Should I be concerned about growth hormones in a cow's milk cappuccino? Are the plant oestrogens in a soya latte affecting my risk of cancer? Will almond milk damage my thyroid? Professor Tim Key and Dr Sarah Bath are spilling the tea (milk, no sugar) and looking at the hormonal impact of plant and cow’s milks.
Sally: Hello and welcome to Hormones: The Inside Story, the podcast from the Society for Endocrinology. I'm Dr Sally Le Page, an evolutionary biologist and science presenter, and I'll be chatting with a whole bunch of hormones scientists - or endocrinologists - to bring you surprising stories and cutting edge research.
Sally: Today we're taking a swig of the white stuff and looking at the hormonal impact of milks - and not just the stuff that comes out of cows. We'll be asking, is my soya latte affecting my oestrogen levels?
Tim: If woman consumed large amounts of soya, you'd have both the natural hormone and the plant oestrogen in the blood as well.
Sally: Do plant based milks mess with my thyroid?
Sarah: A soya latte, if it's not fortified, would not replace the iodine as if it was a latte made with cow's milk, and we know that iodine is essential for thyroid hormone production.
Sally: Or is it actually the hormones found in cow's milk that I should really worry about?
Tim: Cows will secrete IGF-1 in their milk. But it's not a simple story: frustratingly in many areas of science, the truth is quite hard to get to and may be complicated.
Sally: There are few things I appreciate more than the smell of freshly ground coffee as a barista hands me a warm, foamy latte in the morning, perhaps to go alongside smashed avocado on toast if I'm really living up to the millennial stereotype. But how does my coffee order affect my hormones?
Sally: Plant-based drinks have soared in popularity over the last decade, with more and more people turning to non-dairy milk alternatives. For some, the choice is driven by dietary restrictions, such as lactose intolerance. For others, the decision is based on animal welfare concerns, wanting to reduce their carbon footprint or even a simple taste preference. In fact, a 2022 survey found that nearly half of all British adults now use plant-based milk alternatives in their diet.
Sally: Plant-based milks are arguably better for the health of the planet but what about your own health?
Sally: Unfortunately, the evidence is hard to follow. The internet is full of headlines either touting the health benefits of cutting out dairy milk, or warning that shunning animal products will increase your risk of various diseases. It can get a little confusing, so today, we are spilling the tea - milk, no sugar, thank you - on dairy alternatives, starting with soya milk.
Sally: Soya is a type of bean used to make many food products, including tofu, tempeh and of course soya milk. Here in the UK and the rest of Europe it's called soya, whereas in the US it's called soy, but it's all the same thing.
Sally: If you read the news headlines about soya though, I don't blame you if you're confused. Consuming soya products has been linked to a lower risk and higher risk of certain cancers, particularly breast cancer. So should we be eating more of it, or avoiding it altogether? To cut through the contradictions I'm going to need an expert on the subject, and I've found the cream of the crop:
Tim: I'm Professor Tim Key. I'm deputy director of the Cancer Epidemiology Unit at the University of Oxford.
Sally: Tim is one of the lead scientists on the European Prospective Investigation into Cancer and Nutrition - or EPIC for short. It's a huge study involving over half a million people across Europe, and looks at the relationship between diet and incidence of certain cancers, and Tim's especially interested in whether there's any difference between meat eaters and vegetarians or vegans.
Sally: Now science often starts with an observation. It could be an apple falling on your head or the water level rising when you get in the bath. Well a few decades ago, researchers noticed an unusual data point coming out of East Asia:
Tim: The rates of breast cancer in Japan, for example, were 40-50 years ago, substantially lower than in many Western countries, such as the UK.
Tim: And that sparked various hypotheses, including ideas that maybe they ate less fat and that caused lower risk. But another idea was maybe it was because they ate quite a lot of soya beans, had a high intake of phytoestrogens, and possibly that might have a protective role.
Sally: Let's unpack this hypothesis for a moment. Oestrogens are sex hormones found in humans and other animals, but soya beans come from plants, not animals. How could eating soya have an impact on oestrogen levels?
Tim: There's compounds naturally present in soy and in some other plants called phytoestrogens. The phytoestrogen means plant oestrogen. And they were given that name because if you look at the chemical structure of these compounds, they're quite similar to the natural oestrogen present in humans.
