Soybeans and soy foods

Soybeans are members of the pea (legume) family and have been a staple of Asian cuisines for thousands of years. This is partly because of their high-quality (‘complete’) protein and their ability to be processed into milk and meat substitutes. Soybeans also contain hormone-like substances called phytoestrogens, which mimic the action of the hormone oestrogen and have been linked with beneficial health effects.

Eating soy foods may reduce the risk of several health problems, including cardiovascular disease, stroke, coronary heart disease (CHD) and some cancers, and may improve bone health. Evidence also suggests soy is beneficial for peri-menopausal and postmenopausal women, who may have fewer and less severe hot flushes.

More research is needed, but the evidence so far suggests it is wise to include whole soy foods in your daily diet. See your doctor or dietitian for further guidance.

Nutrition profile#

Soy is a high-quality protein. It is one of the few plant foods (the others being amaranth seed and, to a lesser degree, quinoa) to contain all the essential amino acids, like those found in meat. Soybeans are:

• High in fibre
• High in protein
• Low in saturated fat
• Cholesterol free
• Lactose free
• A good source of omega-3 fatty acids
• A source of antioxidants
• High in phytoestrogens

Types of soy food#

Foods made from soybeans can be divided into unfermented and fermented foods:

• Unfermented foods include tofu, soymilk, edamame, soy nuts and sprouts.
• Fermented foods include miso, tempeh, natto and soy sauce.

Some soy products are also sources of calcium and iron — for example, tofu or tempeh made with a calcium coagulant, and calcium-fortified soy drinks.

There is also a class of ‘second generation’ soy foods. These include tofu sausages and burgers, soy breads, soy pasta, and soymilk yoghurts and cheeses. They also include products that contain soy ingredients such as lecithin (food additive 322), which can be found in some chocolate and baked goods.

Soybeans and phytoestrogens#

Phytoestrogens (‘phyto’ means plant) are naturally occurring chemicals found in plants. Under the right conditions, these compounds copy the action of the female hormone oestrogen, but are far weaker — about 1,000 times less powerful.

There are many types of phytoestrogens, one example being isoflavones. Isoflavones are strong antioxidants and can mimic the effects of oestrogen, although their full physiological effects are still being investigated. Soybeans are the most common source of isoflavones in food, but the amount varies with the type of soy food, the preparation method and the brand. Some of the richest sources are soy flour and soy nuts.

Health benefits of soy foods#

Research suggests soybeans and soy foods have a range of health benefits.

Heart disease#

A diet rich in soy foods is linked with a lower risk of cardiovascular disease, including stroke and coronary heart disease. Oestrogen may protect women against heart disease during their reproductive years, but rates of heart disease increase after menopause.

Soy has been shown to lower total and LDL cholesterol, both known risk factors for heart disease. An analysis of clinical trials suggests 14g to 50g of soy protein can significantly reduce total blood cholesterol, LDL (‘bad’) cholesterol and triglycerides, while moderately increasing HDL (‘good’) cholesterol. Research has also found that adults who include at least 25g of soy protein (about 4 servings) daily in a diet low in saturated fat and cholesterol can reduce LDL cholesterol by about 3 to 4 per cent.

Whole soy products (such as soymilk, soybeans and soy nuts) have a greater effect on cholesterol than processed soy products. It is not known exactly how this works — it could be the phytoestrogens or the soy proteins acting alone or together, or factors such as soy’s high fibre or low saturated fat content. Replacing animal protein (and the saturated fat and cholesterol it carries) with soy may also indirectly improve blood cholesterol. Notably, eating soy protein without isoflavones produces only small reductions, and isoflavone supplements alone have minimal cholesterol-lowering effects.

Menopause#

Because of their phytoestrogen content, the compounds in soy appear to act like a mild form of menopausal hormone therapy (MHT) and may reduce menopausal symptoms such as hot flushes. This is based on observations of women in some parts of Asia, who tend to eat soy-rich diets and typically report fewer hot flushes during menopause than women on meat-rich diets.

However, compared with conventional MHT, it would take almost a year of regular soy consumption to achieve similar effects. More research is needed, but soy may help some women manage hot flushes, even if only modestly.

