Ask The RD: Soya & Fertility

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Hello there! Happy Friday! Oh ok ok…it’s MONDAY but after 3 days of stormy weather interfering with my internet connection i’m a bit late. Sorry about that!

Today I’m going to answer a question from a friend of mine Ruth.  Ruth has suffered from eczema for most of her life and after some experimenting with her diet (more on that another time!), she removed some foods she felt were contributing to her flare ups.  One of the things Ruth removed was dairy produce, and so in order to make sure she was still getting enough calcium in her diet, Ruth (sensibly) substituted diary with calcium fortified products such as soya milk and yoghurt.

Now, I don’t know about you, but when I think about soya, I think about plant based lifestyles, fibre and the all the possible health benefits it’s associated with (such as lowering cholesterol and reducing menopausal symptoms).  However, more recently there has been an interest in the potential effects of high soya consumption on fertility…..


So, How Could Soya Affect Fertility?

Soya contains biologically active compounds called Isoflavones.  Isoflavones have a similar chemical structure to human Oestrogen (or Estrogen as you US folks know it!) and can mimic it’s effects in the body.  Oestrogen is an important female sex hormone (involved the development and release of eggs from the female ovaries during the menstrual cycle) so having high levels of circulating isoflavones could, in theory, disrupt other hormones and interfere with things like sexual development, the menstrual cycle and fertility.


What Foods Contain Soya?

Soya and soya products are made from the soya bean.   In Asian populations they have always been widely consumed in the form of tofu and tempeh, but these days soya products are also consumed regularly in western diets.  Soy is also used widely in the food industry and it can be found in all sorts of products, even those you wouldn’t expect, such as biscuits and burger buns. This means that even those people that don’t consciously eat soya based products are probably getting small amounts of soya in their diet.

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The amount of isoflavones you get from eating soya is very variable.  The type and amount of soya you eat and the processing of the soya bean will affect how much ends up circulating in your system!


What Does the Science Say?

Most of the studies looking at the effects of soya on fertility have been carried out in animals and many have shown that exposing animals to high levels of isoflavones can have a negative effect on their fertility. However, while these studies are a useful insight into the mechanism by which isoflavones may affect humans, as I have mentioned before, the results of animal studies can’t be directly translated to humans. Differences in how animals metabolise the isoflavones, the route by which the soy enters the body (we don’t inject pure isoflavones like in some of these studies, we eat it as a whole soy product), and the amount of isoflavones ingested and absorbed mean the effects could be totally different in us.


To know how exactly soy isoflavones effect human reproduction. We need to study them in humans.


Unfortunately, human studies on the effects of isofavones on fertility are much fewer in number. The small sample sizes, short duration and variable methodological quality of the studies which have been done make it impossible to draw any firm conclusions.  One review of the literature showed that consumption of soy foods and isoflavone supplements led to small changes in circulating levels of reproductive hormones and an average period increase of just over a day, but authors could not say clinically what this would mean in the long term for fertility. This most recent review of the data suggests that consuming high levels of soy (>100 mg soy isoflavones/d) can lead to reduced ovarian function in women during their reproductive years, but again the data is not conclusive.

Unfortunately, the long term effects of consumption of soya and soya products on fertility are relatively unknown. Larger scale, well designed studies are required to answer many questions that need answering about soy consumption.


Okaaay…But Should I Be Worried & How Much Is Should I Eat?

Although the isoflavones found in soy have the potential to interfere with our reproductive cycles, the literature as it stands suggests that this would only occur at very high levels of soy intake (which could lead to high circulating levels of isoflavones). Diets containing low or moderate amounts of soy, for example 1 or 2 servings per day (as part of a well balanced diet), should not have a significant effect on reproductive hormones or fertility. This is about equivalent to 10-25mg of isoflavones per day. Higher intakes may have a significant effect on circulating reproductive hormones, but without further research, we can’t tell if that would have a long term effect on fertility.

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Bottom Line

There is nothing to suggest that including low -moderate levels of soya (1-2 servings per day) as part of a healthy balanced diet would be harmful to your fertility.  For groups that tend to have a higher than average soya intake, such as non-meat eaters, be wary of reducing or removing soya from your diet without replacing it with a suitable alternative (discuss with a registered dietitian). 


Note: Removing whole food groups from your diet can lead to nutritional deficiencies. Before you start a new diet, you should discuss it with your doctor or a registered dietitian.
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6 Responses

  1. Maureen Morgan
    February 17, 2014 at 2:27 pm

    Hi Helen
    This is a good balanced article which I have enjoyed reading and with all the hype for and against soya in our diets it is good to know what is considered a moderate amount for a balanced diet.

    Another great article

    Maureen M

    • helen
      February 18, 2014 at 6:58 am

      Thanks mum ;) xx

  2. Lauren
    February 17, 2014 at 2:36 pm

    Soy can be so confusing! I definitely lean towards whole organic soy like edamame, miso soup (so good!), and tempeh over soy protein powders and other processed soy. Good article!

    • helen
      February 18, 2014 at 6:57 am

      I know Lauren! the literature is tricky! I’m like you, I like whole soy foods…the tempeh out here in Bali is AMAZING. That said I did read somewhere that Indonesian tofu contains formaldehyde?! I really should look into that more! :)

  3. Rachel
    April 12, 2016 at 4:07 am

    On point, I’ve got to eat more soy so I can stop having kids.

  4. Jen
    July 25, 2016 at 7:47 am

    I think it all depends on YOU! I’m severely lactose intolerant.. I’ve avoided dairy and cheese for so long I can’t even stand the smell! I try to alternate between different plant based yogurts and milk so I’m not consuming too much of anything but soy will always be my favorite and I have no issues consuming 3 servings daily(doesnt happen that often) There’s a lot of studies showing how damaging cow dairy is so it’s really all up to you…
    I haven’t drank milk, cheese, yogurt, etc. in over 5 years but my calcium levels are very good! Yay for a balanced diet!

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