Friday Favorites: The Effects of Hormones in Dairy Milk on Cancer

Below is an approximation of this video’s audio content. To see any graphs, charts, graphics, images, and quotes to which Dr. Greger may be referring, watch the above video.

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, but there are also a bunch of estrogens, which then concentrate further when you make other dairy products—like five times more concentrated in cream and cheese, 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. 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’s organic. Animals produce hormones because they’re 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 in ya know, telling the public all our new technologies, like transgenic animals, meaning genetically engineered farm 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, ixnay on the educationay.

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. “Modern genetically improved dairy cows, such as the Holstein”—your standard black and white cow—can get reimpregnated 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.”

Here’s the estrogen levels in milk 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 a birth control pill. So, would it really have an effect on human hormone levels drinking it?

Here are the average levels of three different estrogens and a progesterone metabolite flowing through the bodies of seven men, who then proceeded to drink about a liter of milk. Within hours, their hormone levels shot up.

Here are the average levels of these female sex steroids flowing through the bodies of six schoolchildren, average age eight, before drinking about two cups of milk, and then after. Within hours, their levels shot up, tripling or quadrupling their baseline hormone levels. So, one can imagine the effects milk might have on men or prepubescent children.

But what about women? Presumably they’d 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 a 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.”

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Image credit: Mehrshad Rajabi via unsplash. Image has been modified.

Motion graphics by Avocado Video