Podcast Episode 43 Transcript – Is Soy Good or Bad for Your Health?

Aidan Muir       

Hello, and welcome back to The Ideal Nutrition Podcast. My name is Aidan Muir and I’m here with my co-host, Leah Higl. This is episode 43 where we’ll be talking about whether or not soy is good or bad for your health.

Personally, I’ve got a bit of general knowledge about this, but it’s not something I’m really a deep, deep expert in, because I don’t really need to be. I don’t see [00:00:30] many cases where it really impacts my clients or anything like that, because a lot of things are probably going to be more relevant if somebody has a really high soy intake, so I don’t see a lot of that.

Leah sees a little bit more of that I assume, it’s a bit more relevant for the kind of stuff she does. We figured that this is probably going to be a bit of a good episode in terms of something we can point back to anytime anybody asks us a question being like, is soy good or bad for your health, or how much is okay, and anything like that, we can just point to this episode. But because of all of that, we’re going to do it [00:01:00] in a bit of an interview format where I’m pretty much just asking Leah questions and she’s pretty much just answering them.

So starting off, we’re just going to start off with background stuff, very, very general, and then I’m just going to ask stuff that I’m personally interested in. So, the first question is what foods contain soy and how much do people normally consume?

Leah Higl

Yeah, that’s a really good place to start. So if you’re not sure where soy usually comes from, we’re thinking things like edamame, obviously soybeans, tofu, [00:01:30] tempeh, soy milk, textured vegetable protein. So, really thinking about those plant-based proteins that are really common in a vegetarian and vegan diet, not so much in the standard Australian diet.

So we might have a little bit of some soy protein fortified foods in a normal Australian diet, but it’s pretty low. So generally a standard Australian’s going to eat one to two serves per week of soy foods, and usually it’s going to be a soy fortified product rather [00:02:00] than a block or tofu.

Aidan Muir

Yeah. Yeah, cool. All right, so jumping into I suppose the most pertinent kind of topic, but I want to start real vague and then I’ll go more specific. What are the proposed negatives of soy consumption? What are people talking about? What are people caring about?

Leah Higl

I guess I kind of break it up into four main topics usually. So, one that comes up a lot in the fitness space would be feminizing effects in men, so that tends to be an overarching [00:02:30] one. One I see a lot is issues with fertility in women and some worry around that. Third would be just general cancer risk, so soy increasing cancer risk. Four would be issues with thyroid function. Particularly people that already have low thyroid function, does soy then go on to effect it further?

Aidan Muir

Yeah. So starting off with that first one, feminizing effects in men, where does that come from? Why would people think that? What [00:03:00] would the mechanism be? Actually, let’s just start with the mechanism. What would the mechanism be?

Leah Higl

Yeah. Yeah, so let’s start with phytoestrogens, because that’s particularly what people are concerned about when it comes to soy. So there’s a little bit of debate generally around GMO and soy, but we’re kind of going to leave that out of this discussion, because it’s a rabbit hole of its own in terms organic and whatnot. But we really want to focus on phytoestrogens, because they [00:03:30] are … I guess people worry, because they have the word estrogen in them.

So they are a hormone, but they’re a plant hormone. So they can have estrogen type effects in the human body, but they’re really, really minimal. But the effects are still there, so people are still concerned that, oh, maybe they’ll increase estrogen levels in the body, have issues with fertility feminizing effects in men, and that’s kind of where the fear comes from.

Generally, I find there’s that one article [00:04:00] that was in Men’s Health magazine, oh, it would’ve been 2010, something like that, 2008, that was a bit of a fear mongering around soy, where there was this case study of this one 60-year-old man who was drinking three liters of soy milk per day, so a lot of soy milk, and then his estrogen levels did increase. He experienced breast tissue growth, erectile dysfunction [00:04:30] and low libido as well.

So, they’re the four main things he complained of. When looking at his diet, they realized that like, “Man, this guy’s having a lot of soy.” When they took the soy out of his diet, all those issues did go away.

He returned to baseline in terms of his estrogen levels, all the side effects went away. Issue with that case study is it is a standalone one person situation, it’s not obviously a population study, so that’s not the peak of evidence. [00:05:00] The second part of that is three liters of soy milk. So, that’s way more than definitely the standard Australian diet, but even when we’re thinking vegetarians and vegans, we tend to consume about 40 to 60 milligrams of these soy isoflavones in general.