Sally: Soyabean plants produce these phytoestrogens not to help with reproduction in the way human sex hormones do - but as part of their own defense system against certain fungal diseases. It just so happens that they look very similar to our own hormone, oestrogen.
Sally: And that's why researchers thought they might be important, because oestrogens can play a role in the development of some types of cancer.
Tim: There's extensive evidence that women with relatively more natural oestrogen in their blood do have a modestly increased risk of breast cancer.
Sally: Now, there are many things that can affect levels of oestrogen in the blood, from fluctuations during the menstrual cycle and menopause to genetics and body weight, as well as some medical conditions. Certain medications, like the contraceptive pill or HRT, contain hormones that can affect blood oestrogen levels too.
Sally: So coming back to my soya latte, does this mean that consuming soya products - and therefore soya phytoestrogen - will boost my blood oestrogen levels and increase my risk of breast cancer?
Tim: If a woman was producing certain amount of oestrogen on her own, and then she consumed large amounts of soya, you'd have both the natural hormone and the plant phytoestrogen in the blood as well. So the total amount could be a bit more.
Tim: But in terms of what is the consequence of that, it's a bit more complicated. Phytoestrogens can interact with the receptor that binds oestrogen in the body and have a small effect in terms of stimulating oestrogenic activity. But they're much weaker than the natural hormone produced by humans.
Sally: Here's where it gets a bit confusing. Just because soya phytoestrogens can bind to the oestrogen receptor, it doesn't mean that they're having the same effect as normal oestrogen would. In fact, having more phytoestrogens in the body may lead to a lower oestrogen response overall, as the less efficient phytoestrogens crowd out human oestrogen hormones. Kind of like how having more people in a kitchen might not mean that dinner gets made faster - or better - if the extra helpers can't cook very well.
Sally: In other words, consuming more soya has the opposite effect to what you might expect: more soya leads to more phytoestrogen in the blood, which in turn may lead to less oestrogen activity in the body overall. Does that mean eating more soya actually reduces the risk of breast cancer? Could this explain the historically low cancer rates observed in Japan?
Tim: So that sort of observation can never establish cause and effect, but it can stimulate hypotheses.
Tim: You can speculate maybe the low cancer in Japan could be due to the high soya, it could be due to the low dairy milk. So that's a speculation, but you can imagine there's hundreds of other reasons that might explain that difference.
Sally: There's a golden rule in scientific research: correlation does not imply causation. Or to put it more simply - two things may have happened together but that doesn't mean one caused the other.
Sally: In this case, traditional Japanese diets may be high in soya and people in Japan may have a lower incidence of breast cancer, but that doesn't prove that lots of soya intake leads to less cancer. There are many other differences between traditional Japanese and Western lifestyles that might explain the discrepancy. Japanese diets are usually richer in fish and omega 3s, they also tend to include more green tea, fewer fatty acids, more rice, less dairy... the list goes on - and that's before you even consider the other cultural and lifestyle factors that might be playing a part. So how can we distinguish cause from coincidence and test whether eating soya really is responsible for cutting cancer risk?
Sally: Rather than running clinical trials to test the impact of specific diets, which is really hard to do, researchers have turned to observational studies, looking at what people already eat and do as part of their regular lives over many years, and then seeing how that correlates with their health. It's not as clear-cut as a clinical trial, as there are a lot of variables and you need to recruit a lot of people so you can see any patterns emerging, but Tim and his team have risen to the challenge. They've been conducting a huge study looking at how different diets affect cancer risk across Europe, including looking at the impact of soya.
Tim: We've been working on the EPIC study, following diet and health in half a million people across Europe, where we had recruited a large proportion of vegetarians and also some vegans. And a lot of those women did consume soya products every day; soya milk.
Tim: And in that we saw no clear evidence that there was any difference in risk for those consumed soya versus those who didn't, but the numbers were not huge so we couldn't rule out soya milk.
Sally: So far, no significant conclusions. But this study is running in Europe, which traditionally has a lower soya consumption than Asia. What about the situation over there?
Tim: The better studies have been in the Far East - Japan and China - where some of the populations have had higher intake of soya traditionally. But again, in those studies, there has not been a clear, consistent finding.
Sally: I guess that means the jury's out for now?
