Soy Isoflavones Simply Explained

1. What It Is About

Soy isoflavones are substances found in soya beans and in foods made from soya. The best known are genistein and daidzein; a third is called glycitein. In the plant these substances are bound to sugar; this compound is only cleaved in the intestine. They resemble the body’s own hormone oestrogen. For this reason they are called phyto-oestrogens, “phyto” meaning plant. Because they resemble oestrogen they can bind to the same docking sites in the body, but far more weakly than the body’s own hormone. Research on the subject ranges from cell and animal studies through controlled human trials to observational studies and reviews. One point is important for assessment: how much of these substances reaches the body varies from person to person. Part of this depends on which bacteria live in the intestine. Some people can convert daidzein, with the help of their gut bacteria, into another form called S-equol that acts more strongly. In other people this does not occur. Consequently the same soya products can produce different effects in different individuals.

2. What Studies Show

Hot flushes in the menopause have been studied most frequently. A 2012 review by Taku and colleagues summarised 17 trials 3. The women took soy isoflavones for between six weeks and twelve months. In the pooled analysis hot flushes became less frequent. Because many trials were evaluated together this point is regarded as well established. For bone there is a smaller study with 46 postmenopausal women 4. They took isoflavones from red clover for six months. Afterwards bone density at the forearm and radius had increased. No such difference was seen at the lowest dose, but it was seen at the two higher doses. These are indications, but not yet confirmed, because only a few women took part and red clover is not the same as soya. There is also a study on heart and circulation: 80 men and postmenopausal women each took isoflavones for six weeks and a placebo for six weeks 1. Under the isoflavones the blood vessels were less stiff. Blood pressure remained unchanged. This is a single result in healthy people; whether it benefits people who are ill has not been clarified. In pre- and postmenopausal women a large review of 47 studies observed that two pituitary hormones that regulate the cycle declined 2. Other hormones remained unchanged and cycle length increased slightly. Perhaps the most important observation for understanding is another: only about one third to one half of people can convert daidzein into the more active form S-equol 5. Those who can may benefit more from soya. This question, however, is not yet settled.

3. What Remains Unclear

Many trials tested soya protein or whole soya foods. These contain, in addition to the isoflavones, protein and other substances. It is therefore often impossible to know what proportion of the result is attributable to the isoflavones themselves. Whether a person can form S-equol was not recorded in many studies either. This makes comparison of results more difficult. For hot flushes and for a symptom list called the Kupperman index the analyses reached differing conclusions. Which composition is most suitable for which complaints is an open question; direct comparisons are still lacking. One study on mood and cognitive performance was withdrawn; its results therefore do not count. Long-term observations on higher intakes are likewise still lacking.

4. What This Means in Everyday Life

Soya and soya foods have long formed part of the diet in many countries, and research on the subject goes back a long way. Anyone reading the results should distinguish two things: individual observations in a few people and the pooled evaluation of many trials. The pooled evaluations are more robust, and they show a recurring pattern especially for hot flushes. Second, not all people metabolise soy isoflavones in the same way. Gut bacteria help decide how much reaches the body. In studies the equol-producer status is recorded, that is the question of who forms S-equol and who does not. What follows from this for the individual remains open. The next step would be to clarify whether targeted promotion of gut bacteria achieves more than the isoflavones alone.

All details with study numbers and sources: detailed page on soy isoflavones (Genistein, Daidzein). For children: Soy isoflavones (Genistein, Daidzein) for children.