Even a little bit lower for some vegetarians that are not super soy-focused. This guy would’ve been having 360 milligrams per day of these soy isoflavones. [00:05:30] I guess I should mention that phytoestrogens and isoflavones are interchangeable concepts to a certain extent, so I’m just going to use that interchangeably.

Aidan Muir

Yeah. Yeah, cool. So just out of my own curiosity, because that’s just one case study, but it obviously begs the question of is this a bit of a spectrum? Is this something where it’s like, okay, zero soy intake means you have none of these effects, a moderate soy intake means you probably don’t even notice these effects and a super high one might lead to this? But do we have anything else on [00:06:00] really high soy intakes and that concept? Is there anything on that?

Leah Higl

To be honest, there’s not a lot of research that is someone consuming that amount of soy.

So generally when we’re looking at research, the most that people will usually get is about 100 milligrams of these soy isoflavones. So he was consuming almost four times the amount that we even see in research at its highest amounts, so it’s really hard to say. I’m sure he’s probably not the [00:06:30] only person that’s ever going to experience that from that level of soy intake, it’s just that’s so out of the norm and we don’t have research on it.

Generally research in men looking at effects on estrogen levels and soy intake, when it’s moderate to even mildly high tends to have nill effect.

Aidan Muir

Yeah. Okay, gotcha. Yeah, super interesting. Cool, cool. That’s obviously pretty relevant, because a lot of people are scared of having small amounts.

Leah Higl

Yeah, people are scared of having tofu once.

Leah Higl

Like, “Oh, I’m not going to eat that.” So, [00:07:00] you’re not going to experience these feminizing effects from having a few serves of soy per week.

Leah Higl

If anything, it probably is even good for our health.

Aidan Muir

Yeah, and we’re going to touch on the good for health kind of aspect a little bit later. I want to go through “the bad stuff” first.

Next one, fertility in women. So, that was something you talked about. That’s a bit of a complex area, isn’t it? It’s pretty mixed?

Leah Higl

It’s really, really complex. So if we look at the research as [00:07:30] a whole, mild to moderate soy intake looks like it improves pregnancy and fertility outcomes-

As a whole looking at all of the research. But I don’t like to brush things off and go, “Oh, there’s no effect,” because we know that phytoestrogens can act on pretty much any sex organ that we have, because it does mimic estrogen in some way. So, to say it has no effect I think would be missing the mark a little bit.

[00:08:00] There’s basically three case studies. So very similar to the one 60-year-old man that had three liters of soy milk per day, there was three case studies in the research of women consuming, they estimated it to be over 100 milligrams of soy isoflavones per day on average. These three women experienced loss of period, difficulty with fertility and a few other issues.

Again, once they removed soy from the diet, all those issues went away, so [00:08:30] there was clearly some link between the two.

But generally, the research at a population level says that it’s not really going to be an issue.

So, it’s a little bit of a mixed bag. Most of it’s positive, but that tiny little bit of negative research I think is worth knowing about.

Aidan Muir

Yeah, just worth being aware of I guess. Yeah.

Leah Higl

Yeah, because potentially maybe there are people that are more sensitive than others.

Maybe there is going to be that one woman in a fertility clinic wondering [00:09:00] why she’s not getting pregnant, and maybe she’s vegan and eating a lot of soy foods, so it’s worth considering. It’s probably not the reason most people are infertile obviously, but it’s worth noting I reckon.

Aidan Muir

For sure. That third one you mentioned was cancer risk. How does soy play a role there?

Leah Higl

So in regards to cancer risk, a lot of people assume that soy foods are going to increase the risk of some cancers, but based on the research, we actually [00:09:30] know that it’s probably beneficial for reducing the risk of some cancers. So breast cancer is one, prostate cancer in men is another where it seems quite positive.

So, breast cancer is one. So, the largest and most detailed study to investigate soy intake and the risk of breast cancer developing was the Shanghai Women’s Health Study. So, that had over 70,000 participants and it was long-term, so a huge study. They found that women [00:10:00] who ate the most soy had 59% lower risk of breast cancer compared to those who ate the lowest amount of soy.

Aidan Muir

How much is that? 59%.

Leah Higl

59%.

