So where do these fears stem from? Most are linked to the naturally occurring phytoestrogens (hormone-like plant compounds) in soy, known as isoflavones. Phytoestrogens mimic the effects of oestrogen in the body, leading to concerns that soy may be harmful due to a link between high oestrogen levels and health risks, including breast cancer – oestrogen increases cell division and may therefore contribute to the growth of cancerous cells.
But the plot thickens. In a 2008 study published in the British Journal Of Cancer, researchers found breast cancer risk was significantly reduced in women consuming high intakes of soy (providing more than 10mg isoflavones), compared with those eating smaller amounts. Quite the turnaround. This can be explained by the presence of two oestrogen receptors in the human body – alpha and beta. When it comes to breast cancer, alpha poses the biggest risk. According to experts, isoflavones preferentially bind to the beta-receptor, which may actually have a protective effect. More recent research concluded that for women living with breast cancer, soy intake isn’t detrimental for prognosis or survival.
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Cancer risk aside, a quick google of ‘soy’ and ‘fertility’ is enough to make you ditch those dairy-free milkshakes for good. It comes down to the link between genistein (the primary isoflavone in soy) and reduced fertility in studies done on sheep, mice and rats. But these studies typically involved consuming genistein at levels five times higher than the levels you’re exposed to by eating food containing soy, so there’s an element of scaremongering here.
In fact, more recent research suggests soy might actually be good for fertility. In a 2015 study of women undergoing fertility treatment, isoflavone intake was positively linked with live birth rates. And, according to a 2016 study, soy isoflavones may even protect against the negative effects of BPA (the chemical found in plastic and cans) on fertility.
The upshot is there just isn’t enough convincing evidence in human trials to suggest you need to ditch soy. When it comes to fertility, there are arguably more impactful things to consider; carrying too much or too little body fat, smoking, alcohol and high stress levels can all negatively affect your chances of conception. And aside from isoflavones, eating soy protein as part of a healthy diet has been linked with a lower risk of heart disease, which is another reason not to ditch it. Edamame me up.