The Side Effects of Hormones in Dairy

**Michael Greger** (0:00)
I'm often asked my opinion about the cause or treatment of some medical condition, but the question to instead ask is what does the science say? What does the best available balance of evidence published in the peer-reviewed medical literature have to say right now? Welcome to the Nutrition Facts Podcast. I'm your host, Dr. Michael Greger. Today, some research on the effects of hormones in milk, and we start with a story about what happens to men within an hour of drinking dairy milk. We have known for decades that women who eat meat have higher levels of estrogen in their blood. Compared to non-vegetarians, vegetarian women will find out 45% lower levels of the most potent human estrogen. This may help explain the lower incidence of breast cancer among those eating more plant-based. Even semi-vegetarians who eat less meat than omnivorous women may have significantly lower estrogen levels. Why? Perhaps because vegetarian women excrete two to three times more estrogens in their feces than do their meat-eating counterparts. And why is that? Because the average fecal output of vegetarians can be twice as high. The thought is that because vegetarians eat the amount of fiber close to that which is natural for our species, those relying more on plants have an increased fecal output, which helps pull excess estrogen out of the system. It may also have something to do with the hormones present in animal products. For example, the higher average blood concentrations of estrogen, associated with high dairy consumption, would be consistent with milk as a major source of estrogens in the human diet. The reason we are concerned is that the increased consumption of animal-derived foods may have adverse effects on the development of hormone-dependent cancers. Among dietary risk factors, these investigators were most concerned about milk and dairy products because the milk we drink today is produced by pregnant cows with markedly elevated levels of both estrogen and progesterone. The genetic manipulation of dairy cows enables them to lactate throughout almost their entire pregnancies, which is why commercial cow's milk contains large amounts of estrogens and progesterone these days. That may be why milk drinkers have five times the rate of having twins compared to vegans, and that's how much in effect dairy may have on our hormones. But we don't really know how much the hormones in dairy affects our hormones until we put it to the test. Researchers look to see what happens when men and prepubescent children, who both would normally have low estrogen levels, chug a quart of cow's milk in under 10 minutes. This is what happened to the men. Within an hour drinking the milk, their levels of the estrogen estrone went up, and progesterone and testosterone significantly dropped within 60 minutes. It's funny, there are men who don't want to drink soy milk because of the phytoestrogens, even though soy phytoestrogens normally does not have feminizing effects on men, yet they're perfectly willing to drink cow's milk, which has actual estrogen in it. In children, researchers saw the same thing, the level of estrogens flowing through their body within an hour of drinking milk, more than triple. Estero, the potent estrogen estradiol, the pregnancy estrogen estriol, and a progesterone metabolite. And half the kids couldn't even finish the whole quart of milk. There's also another sextoid hormone present in cow's milk called 5-alpha-P, 5-alpha-pregnantadione, which is a direct precursor to a type of testosterone that may not only drive acne, but also promote prostate cancer. Now, we'd like to see controlled clinical studies, but the problem is there is no control. Dairy without the offending estrogens does not exist. Even organic milk from cows who aren't injected with extra hormones has growth hormones in it, because biologically that's the whole point of milk, right, to put a couple hundred pounds on a baby calf. The problem with humans drinking cow hormones is that there's no natural feedback loop. For our own hormones, if we have too much estrogen or something, our brain dampens it down. But our own protective feedback system is bypassed when our system is invaded by these dihydrotestosterone precursors in dairy product. Let's see, the human endocrine system, our hormonal system, didn't evolve under the influence of ingested dairy and other external hormones and growth factors, and we're just not equipped to cope with such a quote-unquote sneak attack. In summary, evidence suggests that consumption of dairy-sourced hormones, which aren't subject to any innate feedback inhibition, may be the source of the steroid sex hormones that drive acne and at least prostate cancer. This is perhaps the most promising unifying hypothesis or theory to explain the cause of diverse conditions that blemish, scar, shorten or take the lives of millions. In our next story, we look at the effects of the female sex hormones in milk on men, women, and children. All food of animal origin contains hormones. But most of our dietary exposure to hormones comes from dairy products. By quantity, it's mostly prolactin, corticosteroids, and progesterone. There are also a bunch of astrogens, which then concentrate further when you make other dairy products, like five times more concentrated in cream and cheese, and ten times more hormone concentration in butter. So when it comes to exposure to steroid hormones in the food supply, about three quarters of our exposure to ingested female sex steroids comes from dairy, with the rest evenly split between eggs and meat and fish. Eggs contribute about as much as all meat put together, which makes a certain amount of sense, since it comes straight from a hen's ovary, right? Among the various types of meat, you get as much from white meat, fish, and poultry, as you do from pork and beef. And this is just from natural hormones, not added hormone injections like bovine growth hormone. So for these, it doesn't matter if the meat is organic. Animals produce hormones because they are animals, which understandably ends up in animal products. But, only about half of people surveyed seemed to know that, lacking basic knowledge, like not realizing what milk is for. Cows only give milk after having a calf. So these researchers suggested we ought to inform the public about dairy production practices, to which one Journal of Dairy Science respondent wrote, you know, telling the public all our new technologies, like transgenic animals, meaning genetically engineered form animals, or taking away that calf right away, so we can have more of the milk, or not letting cows see grass may not actually result in high rates of public approval. So, ex-nay on the education nay. One thing with potential public health implications that the public may not know about is their exposure to estrogen through intake of commercial milk produced from pregnant cows. See, modern, genetically improved dairy cows, such as the Holstein, your standard black and white cow, can get re-impregnated after giving birth and lactate throughout almost her entire next pregnancy, which means that commercial cow's milk these days contains large amounts of pregnancy hormones like estrogens and progesterone. During the first eight months of a pregnant cow's nine-month gestation, hormone levels shoot up more than 20-fold. But even so, we're still only talking about a millionth of a gram per quart, easily 10 to 20 times less estrogen hormones than what you'd find in like a birth control pill. So would it really have an effect on human hormone levels drinking it? Someone can imagine the effects milk might have on men or prepubescent children, but what about women? Presumably they have such high levels of estrogens in their body in the first place. Well, not all women. What about postmenopausal women and endometrial cancer, for example? Estrogens have a central role in the development of endometrial cancer, which is a cancer of the lining of the uterus. Milk and dairy products are a source of steroid hormones and growth factors that might have these kinds of effects. So, Harvard researchers followed tens of thousands of women and their dairy consumption for decades, and found significantly higher risk of endometrial cancer among postmenopausal women who consumed more dairy. What about dietary exposure to hormones and breast cancer? Unfortunately, understanding the role of dietary hormone exposure in the population burden of breast cancer is not possible at this time. Finally, today, what role cow's milk may play in the aging of ovaries?