Guide to Soy Isoflavones

Soy isoflavones are a potent source of plant oestrogens called phytoestrogens. They are found naturally in soybeans and soy foods including tofu, edamame beans, soy sauce and miso. Plant oestrogens have a similar chemical structure to the human female sex hormone, oestrogen, and it is believed soy isoflavones may help to ease menopausal symptoms such as hot flushes, due to their oestrogenic effects.

What do soy isoflavones do?

The two major components of soy isoflavones are genistein and daidzein, which both weakly mimic the effects of oestrogen to help balance hormone levels. This can have a beneficial effect on hot flushes and night sweats associated with menopause, and may also reduce risk of osteoporosis. This fragile bone condition affects three million people in the UK and can lead to fractures.1

Hot flushes

Three out of every four women in the UK experience hot flushes around the time of their menopause, with sudden feelings of heat often accompanied by sweating, palpitations and a red flush.2 Soy isoflavone supplements may help to reduce these unpleasant symptoms. In a 2012 review of 277 relevant studies,3 it was found that soy isoflavone supplements were significantly more effective than a placebo in reducing the frequency and severity of hot flushes – although the authors said more studies were needed.


A decline in oestrogen levels is regarded as a leading cause of bone loss and osteoporosis in post-menopausal women. Soy foods are associated with improved bone health, especially among Asian women whose consumption of soy foods tends to be high.

A systematic review by researchers in China found women who took soy isoflavone supplements on a regular basis showed a 54 per cent increase in bone density. They also reported that regular isoflavone intake was associated with a significantly lower level of bone re-absorption, the process by which older bone is broken down and transported back into the bloodstream.4

However, the evidence on how soy isoflavones supplements may influence bone health is mixed and experts say more research is needed.5

Getting soy isoflavones from your diet

Soy isoflavones are found in chickpeas, grains and some vegetables, but soybeans (the high protein seeds of the soy plant) are by far the most concentrated source.

Soy-based foods include traditional Asian staples including tofu, tempeh, miso, natto (a fermented soy bean dish) and edamame beans. In a Western diet, we tend to consume a lot less soy based products (although increasingly we are choosing more in the form of soy protein, soy milk and yogurt), so a supplement could be a more convenient way of getting it.


Soy isoflavones can cause mild stomach problems such as bloating and cramping. It is also believed soy isoflavones can interfere with the absorption of synthetic thyroid hormones, so if you are taking thyroxine for an underactive thyroid condition, always check with your doctor first as you may need monitoring. 6, 7 Check with your doctor if you are taking any other prescribed medication before you start taking soy isoflavone supplements.

Some studies have shown that soy food intake is associated with longer survival and low recurrence among breast cancer patients.8

Correct dosage

There is currently no UK or EU recommended daily intake. Dosages available in the UK include one to two 100mg isoflavone tablets daily (each tablet contains 40mg of isoflavones).

Soy isoflavones are a natural source of plant oestrogens that could help reduce symptoms associated with menopause, including hot flushes, and may help to strengthen bones. Soy isoflavone supplements are popular with women going through the menopause and after.

Dr Sarah Brewer


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