Tim: There's certainly not enough evidence to reach the conclusion that soya would reduce breast cancer. Also, not much evidence that there's any reason that it might increase it, which is also possible.
Tim: So I think in terms of people in this country today, you could consume soya foods if you like them or for other reasons and probably got not much to do with it affecting your risk of breast cancer.
Sally: To sum up, there's no hard evidence saying that you should give up your soya latte if you're worried about your cancer risk. But on the flip side, there's no hard evidence to suggest that switching to soya specifically will cut your risk. However, there's a bigger issue around whether you should ditch dairy milk and switch to a plant-based one for the sake of your health. And if you thought the headlines about soya and cancer risk were confusing, just wait until you hear about dairy...
Sally: "Drinking just two cups of milk a day 'can increase the risk of breast cancer in women by up to 80%'"
Sally: "White Lies: The Health Risks Of Dairy"
Sally: "Drinking Milk Increases Prostate Cancer Risk for Men by 60 Percent"
Sally: These are just a few of the many, many headlines you might read if you were to Google "milk" and "cancer"... and I'll be honest, some of these sound pretty scary - but is there any truth to them?
Sally: This time it's not oestrogen that's in the scientific crosshairs, but a different hormone called IGF-1, which has been implicated in some types of cancer. But before we get to that, let's find out from Tim what IGF-1 is, and how it might end up in that bottle of milk sitting in your fridge.
Tim: Insulin like growth factor 1 is a hormone that is found in humans and all other mammals and its main role is to stimulate the growth of cells and the growth of the body. Without it, animals and humans would not grow.
Sally: IGF-1 is normally produced in the liver, and it plays a whole host of roles in the body from the very earliest stages of development in the womb all the way through to adult life.
Tim: It's a completely normal and essential hormone to have. All cows, all people have IGF-1 in them and both cows and other mammals will secrete IGF-1 in their milk. But all humans will have it, regardless of whether they drink cow's milk.
Sally: IGF-1 is not only naturally produced by our bodies, but it's also found in the animal proteins in food, including meat, eggs and dairy. So why should it matter if we get some more of it in our diet from drinking cows milk? Here's where we come back to the cancer connection.
Tim: Cancer is a disease where growth gets out of control and cells are growing too fast and increased rates of cell division can increase the chance of something going wrong, which might lead to cancer. So it seemed very plausible that IGF-1 might have a link to cancer and that higher levels might be associated with a higher risk.
Sally: There's not only a plausible mechanism for why IGF-1 may be linked to cancer, but we also have the data to back it up.
Tim: There's now quite strong evidence that people with higher levels of IGF-1 in their blood do have a higher risk of some cancers.
Tim: And the most evidence - and it's consistent for all three cancers - is for breast cancer in women, prostate cancer in men, and for colorectal cancer, which affects both men and women.
Sally: That sounds pretty bad - so how much increase in risk are we talking about here?
Tim: So for all three of those, people with relatively high levels, say the top 20% of people, would have about a 30% higher risk of that cancer than the people in the lowest 20%. So it's not a huge difference in risk.
Sally: Now I don't know about you but 30% still doesn't mean much to me. Is that a lot? Is it not that much? How does it compare to a well-known risk factor like smoking?
Tim: Someone who's heavy smoker might have about 40 times the risk of someone who never smoked at all. So that is a very, very large impact.
Sally: To put it into context, 40 fold is a 4000% increase. So if smoking increases your risk of developing cancer by 4000% and high levels of IGF-1 in your blood only raise it by 30%, is IGF-1 really something to worry about?
Tim: You can choose not to smoke, but everybody has to have hormones like IGF-1. So it's a different story, really. Smoking is an extreme case and the solution is fairly clear.
Tim: But for hormones they're much smaller impacts, but it doesn't mean they're not worth pursuing.
Tim: More recently, there have been genetic studies showing that genetic variants that predict higher levels of IGF-1 in the person's blood are also associated with about the same level of increase in risk.
Tim: So it seems to me pretty compelling evidence now that IGF-1 does have a modest impact of increasing risk.
Sally: So we know that high levels of IGF-1 in your blood increase your risk of cancer - albeit modestly. But what about milk? If cow's milk has IGF-1 in it, does drinking a dairy latte increase your blood IGF-1 levels too?
