Food and drink: It's time to spill the soy beans
Soy bean products are growing more popular and, from milk to tofu, there is an entire range of foods available. However, there are some controversies too. So just how healthy and safe is it? And how much should you eat? Dietitian Orla Walsh separates the truth from fiction.
Soy comes from soybean plants, which are part of the legume family. As soy is closely related to the likes of beans, peas and lentils, it too is a great source of plant protein. You will find the same amount of protein in 1.5 cups of cooked soy beans as you would find in 150g of meat. Unlike most other plant proteins, soy is considered a complete protein - meaning that it contains all the essential amino acids or building blocks that make up a really good source of protein.
Not only is it a good source of protein, but it's also jam-packed full of vitamins and minerals. Most notable micronutrients include calcium and iron, making soy an excellent choice for vegans who omit the likes of iron-containing meats and calcium-containing dairy from their diet.
Like most legumes, soy beans are also an excellent source of fibre. Soy provides both soluble and insoluble fibre in excellent quantities. Soluble fibre is the gel-like fibre that helps soften the stool, while insoluble fibre is the roughage that bulks it out. Therefore soy beans can act like a sweeping brush, cleaning our gut and keeping things moving!
There are two things that make soy beans unique among their legume pals - how much fat they contain and their isoflavone content.
1 Soy beans are about 20% fat. Most other beans, peas and lentils are relatively fat-free. However, their fat content is certainly not a negative - the fat they provide is healthy fat, with about 60% being polyunsaturated, and 30% monounsaturated fatty acids.
2 Soy beans are a rich source of isoflavones, which are antioxidants that have many health benefits. Each gram of soy protein is associated with approximately 2.2mg to 3.3mg of isoflavones. Isoflavone content can vary depending on soil and growing conditions. There are many kinds of soy foods that are made from soy beans, such as tofu, soy milk, textured vegetable protein (TVP) and soy sauce. They are not created equal - their level of processing changes their nutrient profile.
Firstly, the fat content of soy foods differs depending on whether the whole bean is used. For example, TVP is fat-free, while other soy foods such as tofu and soy milk contain a little more fat.
Secondly, their isoflavones content varies, depending on how processed the food source is and how the soy protein was extracted from the soy bean. Tofu, soy milk, soy nuts, tempeh, edamame, soy flour and textured soy protein all contain significant amounts of isoflavones. Very processed soy-containing fast foods like soy burgers contain considerably less isoflavones than the more whole food varieties, while the likes of soy sauce contains none.
Over the last number of years, soy has gone from in-vogue to a queried food. There are a lot of myths about it, so let's get to the bottom of each.
Is soy good for menopause?
Most women in the Western world experience hot flushes, while only about 20% of Japanese and Chinese women are thought to, possibly in part due to their traditional diet being based on soy foods. On average, Japanese women are thought to eat about 15mg-40mg of isoflavones per day, which is approximately 10 to 20 times higher than the typical diet of a Western woman.
Other benefits proposed to be due to this higher isoflavone intake include lower risk of heart disease and osteoporosis as well as cancers like breast, colon, endometrial and ovarian. Hormone replacement therapy remains a popular and effective treatment of symptoms associated with menopause.
What may add additional benefit is upping soy protein intake. Studies have shown that eating 50mg to 80mg of soy isoflavones each day for eight to 12 weeks helped lower the frequency and severity of hot flushes.
Is soy good for your heart?
A wealth of research led to the US Food and Drug Administration (FDA) encouraging people to eat 25 grams of soy protein each day to help lower cholesterol. This is the amount found in about three servings of tofu or soymilk.
How it's thought to work its magic is two-fold. Firstly, soya foods are eaten instead of foods higher in saturated fat, thus displacing some of the cholesterol-increasing foods within the staple diet. Secondly, it's thought to reduce the body's natural cholesterol-producing capacity within the liver.
A meta-analyses (a way of grouping and assessing clinical trials together) reported that soy protein lowers LDL 'bad' cholesterol by 4% to 6%. This is a similar effect to cholesterol-lowering effects of soluble fibre, as found in oats. To put this in real terms, for every 1% reduction in cholesterol, there is an estimated 1% to 2% lower risk of heart disease.
Furthermore, soy protein has been shown to lower circulating triglyceride levels by approximately 5% to 10% and may increase HDL 'good' cholesterol by 1% to 3%. This again may be clinically relevant, since each 1% increase in HDL cholesterol has been said to lower heart disease risk by 2% to 3%.
The heart-healthy benefits from soy are thought to extend further than blood cholesterol results. Researchers from Harvard University reported that soy dramatically lowered blood pressure, too.
In this study, 25g of protein intake per day from soy nuts lowered systolic and diastolic blood pressure by 10% and 7%, respectively. Additionally, small studies suggest that the isoflavones of soy improve the flexibility and function of the blood vessels.
There are some discrepancies out there, put down to things like non-human studies, the source of the soybeans used, different processing methods of soy and more studies showing results in women than men. However, the bulk of research is pointing to soy being good for the heart.
Is soy bad for the thyroid?
A review of 14 studies suggests not! The researchers found that there was no harmful effect experienced from eating soy food in those that were healthy and had a normal functioning thyroid gland.
If you are taking synthetic thyroid hormones, be sure to avoid eating soy foods at the same time that you take this tablet, because soya isoflavones can interfere with the absorption of your medication. However, do note that your medication is generally supposed to be taken on an empty stomach.
In one study, six out of the 60 women with subclinical hypothyroidism developed clinical hypothyroidism after eight weeks of supplementing their diet with 30 grams of soy protein and 16mg phytoestrogens. It is thought that this only occurs in some people who take a very high dose of phytoestrogens. Interestingly, in the exact same study, those that consumed the higher dose of soy and phytoestrogens experienced significant reductions in insulin resistance, blood pressure and inflammation markers. More studies are needed to optimise the dose and timing of soy supplementation in order to maximise the potential benefits and minimise the potential downsides.
Does eating soy increase the risk of breast cancer?
There have been concerns surrounding soy due its phytoestrogen or plant osetrogen content. Large epidemiologic studies that followed large numbers of healthy women over the years have shown no association between soy and breast cancer.
Other studies have looked at the eating habits of people after a breast cancer diagnosis and found that in the 9,000 breast cancer survivors studied, eating soy lowered the risk of breast cancer recurrence. Eating at least 10mg of isoflavones daily has been linked to a 25% decrease in breast cancer recurrence. Therefore soy foods, not soy supplements, may help to prevent breast cancer.
There is a host of research ongoing to try and understand it further. It does appear that the type of soy product used in studies may be a factor, with the less processed foods winning out.
Where the concerns around soy and breast cancer came from were earlier studies that suggested that genistein, an isoflavone found in soy, increased growth of oestrogen receptor-positive breast cancer cells and promoted breast cancer growth in some rats and mice. However, further research suggests that rodents metabolise phytoestrogens differently to us humans. Recent studies suggest that oestrogen-sensitive breast cancers do not need to avoid soy foods.
How to add more soy into your daily diet
1. Make your breakfast porridge with oats and soy milk, as an alternative to cow's milk.
2. Enjoy a soy yoghurt with fruit at your mid-morning break.
3. Try adding some tofu to your salad instead of meat or chicken
4. Enjoy some tasty soy nuts as your afternoon snack.
5. Add some edamame beans to your dinner - they're lovely in a stir-fry!