Aidan Muir

That’s pretty huge, yeah.

Leah Higl

So, that’s huge. So this was in China, so it’s probably a little bit higher intake of soy than what we’d have here at Australia generally as well, but that’s a huge statistic. So not only does soy not cause breast cancer, it’s maybe protective [00:10:30] against breast cancer-

Aidan Muir

Yeah. Yeah, that’s interesting, hey?

Leah Higl

Which is interesting.

Aidan Muir

Because I have heard people link the two, but it’s interesting to see in breast cancer in that large scale study, not only was it not increasing the risk, it was-

Leah Higl

Is decrease and by a fair amount.

So overall when it comes to cancer and soy intake, it seems pretty positive and beneficial rather than the other way around.

Aidan Muir

Yeah, super interesting. Then the last one that we were going to touch on, thyroid [00:11:00] function. We’ve spoken off air about this, but it is a complex topic. But do you want to give a bit of a broad overview on that? I might ask some more specific stuff if anything comes up.

Leah Higl

Yeah, let’s start with people taking thyroid medication, because I feel like that’s the easiest place to start. So, we know that soy does interact with thyroid medication to a certain extent. So if you are on thyroid medication and you eat a lot of soy, it’s probably worth speaking to your doctor about if you’re going to have a sudden increase [00:11:30] in soy intake. It’s also best to not have soy three to four hours around the time you take medication.

So, there has been some worry around a high soy intake potentially reducing thyroid function in people with already compromised thyroid function. Once again, looking at the breadth of the research, it doesn’t seem that it does that. So a moderate soy intake does seem to be okay for people that do have [00:12:00] hypothyroidism, but there are some studies where people did have moderate soy intakes and their thyroid hormones did change in a negative way, but the researchers didn’t deem it clinically significant enough to kind of say, “Oh, X equals X.” But I think, again, it’s worth noting that there is some change in thyroid hormones at a certain point in some people with soy intake.

Aidan Muir

Yeah. [00:12:30] Yeah, I wrote a blog post on Hashimoto’s, so underactive thyroid in that case, and it was something that I kind of saw as well where a lot of people in the thyroid space who are not necessarily “evidence-based practitioners” will very much be like, “No soy ever, that’s going to make things worse.” Then there’s a lot of people in the evidence-based space who are just like, “No, soy’s completely fine.” But-

Leah Higl

I think it’s more nuanced than that, to be honest.

I think for most people, particularly [00:13:00] if you’re on thyroid medication, it’s probably not going to make a huge difference to have a mild to moderate soy intake.

But even thinking about some of the vegans I work with in the sports space, some of them do have underactive thyroid and trying to eat a lot of protein and therefore having a lot of soy in their diet. So I do wonder for those people, is it an issue at that point?

With the research, there’s not a lot of research that has a really high soy intake the way that vegans [00:13:30] are starting to consume it, or the way at least I consume it and other people in the sports space that are vegan consume it. So, I think it’s just worth putting a bit of a question mark on and going, yeah, okay, it’s not as clear cut as maybe we think.

Aidan Muir

Yeah, and to the best of my knowledge, I read every study on that topic that I could find-

Leah Higl

Every study.

Aidan Muir

Which is a bit of a sign that there’s not that much research. I mean, I do try hard, but if there was hundreds of studies, I wouldn’t have read every study.

Leah Higl

This is true.

Aidan Muir

[00:14:00] It’s hard, because there’s different levels to this. Really high soy intakes, moderate, low soy intakes, all these kind of things, medications. What if you’re just on the border of having underactive thyroid, but your thyroid simulating hormone is high? There’s so many different presentations of this, and how does soy affect all of those? Yeah, nuanced, as you said.

Leah Higl

Very nuanced. I think there was one study that I came across that kind of changed my mind on things, because previously I was kind of in the camp of, oh, [00:14:30] you don’t need to worry about it. It’s not an issue. I’m probably a bit biased, because I love soy.

But looking into it a bit more, I found one study where they had a group of people that had hypothyroidism and they put one on a diet that had the amount of phytoestrogens that was normal for the Australian diet, so one to two milligrams per day. The other group was simulating a vegetarian diet where they were having about 16 to 20 milligrams of these phytoestrogens per day, [00:15:00] and then they measured their thyroid function over six months, I believe. There was a threefold increase in progression from subclinical to overt hypothyroidism in that vegetarian group.