Tim: Well, that is an obvious, important question to answer. There's quite a lot of evidence showing that people who consume more milk do have modestly higher levels of IGF-1 in their own blood.
Tim: There has been one study where athletes in Finland drank bovine colostrum, which is the milk produced by the cow immediately after the birth of the calf - just the first few hours - which has been shown to have much higher levels of IGF-1 than the traditional milk that people eat that the cow produces. It can be more than a hundred times higher. But the changes they saw in the IGF-1 in the blood were not very large.
Tim: It's probable that milk and maybe other dairy products, if consumed in quite substantial amounts, may result in a modest increase in the IGF-1 in the person's blood. It's an interesting study, but I think it's of unclear relevance to people who simply drink ordinary milk.
Sally: Assuming you're not starting your day with a bovine colostrum cuppa, drinking dairy milk might slightly affect levels of IGF-1 in your blood. -But is it enough to be worth worrying about?
Tim: People who drink more milk, maybe it'll have a small impact on their IGF, but whether that has a net positive or negative effect on their risk of future diseases is not clear, and it seems likely that any impact is quite small, if there is one.
Sally: Now, IGF-1 isn't the only thing found in dairy products. Do any other components of milk influence your risk of cancer?
Tim: There's a lot of evidence that's generally consistent that drinking milk, particularly, is associated with a lower risk for colorectal cancer. That's cancer of the large bowel, large intestine.
Sally: If you cast your mind back about five minutes, you might remember that Tim previously told us that having higher levels of IGF-1 in your blood is associated with an increased risk of bowel cancer, but now drinking milk reduces your risk of bowel cancer? I did warn you the evidence was quite confusing! How does dairy milk protect you from bowel cancer?
Tim: It's not absolutely clear why that is that. Probably the favourite hypothesis is that it's due to the large amounts of calcium. And there is separate research showing calcium has impacts that could be beneficial in terms of improving the state of the intestine.
Tim: Consuming more rather than less milk is associated a lower risk of colorectal cancer. Again, it doesn't prove it's the milk. It could still be calcium, but the milk had that impact.
Sally: If it's the calcium in the milk that's having the protective effect, that means that people who drink plant-based milks that have been fortified with calcium may also see the same benefits. But there's one nutrient that's commonly missing from these drinks and that could be having an impact on your hormones.
Sarah: Iodine is a mineral that's found in our diet and is essential for production of thyroid hormones. So it's something that's needed in very small amounts from the diet actually, but it is essential in order to produce thyroid hormones that are essential for so many aspects of life.
Sally: That's Dr Sarah Bath, a lecturer in Public Health Nutrition at the University of Surrey and a registered dietitian. She's worked on iodine and plant milks for over a decade.
Sally: If you listened to last week's episode, you'll know what a marvellous - and sometimes temperamental - little organ the thyroid is. It controls everything from your metabolism to your mood by producing two thyroid hormones, T3 and T4.
Sarah: Iodine is an essential component of those thyroid hormones and therefore the role of iodine is the production of those thyroid hormones.
Sally: Given that iodine is so important for health, you're probably wondering where you can find it.
Sarah: Iodine is in quite a few foods in the diet. The main sources, though, tend to be animal sources; primarily milk and dairy products. It's also commonly found in reasonable concentrations in fish and also in eggs.
Sarah: And you can see from that list, actually, that they are all animal sources. So the majority of iodine is in animal-based foods. That's not to say there's none in other foods. Iodine is present in small quantities in many foods. There are small amounts in plant-based foods, it's just they're not the richest sources.
Sally: If you don't get enough iodine from your diet, you can't make enough thyroid hormones. And that can have some serious consequences.
Sarah: Typically the symptom of deficiency is goitre, which is the quite visible, enlargement on the neck that's very obvious.
Sally: A goitre is easy to spot, but iodine deficiency can also cause more insidious symptoms.
Sarah: If the deficiency continues for a prolonged period of time, then there can be effects on thyroid hormone production and underactive thyroid can result from prolonged deficiency of iodine.
Sarah: And a lot of the effects of underactive thyroid are quite subtle and quite vague so it may not be immediately obvious to people.
Sally: You'll remember from our previous episode that an underactive thyroid can make you feel lethargic and put on weight. These kinds of symptoms are common and therefore easy to miss. However, it's important to catch the early signs of iodine deficiency, especially during pregnancy.
