Hi Dan, I've seen some articles online that suggest males should limit their intake of certain foods because they can lower testosterone. Examples are soy products and flaxseed, both of which are often touted as healthy. Is that true, and if so, how much is too much?

You’ll find this being written in many articles, so I thought I’d have a proper dig into what evidence their words are based on. There’s a study from 2001 that is cited by several of these articles where men with prostate cancer consumed flaxseed before surgery and their testosterone levels decreased. Pretty clear cut right? Not at all because the participants also decreased their fat intake and we know this lowers testosterone levels. 

The good news is the same scientists that did the 2001 study conducted a follow-up study in 2008 and found that flaxseed did not result in lower testosterone and the effects were most likely due to the stress experienced before surgery. 

When we look to rats, even when flaxseed made up 25% of their diet – which if you hadn’t guessed is a whole lot of flaxseed – testosterone levels were not affected. The same is true for soy products as shown by a review that looked at lots of different studies and put them together.

There are a handful of studies and isolated case studies which suggest a negative effect on testosterone levels such as a man who consumed nearly 3 litres of soy milk a day. However, when we look at all the evidence together these are seen as exceptions to the rule. You’ll also find these articles quote studies looking at women with PCOS or breast cancer but they are not relevant to a discussion about testosterone.

If you want my honest opinion most articles just copy one another without doing the due diligence to check if what’s being said is actually true. The link of testosterone to these foods is as much cultural as it is scientific because meat is seen as “manly” while plant-based foods are “feminine”. Honestly if a seed can crack someone’s fragile view of masculinity that easily then they’ve got other issues. 

So onto the health side. Yes, both flaxseed and soy are healthy due to the variety of micronutrients they contain. They’re also high in fibre and good sources of protein and unsaturated fats. 

In short, both flaxseed and soy are great additions to your diet and the effects on testosterone aren’t something to worry about.

TL;DR

I wouldn’t worry at all. These articles often copy one another and with a little bit of digging you’ll find the evidence these claims are based on just isn’t there.  

Dan Clarke, RNutr

Junior Sustainable Nutrition Manager