Information sheet: Soy & isoflavones

Key messages

• There is insufficient evidence for Cancer Council to make a recommendation on the consumption of foods containing isoflavones, including soy products, for cancer prevention.
• However, the available evidence does not suggest that these foods need to be avoided, including by breast cancer survivors.
• Caution is recommended in the consumption of traditional fermented soy foods preserved with salt (such as soy sauce, natto, and miso) as a probable link has been found between salt-preserved vegetables as traditionally prepared in East Asia and stomach cancer risk.
• Cancer Council does not recommend or support the use of supplements such as soy protein isolates or isoflavone capsules to prevent cancer.

Background

Soybeans are a legume native to Asia. Soy can be consumed as whole beans (edamame) or processed into a variety of products.

Soy-based foods such as miso, natto, tempeh and soy sauce are created using traditional fermentation methods. Soy can also be processed without fermentation into a variety of food products including tofu, soy milk, textured vegetable protein, meat substitutes, soy breads and crispbreads, soy cheese, soy yoghurt, soy pasta, and soy additives used in non-soy foods.

Soybeans and soy products contain plant compounds called phytoestrogens that can act like a weak form of oestrogen in the body. Phytoestrogens can be classified into four groups⁽¹⁾:

• Isoflavones/isoflavonoids – found in legumes, with soybeans being the richest source
• Lignans – a constituent of dietary fibre and found mainly in nuts, seeds, legumes, cereals, vegetables and fruits
• Coumestans – found in sprouted beans, alfalfa, spinach and clover
• Stilbenes – found in red wine and peanuts

As oestrogen has been linked to certain cancers, there have been concerns that eating foods containing phytoestrogens may affect your risk of developing or recovering from cancer.

Epidemiological evidence

The World Cancer Research Fund (WCRF) have reviewed the available evidence on the association between the intake of foods containing isoflavones and the risk of cancer⁽²⁾.

The 2017 review by WCRF concluded that there was not enough evidence on the link between the consumption of foods containing isoflavones and risk of pre- and post-menopausal breast cancer to make an evidence statement⁽²⁾. In studies on dietary intake there was limited suggestive evidence that foods containing isoflavones decreased the risk of lung cancer in people who have never smoked⁽²⁾ but they did not find sufficient evidence to make a recommendation⁽²⁾.

A 2014 review of the available evidence by the WCRF found that breast cancer survivors who ate more soy foods after diagnosis may have a lower risk of dying, however the evidence is listed as limited suggestive and not sufficient to make a recommendation⁽³⁾.

Biological mechanism

The biological mechanism underpinning the observed relationships between isoflavone-containing foods and cancer risk is not entirely known. Isoflavones share a similar structure with a hormone (a form of oestrogen)⁽²⁾. It has been hypothesised that isoflavones may reduce the activity of oestrogen by attaching to oestrogen receptors in the body (thereby blocking internally produced oestrogen from attaching to the receptor) or inhibiting oestrogen production⁽⁴⁾. Isoflavones have been found in some studies to reduce the activity of cellular processes that promote cancer growth⁽⁴⁾.

People who consume higher amounts of soy foods, dietary isoflavones and phytoestrogens generally have better diets, which could impact on any observed relationships found between these foods/food components and cancer outcomes⁽²⁾.

Current levels

The intake of legumes, which includes soy, is relatively low in Australia, with 6% of vegetable intake and 5% of the intake of meat and alternatives coming from legumes⁽⁵⁾. Approximately 6% of Australians aged 14 years and over drink soy milk at least once a week⁽⁶⁾.

Although soy products are a major dietary source of isoflavones, other foods such as pistachios, adzuki beans and broad beans also contain appreciable amounts⁽⁷⁾. This means that ‘foods containing isoflavones’ and ‘soy foods’ are distinct, but overlapping, dietary intake categories.

Dietary recommendations

The Australian Dietary Guidelines include soy foods within three of the five essential food groups of vegetables (i.e., soybeans), meat and alternatives (i.e. soy beans, tofu and tempeh) and dairy and alternatives (i.e. calcium-fortified soy milk and yoghurt)⁽⁸⁾.

The available evidence suggests that soy and other isoflavone-containing foods are unlikely to increase cancer risk and may in fact decrease the risk of developing some cancer types, although the evidence is not strong enough at this stage. Cancer Council recommends that people who already consume soy foods as part of their normal diet, including those who have been diagnosed with breast cancer, should continue to do so.

Salt-preserved fermented soy foods, such as natto, soy sauce and miso, should be consumed only in small amounts, as there is some evidence linking these foods to an increased risk of stomach cancer.

Some experimental studies have found adverse effects of high doses of phytoestrogens on cancer risk. Therefore, the use of supplementary sources of soy and phytoestrogens, including isoflavone supplements and soy protein isolate, is not recommended, particularly for women with a history of breast cancer.

Policy context

Cancer Council Australia supports:

• Investment in regularly updating and promoting national dietary guidelines and
• Investment in conducting regular dietary surveys to understand current dietary patterns and inform the promotion of dietary guidelines.
• The implementation of a sustained, effective, and adequately funded social marketing campaign, focused on the adverse health outcomes associated with obesity, poor diet, and physical inactivity.
• Mandating the Health Star Rating system