Do not take highly processed soy supplements if you have a high risk of breast cancer or are a breast cancer survivor. Eating moderate amounts of whole soy foods, however, appears to be healthy.

Other possible benefits#

Other possible benefits of soy foods include:

• Improved cognitive function and visual memory
• Lowered blood pressure
• Improvements to blood vessels (such as greater elasticity of artery walls)
• Improved bone health
• Protection against some cancers, including breast cancer

Further research is needed before these benefits are conclusively known.

How much soy?#

Generally, around 30 to 50mg of isoflavones is enough to offer health benefits. The average isoflavone content of some foods includes:

• Half a cup of soybeans – 40 to 75mg
• Quarter cup of soy flour – 45 to 69mg
• One 250ml glass of soy drink – 15 to 60mg
• One 115g block of tofu – 13 to 43mg
• One 110g block of tempeh – 41mg
• One container of soy yoghurt – 26mg
• 2 slices of soy bread – 7 to 15mg
• One teaspoon of soy sauce – 0.4 to 2.2mg

Some studies suggest traditional soy foods tend to have more beneficial effects than ‘second generation’ soy foods. Don’t forget to check food labels — many fermented soy products, such as soy sauce, are high in salt.

Tips for increasing soy intake#

• Choose whole soy foods like soymilk, tempeh, soy bread and tofu.
• Check ingredient lists to make sure soy foods are made from whole soybeans and not soy isolates.
• Make sure products (such as cereals) contain soy protein and not just added isoflavone.

Foods other than soy that contain phytoestrogens include:

• Other legumes – such as chickpeas, lentils and kidney beans
• Wholegrain cereals – like oats, wheat, corn, barley, rye and buckwheat
• Nuts and seeds – such as almonds, linseed, flaxseed, sunflower, pumpkin and sesame
• Sprouts – such as alfalfa
• Oils – such as extra virgin olive oil

Soy and babies#

Eating soy foods as part of a normal diet is generally more beneficial than harmful. There has been concern about high soy consumption for some men and for people with thyroid conditions, but research suggests this is not a problem.

Studies have shown that soy infant formula does not appear to be harmful for healthy, full-term babies. However, soy formula may be harmful for premature babies and is best avoided. Check with your child health nurse or doctor.

Genetically modified soy#

Some soybean crops have been genetically modified (GM) to resist herbicide, but are otherwise identical to non-GM soybeans. GM soy can be found in primary soy products (such as tofu or soy flour) and in a wide range of other foods (such as chocolate, potato chips, margarine, mayonnaise, biscuits and bread). Some people prefer not to eat GM foods. Labelling rules for GM ingredients vary between countries, so check product labels if this matters to you.

Soy allergy#

Soy allergy can occur when the immune system reacts abnormally to soy protein. Soy allergies are common in babies and children. Symptoms can include:

• Itching, or a tingling sensation of the mouth and lips
• Flushed skin and hives
• Nausea and diarrhoea
• Wheezing and shortness of breath
• Dizziness and confusion

Lecithin (food additive 322) is made from soybeans and is often used in processed foods such as chocolate, baked goods, margarine, sauces, cheese and dairy products.

Not all soy products cause a reaction. Many people with a soy allergy can safely eat soy lecithin, soya bean oil and some fermented soy foods, as these are less allergenic than products containing raw soybeans.

If you have a soy allergy, seek advice from your doctor. Always read food labels and avoid soymilk, soybean, soya, soy nuts and soy sprouts, soy sauce, tofu, miso, tempeh and tamari.

Key points#

• Soybeans are the most common source of isoflavones in food.
• A diet rich in whole soy foods is linked with a lower risk of cardiovascular disease and lower cholesterol.
• Because of their phytoestrogen content, soy foods may reduce menopausal symptoms such as hot flushes.
• Do not take highly processed soy supplements if you have a high risk of breast cancer or are a breast cancer survivor.
• Soy allergy is common in babies and children; if you have a soy allergy, read labels and seek advice from your doctor.

Where to get help#