Got Milk, Got Unwanted Hormones?

Factory farming churns out cow’s milk laced with steroid hormones.
Published on March 22, 2014 by Robert D. Martin, Ph.D. in How We Do It

Holstein dairy cows produce significantly more milk for female offspring than for males. Harvard anthropologist Katie Hinde and colleagues reported this finding in a recent paper, deservedly attracting considerable attention. The results were based on a gigantic dataset beyond the wildest dreams of most investigators: 2.5 million lactation records from 1.5 million cows. Offspring sex influences milk output in two ways. First, the mother produces more milk after giving birth to a female calf than after a male. (Calves are removed soon after birth and cannot influence subsequent milk production.) More surprisingly, ongoing milk production by a pregnant cow is enhanced or diminished according to the sex of her fetus. As Hinde and colleagues concluded, this indicates that offspring in the womb program mammary function. But wait: That means that much of the milk we now drink comes from pregnant cows!

Milking cows during pregnancy?

An avid milk drinker in my youth, I never really thought about how it was obtained, and it certainly never occurred to me that farmers might milk cows in late pregnancy. I first learned of this a few years back. In fact, the average yield of Holstein cows   —  specially selected for milk production  —  exceeds 22,000 pounds a year. Modern dairy farmers harvest milk from a Holstein cow for about ten months, seven during pregnancy. Like humans, cows are pregnant for about nine months, so milking ceases only for the last two. But levels of steroid hormones, notably estrogens and progesterone, soar as pregnancy progresses.

Health risks posed by estrogen-mimicking organic compounds in the environment, such as bisphenol A (BPA), have been much discussed (see my June 11, 2013 post: Spectators on Steroids). But natural estrogens can be 100,000 times more potent.

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It is tempting to conclude that, because excess hormones in cow’s milk result from milking late-pregnant cows, all we need to do is to stop this recent practice. But such a major upheaval in the economics of dairy farming is unlikely to happen any time soon. Moreover, milk contains high levels of beneficial nutrients such as calcium and vitamin D, so eliminating dairy products is not a desirable option. However, evasive action, such as drinking skim milk, is possible because steroid hormones are primarily located in milk fat. In addition, a 2012 paper by Farlow and colleagues reported that estrogen levels in goat's milk were significantly lower than in any cow's milk product tested.