The Truth About Hormones in Milk

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Estimated reading time: 12 min

tl;dr: If there is anything to worry about in milk it probably isn’t the hormones.

Do you fear hormones in milk? Then you’re not alone. Like many other foods, dairy products have been the victim of pseudoscientific fear mongering – specifically focusing on the hormonal content in milk, and its proposed propensity to give you cancer. But how much truth is there to these frightening claims? Here are six truths about hormones in milk, that should calm you down.

Growth Hormone in Milk: Dispelling Misconceptions

Growth hormone. The mere idea of its presence in an innocent glass of milk is enough to make many people turn their backs on milk and encourage others to do the same. But is there anything to be afraid of? No. Because even though cow’s milk does indeed contain a little bit of growth hormone, it doesn’t really matter: Growth hormone from a cow has no biological activity in humans [1]

Bovine growth hormone (bGH, also called bovine somatotropin, bST) is a so called peptide hormone. That means, that it is made from amino acids, just like any other protein we eat and digest. There are no studies that indicate that growth hormone from milk should be able to survive our digestion or that fragments from this digestion has any biological activity. In fact, nothing suggests that growth hormone from cows even have an effect on our own human growth hormone receptors.

But even if this was the case – even if bGH was biologically active in humans – the amount of growth hormone in cow’s milk is miniscule (approximately 1/1000 of a gram per liter of milk [2]), and 85-90% of it is destroyed during the heat treatment milk goes through during production [3]. The tiny amount that is left is in all likelihood readily digested in the intestine and absorbed as amino acids, as is the case with any other dietary protein.

(Some blogs make the opposite statement, that growth hormone is not broken down neither through digestion nor heating, but this is because many people confuse growth hormone with another hormone called IGF-1 which is a bit more durable than bGH. You can read more about the myths of IGF-1 later in this article.)

It might help a little to know that growth hormone in milk is not there to make the calf grow any faster. The hormone is in the cow’s milk as it is in the human mother’s milk because there is a passive transport from the blood into the milk. The minute amount of growth hormone in the cow’s milk has no effect on the calf because even though its growth is dependent at least partly on growth hormone (and IGF-1) milk is not the primary source: the calf produces far greater amounts in its own pituitary gland.

Milk contains the same amount of IGF-1 as your own saliva

IGF-1 (insulin-like growth factor 1) is a peptide hormone just like growth hormone, but the two hormones are not the same. Actually, IGF-1 is produced by the liver as a response to being exposed to the pituitary’s own production of growth hormone, and a lot of the effects of growth hormone is mediated through IGF-1. Even though there is an observed connection between IGF-1 levels in the blood and the development of certain cancers (explained later in the article) this fear of IGF-1 in milk is unnecessary. Studies show that intake of IGF-1 cow’s milk has no discernible biological activity in humans [4] – just as with growth hormone – even though IGF-1 from cows and humans are identical.

This could simply be due to the fact, that concentrations of IGF-1 in milk is much lower than the concentrations of IGF-1 in our own digestive fluids present in our gastrointestinal tracts [4]. The amount of IGF-1 in dairy milk is about the same as the amount in human mother’s milk [5] and far below our own daily hepatic production.

Since the concentration of IGF-1 is about 100 times higher in our blood than it is in milk, theoretically speaking if an infant drank one and a half liter of dairy milk a day – from American cattle treated with synthetic growth hormone* to produce more IGF-1 and therefore more milk – the amount of IGF-1 that the infant would get from the milk would still only amount to 1% of its own production [7]. The amount of IGF-1 in milk is simply too small to have any relevance.

As a parent, you might be extra concerned about what your kids gets exposed to, making you consider even minute amounts of hormonal exposure undesirable, and then choosing not to use formula. That worry is understandable, but there is no reason for it; even though IGF-1 is more resistant to heat treatment than bGH, it is broken down during the special heat treatment that infant formula undergoes [8].

*European dairy cows does not get growth hormone treatments among other things because the EU does not allow it in the production of milk [9].

Soy protein makes your IGF-1 levels rise more than milk does

Even though milk doesn’t contain relevant amounts of IGF-1 the amount of the hormone actually rises after drinking milk. A rise of only 14 micrograms per liter blood [10] (which already contains between 135 and 600 micrograms per liter [6]) but a rise nevertheless. This kind of rise in the circulating levels of IGF-1 is not a unique consequence of drinking milk. There are lots of foods that boost our own production of IGF-1.