Sarah: Iodine is required at all life stages, but thyroid hormones are particularly important for brain and neurological development.
Sarah: So iodine is needed in the early stages of life, in early pregnancy and throughout early infancy and childhood.
Sally: And unfortunately, people who are pregnant are also at higher risk of iodine deficiency.
Sarah: The demands for iodine are higher in pregnancy so it's harder to meet that and therefore there's greater prevalence or risk of deficiency in that group.
Sarah: We've seen in research that we've been doing at the University of Surrey that even in cases of mild deficiency, there's effects on offspring in terms of IQ and in terms of reading ability in school age. And that's been shown in a number of other studies throughout the world.
Sarah: There's a quote from the World Health Organization which says, "Iodine deficiency is the world's most prevalent yet easily preventable cause of brain damage."
Sally: It sounds almost unbelievable but it's true. Iodine deficiency is the main cause of brain damage in childhood, with knock-on effects in adult life. Globally, nearly 50 million people worldwide are thought to suffer from some level of brain damage related to iodine deficiency. That's roughly the population of Spain.
Sally: And iodine deficiency affects countries all around the world, both rich and poor. In the UK, we're lucky that iodine-rich foods like dairy, fish and meat are common in our cultural diet and relatively affordable for the majority of people. But this isn't the case in many other parts of the world.
Sarah: In terms of deficiency in countries like the UK, we are now looking at mild or moderate deficiency rather than a very low intake or severe deficiency. It may be more common in other countries. The dietary sources that are in each country and in each population vary.
Sarah: So in the past, many countries had extremely low intakes of iodine and more of a severe deficiency problem. And there the effect on IQ is more pronounced and more obvious in terms of the effect on the offspring and IQ and that can have effects on the economy. So there's a huge public health need to prevent iodine deficiency.
Sally: In response to such widespread iodine deficiency, many countries turned to a simple solution: salt.
Sarah: One of the ways that there's been such a huge improvement across the world is through iodised salt programmes. So that's where normal table salt has been iodised, i.e. a small amount of iodine has been added to it.
Sarah: And salt was chosen as a vehicle for many reasons; it was widely used. From a technical point of view, it was easy to add the iodine to salt and it was a vehicle that was quite effective.
Sarah: And really it's a huge public health success story because the number of countries with deficiency and particularly the number of countries with severe deficiency of iodine has greatly reduced.
Sally: 125 countries around the world, from Canada to Kazakhstan, opted to add iodine to salt to combat iodine deficiency, and now four in every five households around the world consumes iodised salt.
Sally: Here in the UK though, you've probably never even heard of iodised salt, and that's because of another public health issue.
Sarah: Whenever I talk about salt and iodine, particularly in the UK, I often then get questions about people who know that salt is something that we've been told to reduce in the diet for different reasons, for cardiovascular reasons, and therefore that it seems a bit strange to be talking about salt with something beneficial in it when we've heard so much about needing to reduce salt intake.
Sarah: We didn't add iodine to salt, we have never had an iodised salt programme in the UK.
Sally: So if we don't have iodine in our salt, how come we don't have a major iodine deficiency problem here?
Sarah: What's happened is that other sources have played a more important role, so milk and dairy products. The effect of deficiency went away through changes in the dairy farming industry that meant milk and dairy products have a higher amount of iodine in the milk than in other countries.
Sarah: And around the same time, people started drinking more milk. So we had schemes like the school milk scheme. We had marketing of milk and “Drinka Pinta Milka Day” slogans that came in in the fifties and sixties. And what that's meant is that we've had a sort of protection against deficiency through milk and dairy product consumption.
Sarah: It has been termed an accidental public health triumph in the UK because it happened for these underlying changes that meant actually we didn't go down the salt route because it appeared to be unnecessary given that we had this alternative vehicle, I suppose you could see it as; we had iodine in the milk.
Sally: The British government may have struck it lucky by putting more milk on the table rather than in salt shakers, but that's no longer cutting the mustard.
Sally: In 2011, researchers reported the findings of the first national survey of iodine levels for 60 years, and found a worrying trend of mild deficiency. And what's even more concerning is that in many parts of the UK, iodine deficiency has been found in pregnant women; the very group of people for whom iodine is most important. And now that more and more people are switching from iodine-rich dairy milk to plant milks, what does that mean for the country's iodine levels?
