diet are replaced with the equivalent quantity of soya foods – with no dose dependent effect.(19)

The cholesterol lowering benefits of soya foods are thus attributed to both:

• Extrinsic factor: as mentioned above, soya foods often displace higher saturated fat animal foods in the diet(19) and they have a naturally cardioprotective fat profile: low in saturated fat whilst providing unsaturated fats. This combination is one of the most powerful dietary factors for lowering LDL cholesterol and cardiovascular risk.(14-16,19)

• Intrinsic factor: soya protein per se, has been shown to directly lower serum cholesterol. 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.(7,18-20)

Soya’s cardioprotective benefits beyond cholesterol-lowering

Blood pressure: hypertension is one of the primary modifiable risk factors for CVD; attributable to 49% of CHD and 62% of stroke incidences.(21) Additionally, modest reductions in blood pressure can result in significant benefits to cardiovascular health with the latest findings from the large Blood Pressure Lowering Treatment Trialists' Collaboration demonstrating a 9-11% reduced risk of CVD events for every 5mm Hg reduction achieved.(22)

A number of studies have demonstrated soya foods to produce a moderate hypotensive effect. A recent meta-analysis of 15 randomised double-blinded controlled trials comparing soya consumers with non-consumers demonstrated a 1.7mm Hg and 1.27mmHg reduction in systolic and diastolic blood pressure respectively.(23) These are promising results, and more research would help identify effective doses.

Improved endothelial function: although more evidence is needed, soya isoflavones’ anti-inflammatory properties have been associated with improved endothelial function, reduced inflammatory markers and reduce arterial stiffness.(8,24)

Reducing waist circumference: visceral fat even in the presence of a healthy body mass index, significantly increases an individual’s risk to CVD and metabolic syndrome. Visceral fat, in contrast to endogenous fat, is ‘biologically active’ disrupting normal metabolic activity and promotes systemic and vascular inflammation, which is fundamental to all aspects of the atherosclerotic process, from fatty streak development to atherothrombosis.(25)

Studies have demonstrated soya protein to be effective at producing modest waist circumference reduction compared to controls.(26) A 2019 meta-analysis of 22 randomised placebo-controlled studies with overweight or obese participants demonstrated a significantly greater waist circumference reduction in the groups randomised to consume soya foods or drinks providing an average daily dose of 25.5 g soya protein, 60–135mg isoflavones and 240–720 mL of soya drink.(27) These findings demonstrate that soya foods can play a complementary role when included in calorie balanced programmes to help reduce body weight and visceral adiposity.

Conclusions

Soya, as part of a healthy balanced diet, can enhance cardiovascular health. The evidence supports soya’s positive effect on a number of cardiometabolic factors including improved LDL cholesterol and blood pressure, anti-inflammatory properties and favourable impact on waist circumference.