An intervention study from 2011 showed, that a daily intake of 50 grams of soy protein raised the level of IGF-1 in the blood by 21 micrograms per liter [11]. Since soy contains no IGF-1, the rise must have been due to it being able to boost the pituitary production. Generally it seems that our own production of IGF-1 is dependent on our intake of protein [12]. Whether this comes from animals or plants is irrelevant, and it is reasonable to believe that a low production of IGF-1 is a sign of malnutrition – especially in the elderly [13]. But why are we so concerned about IGF-1 at all?

IGF-1 does not give you cancer

Basic scientific misunderstandings are at the root of most health myths. When it comes to disease, one of the most predominant is, that complex diseases have a single cause. Many believe that diabetes is caused by sugar (or even carbohydrate) intake, that salt is the cause of high blood pressure, and that IGF-1 causes cancer. But cancer – as with any of the aforementioned ailments – is a multifactorial disease [14] and any form of cancer is most likely caused by a host of interconnected factors. So when someone claims that one thing – for instance growth hormone or IGF-1 is the cause of cancer – it is always untrue (and easy to disprove).

What we know is that a high level of circulating IGF-1 might increase the risk of one type of cancer (in the prostate) – not cancer in general. That high IGF-1 levels might promote a single type of cancer is a hypothesis, not a fact, and under all circumstances a growth factor as this one would most likely only play a part of an intricate confluence of factors.

Most claims that IGF-1 is the primary cause of any cancer is based on cell studies, which can never be relied upon to establish cause and effect in the complex interactions inside the human body. Cell studies can only give rise to hypotheses (or help to explain the mechanisms behind and observed correlation) that can be subsequently tested in further scientific studies. These scientific studies have been done and they show (among other things) that even when the contents of IGF-1 in the blood is raised a bit, the intake of low fat dairy milk is associated with a lower risk of colorectal cancer – in particular in people with a high level of circulating IGF-1 in the blood [15]. This is the exact opposite result of the hypothesis that milk consumption causes cancer in general and, in the context of the mentioned article, that this process is driven primarily by IGF-1.

In the end our primary concern is to know whether certain foods increase the risk of cancer – and not whether certain components of our food does so in isolation or in a petri dish. Before describing what we actually know about the connection between milk and cancer, we have to first turn our eyes to the last group of milk hormones: estrogens.

The hormones in milk do not increase your estrogen levels

bGH and IGF-1 are not the only hormones making people fearful of milk. A lot of the milk that we drink comes from pregnant cows and as with humans, this means that the levels of circulating estrogens are higher than normal. Since that which happens in the blood is usually mirrored in the milk – in cows as in humans – the milk of cows pregnant in the third trimester contains as much as twenty times as much estrogen found in milk from non-pregnant cows. But as you might have caught on to at this point “twenty times as much” doesn’t mean anything in itself.

A new study in mice [16] tested whether the amount of estrogens found in regular dairy milk from pregnant cows had any effect on the circulating levels of estrogens and the genitals of the mice.

It didn’t.

After that, the mice were given 100 times as much estrogen as they had found in the milk samples with the highest amount of estrogen. Again, nothing happened.

Only when the amount of estrogen reached 1000 times the amount found in milk, it became possible to detect effects on the blood and genitals. This observation is really no mystery: estrogens are steroid hormones and they are, by and large, all broken down during first-pass metabolism in the liver [17] – until the concentrations overwhelm this system.

Everything that we absorb across the gastrointestinal membrane is transported directly to the liver through the portal vein system before it enters the general circulation. The liver in this effect is our “detox-organ” ensuring that we are not exposed to substances that could hurt us.

That the liver is so effective at metabolizing steroid hormones is also the primary reason that bodybuilders usually have to inject steroids instead of ingesting them. If they do ingest them it is because the steroids have been chemically modified to be able to resist the passage through the intestines and liver, which is also the reason why ingestible steroids can have a toxic effect on the liver [18].

All these myths about hormones in cow’s milk are designed to make you believe that milk causes cancer. And that is really the real question we want answered – not whether certain hormones could theoretically raise our risk of cancer.

If there was any strong evidence that milk raised the risk of cancer in general, it would be listed at the top of every article on the internet trying to persuade you or scare from drinking milk. You would see meta analyses of studies on milk that showed a clear connection between milk and cancer.