Sarah: This is something that we were really interested in a few years back when we saw this gradual, but increasing now, rise in popularity of milk alternative drinks.
Sarah: So a few years back we measured the iodine concentration in a number of these different products, so the soy milk and the almond, the oat, coconut milks that are available on the shelves. And what we found was that the concentration in those drinks was very low, at just around 2% of the value that we found in the cow's milk, unless they had iodine added to them.
Sarah: And at the time, very few did. In other words, people would not be replacing the iodine that they were having in the cow's milk with these milk alternative drinks.
Sarah: And the situation has improved a little bit. Now in recent years, more drinks have iodine added to them. But it's still unfortunately the case that most of the drinks on the market do not.
Sarah: Hopefully, with time, the situation improves and as we've seen over the years, the number of products on the market with added iodine is increasing. The number of brands that are adding iodine is increasing so consumers are now having more choice across a range of products and also not just the milk alternatives but some of the alternatives to yoghurts and cheese.
Sally: According to the Vegan Society, the number of people who choose not to eat any animal products at all quadrupled between 2014 and 2019. If you're completely cutting out these sources of iodine from your diet, it's vital that you get your iodine from somewhere else. But if you can't rely on a dairy-free milk substitute to provide your recommended daily dose, where else should you be looking?
Sarah: Unfortunately, it's quite difficult to find reliable sources of iodine in a vegan diet. There is one plant source that I haven't mentioned that is seaweed. So some seaweeds, particularly brown seaweeds like kelp, have extremely high concentrations of iodine. But it's highly variable and can actually be too high and can lead to excess intake.
Sarah: And so for that reason, it's generally not recommended to rely on seaweed products for iodine.
Sarah: If people are avoiding or can't have cow's milk and if there isn't any source that they can take, a rich source of iodine, then consider small dose supplements of iodine to just give enough iodine in that diet to meet the needs.
Sally: All of this discussion has left me gasping for a cuppa. But before I head out to my favourite coffee spot, should I be concerned about a cow's milk cappuccino upping my cancer risk, or suspicious that a soya latte is screwing with my hormones? Tim isn't convinced.
Tim: I think that the evidence we have at the moment is that although there could be some impacts of soya on hormones and of dairy on hormones, for both of them in realistic intakes in this country at the moment, it seems unlikely that, if there is an effect, it would be big enough to be detectable or anything to be worried about.
Sally: But if you have switched to dairy-free drinks, keep in mind that not getting enough iodine could be an issue.
Sarah: People know calcium is important in cow's milk, but have not perhaps had iodine on the radar in the same way. A soya latte, if it's not fortified, would not replace the iodine as if it was a latte made with cow's milk, and we know that iodine is essential for thyroid hormone production. So if somebody is having a soya-milk latte, but having plenty of fish and eggs and other sources, then that's less of a concern than if somebody is not replacing or not having any other sources of iodine. Then indeed, that may put them at risk of low iodine intake and deficiency.
Sarah: But in a nutshell, think of iodine, be more aware of iodine and look at where else there may be iodine in the diet, and if there isn't any source then consider a suitable supplement.
Sally: That's all for now. Thanks to our guests, Professor Tim Key and Dr Sarah Bath.
Sally: We'll be taking a short break before our final episode of this series, which is all about the future of hormone technology and blurring the line between science fiction and science reality.
Sally: Can AI help doctors make better decisions during IVF treatment?
Simon: If we personalise the dosage of drugs maybe we see that actually I can give a lot less and we can save maybe 10-20% of costs
Sally: Can 3D printed stem cells finally offer a cure for type 1 diabetes?
Vicky: It's something that when I left medical school 20 or so years ago, felt like science fiction is now really tantalisingly close.
Sally: And why have scientists created a remote control adrenal gland?
Polina: How can we hijack this whole circuit because if we could do that, then we will be able to control them releasing their hormones.
Sally: Hormones: The Inside Story is a podcast from the Society for Endocrinology. Explore more about the world of hormones at yourhormones.info and follow them on Twitter @your_hormones. The show is a First Create the Media production. It was researched, written and produced by me, Sally Le Page and Emma Werner. Our executive producer is Kat Arney. Thank you for listening and we'll see you again soon.