Soy contains isoflavones which are phytoestrogens with potential hormonal activity due to their similar chemical structure to estrogen (17-β-estradiol) and compounds that modulate estrogen receptors. This biological activity of soy derivatives has raised the question of whether dietary soy intake influences fertility. The answer is not a definitive yes or no but below is a summary of key evidence investigating soy intake and chance of conception.
In cows and sheep there is clear evidence that grazing on pastures high in phytoestrogens (which are found in soy) impairs ovarian function, reduces conception and increases embryo loss  and may lead to permanent infertility . Given this realisation in livestock it became a focus of research in humans to determine if there was an association between dietary soy intake and fertility, in both men and women.
Intakes in Women
The intake of soy products is most often higher in vegetarian diets where soy is included in the diet as a source of protein. The intake of vegetarian meat replacers, soy milk, and tofu/other soybean products all increases levels of isoflavones. On average, European intake of isoflavones is <1mg per day while women living in China and Japan consume 5–27mg per day. a recent British study reported intakes of ~10mg/day in vegetarian women, compared to an intake of 0.23 mg per day in non-vegetarians , but vegetarian intakes of isoflavones have been reported as high as 150mg a day.
A recent study carried out in North American Adventist women suggests a relationship between isoflavone intake and the chance of having a child . In women with high isoflavone intakes (≥40 mg/day) the lifetime probability of have a live birth was reduced by 3% compared to women with lower intakes (<10 mg/day). Women with higher isoflavone intakes also had a 13% risk of never being pregnant.
Bisphenol A, which you have heard of as BPA, is a chemical found in plastics that has detrimental effects on reproduction, in animals and humans. Data in rats and mice show that certain dietary factors can reduce the harmful effects of BPA and one of these is isoflavones . Does the same hold true in humans?
One study has investigated if soy intake modifies the association between BPA and fertility in women undergoing IVF . In women that did not consume soy products BPA concentrations were related to the chance of a live birth. Women with high BPA concentrations had a 17% live birth rate compared to a much higher birth rate of 54% in women with low BPA levels. In women that did eat soy products (and therefore had a higher intake of isoflavones), there was no relationship between BPA levels and birth rate, suggesting that soy was somehow protective against the adverse effects of BPA on fertility. In women that did eat soy and had low levels of BPA, fertilization rates were similar to women that did not eat soy (72% v 74% in non-soy consumers). A take home message from this study would be to avoid BPA!
Intakes in Men
A comprehensive study that looked at semen quality, morphology and DNA fragmentation has shown that isoflavone concentrations were associated with a lower percentage of normal sperm and increased abnormalities in other sperm health markers . In very overweight men however this association was not as strong, and this is most likely attributable to the fact that being overweight reduces fertility in men by ~50%, regardless of how much soy you eat!
Interestingly another study has shown that Male partner's intake of soy foods was unrelated to fertilization rates, the number of poor quality embryos, accelerated or slow embryo cleavage rate, and implantation, clinical pregnancy and live birth . Isoflavone intake was ~24mg per day in the highest intake group which is much higher than typical western intakes of around 1mg per day but still lower than intakes in Asia which are ~40mg per day.
Studies in healthy males that have added high amounts of soy to diets claim no impact upon sperm  however these intervention studies are often short (~8 weeks). Sperm takes around 3 months to go through the entire production process (from initial growth to being ready for ejaculation) so these studies should be viewed with caution as longer term impacts (> 3 months) need to be investigated.
Take Home Message
Low intakes (~10mg per day) of isoflavones do not appear to have a negative impact upon fertility but there is a chance that higher intakes for prolonged periods may not be conducive to conceiving.
If you are vegetarian your intake of isoflavones is likely to be higher than someone that includes meat in their diet so it is a good idea to do an audit of how much soy you consume. You could look for alternatives in some instances, such as replacing soy flour with high protein wheat flour, or soy beans with lima beans or chickpeas which have a much lower isoflavone content.
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