I mean, the difference was two people had an underactive thyroid in one group and six in the other group, so it could just be the matter of unlucky splitting of participants.

Aidan Muir

Yeah, random chance potentially.

Leah Higl

Random chance. But it’s also like, [00:15:30] it also could mean something.

Aidan Muir

It’s worth being aware of, yeah.

Leah Higl

I don’t know.

Aidan Muir

Yeah. So, I suppose that’s going through a lot of the common potential downsides that people are thinking of or talking about, or everything like that, so we’re going to look at it from the other perspective. Not just from the health perspective, but also just what are the benefits? What are other reasons to be consuming soy? So going through that, I know rattling off a few things, what could be some of the benefits?

Leah Higl

I mean, if you’re plant-based at all, it’s such a good source of protein that’s super high quality, [00:16:00] it’s so versatile. So, in that way I love soy. I’d die for soy, love it. I eat so much of it. So yeah, it’s great in that way. There’s also quite a bit of research of in menopausal, post-menopausal women consuming soy and it helping with menopausal symptoms like hot flashes, and even bone health post-menopause due to that estrogen effect that phytoestrogens have.

So, it might be protective against bone loss in post-menopause.

Aidan Muir

[00:16:30] That’s interesting.

Leah Higl

So, I think that’s pretty interesting.

Then you have the cancer stuff, so breast cancer in particular and also prostate cancer that it might be protective against.

So there’s a lot of benefits to soy intake, it’s just like can you take it too far is the question?

Aidan Muir

Yeah, for real. Yeah, okay, so one of the obvious questions we’ve got from that quality protein perspective, I assume it’s a complete amino acid profile, it’s high quality protein.

Are there many other vegan sources that fit [00:17:00] that criteria?

Leah Higl

It is pretty much the only one that is really high quality and efficient and has a good leucine content.

So, leucine is the amino acid that is known for triggering muscle protein synthesis. So we know if you’re focused on muscle building, muscle recovery, that leucine content’s quite important. It’s pretty much the only good source of leucine on a plant-based diet.

So, that’s something to consider from a sports perspective.

Aidan Muir

Yeah. Yeah, for real. I feel like this is almost too [00:17:30] simple with the positives, where it’s just complete source of protein, it gives you another option and also taste variety and stuff like that. I think about it, I’m like, if I’m a plant-based diet-

There’s obviously less variety than there is by definition compared to being omnivorous. You’ve literally cut out a lot of foods, just like you have with any [inaudible 00:17:48], but there’s less good protein sources. So even if one just happened to be equal of all the other ones, you’d still want it as an option-

Let alone the fact that it is one [00:18:00] of the only complete protein sources.

From the positive perspective, we have kept it shorter than we have on the other perspective, but what would you say are the key takeaways from all of this? A bit of a summary from what we talked about.

Leah Higl

Yeah, so I think the key takeaways would be generally soy is a great food and the fear mongering that happens around it, not ideal, because it is a really great source of protein, particularly for plant- [00:18:30] based people. But generally, I’d say for women who do want to get pregnant, generally limiting it to four to five serves of soy per day at the most. Probably something to kind of chuck in there in terms of fertility recommendations just in case.

I want to kind of reiterate that feminizing effects in men is just not something we’ve ever seen in the research, outside of that one single crazy case study. Then yeah, generally phytoestrogens can [00:19:00] be protective against cancers and that post-menopausal stuff, so I think those would definitely be the key takeaways.

Aidan Muir

Yeah, I see it as we’re kind of reaching to find downsides. We’ve really gone out of our way to find any downsides yet. The way it’s kind of messaged is if the average person needs to be concerned about it-

Which is not the case. Yeah.

Leah Higl

Yeah, like I said, I don’t want to be that person that brushes off everyone’s I guess worries about soy, I want to address them.

It’s just [00:19:30] that you’ve probably noticed that most of the negative stuff is a handful of case studies, which we know is not going to be the peak of evidence-based research. So, we are really reaching to find negative things about soy.

Aidan Muir

Yeah, easy. Well, we’ll wrap things up there. So, this has been episode 43 of The Ideal Nutrition Podcast. Thank you to everybody who’s been listening, and as always, if you could please leave a rating and review, that would be massively appreciated.