But that is not what you see.

Instead you see that one study that detected a correlation (and not all the ones that didn’t) along with a lot of cell- and animal studies that in no way proves a connection, but instead uses circumstantial evidence to imply a connection that we could actually just study directly.

Circumstantial evidence is ok to base our beliefs in when we have no other alternative. But we do have alternatives. Plenty actually.

Milk does not increase your risk of getting cancer

Figuring out of there is a connection between a specific food and cancer is much harder than many people assume. Even if we observe an association, it is not safe to assume that the one thing causes the other – it might as well be the other way around (this is called reverse causality – which probably explains why artificially sweetened beverages and obesity are correlated [19]) or it could be something else entirely that causes both the exposure and the outcome (this is called a confounder). Observe that an exposure and an outcome that seems connected, does not mean that one thing caused the other.

When it comes to research into what raises the risk of cancer, we usually have nothing else than observations like these to go on. We cannot make longitudinal controlled studies because it would be utterly unethical to try to actively provoke cancer in a group of people and if we did and saw even a small rise in cancer incidence, we would have to end the study immediately. So how can we know whether something causes cancer or not?

In 1965 Sir Austin Bradford Hill developed a set of criteria [20] that needs to be fulfilled for a causality between an assumed cause and effect to be established. The Bradford Hill-criteria in summary looks like this:

  • The effect size (the increase in likelihood) has to be large
  • The observed connection needs to be consistent across multiple studies
  • There needs to be an absence of other likely explanations for the observed connection
  • The effect (the increase in likelihood) has to be observed long enough after the exposure so that it fits our current understanding of the etiology and pathogenesis of the disease
  • There should be a dose-response relationship (that greater doses the greater the observed risk)
  • There should be a possible or known explanation to the mechanism behind the observed connection (biological plausibility)
  • There should be a coherence between what we find in observational studies as well as in intervention studies, animal studies and cell studies

A good example of an association that fulfills the Bradford Hill-criteria is smoking and lung cancer. Smoking increases the risk of protracting lung cancer between 1500 and 3000% [21] – an effect seen in all observational studies and all populations. Lung cancer is largely absent in the non-smoking population while the lifetime risk of developing a lung cancer in smokers soar around 17%. The observed period of time between first exposure and the appearance of diagnosable cancer is congruent with what we would expect. There is a clear dose-response relationship (more cigarettes lead to higher risk) and there is a good explanation to what we observe, since cigarette smoke contains a long range of strong mutagens that is carcinogenic in both animal and cell studies.

To the best of my knowledge there is not a single food that fulfills the Bradford Hill-criteria for causality.

(Processed red meat comes close but even in that case the effect size is small.)

To be able to claim that milk increases the risk of cancer, it would take (among other things) that milk consumption was consistently correlated with a marked increased risk of cancer and that this effect was dose dependent.

This is not what we see.

Different studies point in different directions and with some types of cancers it seems like it is protecting against that specific type of cancer but seems to promote other types of cancer.

If you are anti-milk (or just pro-fearmongering) you would probably feel inclined to point to the elevation of the risk of protracting prostate cancer that seems to follow a high consumption of milk, and exaggerate it – even though the literature shows that the increased risk is a mere 3% per 200 mL milk per day [22]. At least that is the focal point of the articles that through blatantly selective presentation of the literature reveals themselves as anti-milk ideologues.

If you are pro-milk you might emphasize the reduced risk of colorectal cancer highlighted in an analysis of 10 studies that showed a 15% reduced risk when consuming 250mL/day versus less than 70ml/day [23]. Or you might focus on the newest meta-analysis regarding breast cancer that – contrary to what the fearmongering about hormones in milk would indicate – shows that consumption of dairy products is correlated with a dose dependent reduction in risk at up to 24% [24].

But you don’t have to be neither pro nor anti milk. Science doesn’t encourage any of these black and white opinions, and I am neither. There are lots of middle roads between the two extremes that doesn’t force you to choose side.

Maybe the observed correlations are causal. Maybe milk plays a (small) part in both protecting against some cancers, and promoting others. But it is factually wrong to hardheadedly claim that milk causes cancer. The criteria to claim causality simply are not there.

A current systematic review and meta-analysis of observational studies on milk consumption and mortality showed, that milk neither lowers or raises the overall risk of death neither before time or by cardiovascular disease or cancer [25].

So milk in all likelihood is not relevant to worry about when it comes to cancer in humans. And that is perfectly fine because we have other things to worry about – such as smoking, alcohol and obesity – if we really are interested in lowering our risk of getting cancer.

I know this is an inflamed subject

First of all, it is inflamed because there exists a plethora of truly horrifying untruths n articles, magazines and books.

These untruths about milk, hormones and cancer is convincing enough to fool even reasonable and rational people. I was myself once fooled by them. Therefore I would never dream of judging others for believing the same untruths nor for sharing them. Because if it was indeed true that milk was chock-full of hormones that gave us cancer it would be amoral to not shout this information from the rooftops and demand that health professionals took action. But it just isn’t true – and thank God for that!

The subject has also been inflamed because milk and health have been mixed with milk and climate, milk and animal welfare or milk and “Big Food”.

It is perfectly fine to think that the production of milk is unethical or that the people responsible for the production of milk or the dairy lobbyists are exaggerating the health benefits of milk consumption. It is also perfectly fine to question the necessity of milk consumption and whether milk consumption is bad for the environment. As long as you remember to separate the different issues, because none of these attitudes changes what science tells us about milk, hormones or cancer.

Author Bio: Morten has a masters degree in molecular nutrition and food technology, and spends approx all his time refuting pseudoscientific health BS.


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  2. Groenewegen et al. (1990) Effect of bovine somatotropin on the growth rate, hormone profiles and carcass composition of Holstein bull calves. 
  3. Groenewegen et al. (1990) Bioactivity of milk from bST-treated cows. 
  4. Collier R.J. & Bauman D.E. (2014) Update on human health concerns of recombinant bovine somatotropin use in dairy cows. 
  5. Collier et al. (1991) Factors affecting insulin-like growth factor-I concentration in bovine milk. 
  6. Lægehåndbogen på (2014) IGF-1. 
  7. FDA (2009) Report on the Food and Drug Administration’s Review of the Safety of Recombinant Bovine Somatotropin.
  8. Collier et al. (1991) Factors affecting insulin-like growth factor-I concentration in bovine milk.
  9. Brinckman, D. (2000) The regulation of rBST: the European case. 
  10. Qin et al. (2009) Milk consumption and circulating insulin-like growth factor-I level: a systematic literature review.
  11. McLaughlin et al. (2011) Effects of Tomato- and Soy-Rich Diets on the IGF-I Hormonal
    Network: A Crossover Study of Postmenopausal Women
  12. Norat et al. (2007) Diet, serum insulin-like growth factor-I and IGF-binding protein-3 in European women.
  13. Maggio et al. (2013) IGF-1, the Cross Road of the Nutritional, Inflammatory and Hormonal Pathways to Frailty.
  14. Cancer research UK (2016) Causes of cancer and reducing your risk. 
  15. Ma et al. (2001) Milk Intake, Circulating Levels of Insulin-Like Growth Factor-I, and Risk of Colorectal Cancer in Men. 
  16. Grgurevic et al. (2016) Effect of dietary estrogens from bovine milk on blood hormone levels and reproductive organs in mice. 
  17. Pond, S.M. & Tozer, T.N. (1984) First-pass elimination. Basic concepts and clinical consequences. 
  18. Kafrouni et al. (2007) Hepatotoxicity Associated With Dietary Supplements Containing Anabolic Steroids
  19. Koning et al. (2011) Sugar-sweetened and artificially sweetened beverage consumption and risk of type 2 diabetes in men.
  20. Austin Bradford Hill (1965). The Environment and Disease: Association or Causation?
  21. Centers for disease control and prevention. (2016) What Are the Risk Factors for Lung Cancer? 
  22. Aune et al. (2015) Dairy products, calcium, and prostate cancer risk: a systematic review and meta-analysis of cohort studies.
  23. Cho et al. (2004) Dairy Foods, Calcium, and Colorectal Cancer: A Pooled Analysis of 10 Cohort Studies. 
  24. Zang et al. (2015) The Association between Dairy Intake and Breast Cancer in Western and Asian Populations: A Systematic Review and Meta-Analysis.
  25. Larsson et al. (2015) Milk Consumption and Mortality from All Causes, Cardiovascular Disease, and Cancer: A Systematic Review and Meta-Analysis.
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