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 programme 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 oestrogens and progesterone, soar as pregnancy progresses.

Health risks posed by oestrogen-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.

The topic of hormones in milk also has a strong Harvard link, through the School of Public Health. Ganmaa Davaasambuu, a Mongolia-trained medical doctor with a PhD in environmental medicine, has spearheaded pioneering research into hazards of steroid hormones in cow’s milk. I first learned about her research from a 2006 article in Harvard University Gazette by Corydon Ireland, reporting on a seminar she gave on the potential rôle of dairy products (e.g. cow's milk or cheese) in hormone-dependent cancers affecting the reproductive system. Oestrogen levels in milk from a late pregnant cow can be more than thirty times higher than from a non-pregnant cow, and in the USA dairy products account for two thirds of individual dietary oestrogen intake.

Societies maintaining traditional herding practices, such as nomadic inhabitants of Ganmaa’s Mongolian homeland, draw milk from cows for only five months a year, including at most the first three months of pregnancy. A survey in Inner Mongolia revealed that cows fed exclusively on pasture lactate only during the first half of pregnancy. Conceiving through natural mating in July/August and giving birth to calves in April/May, they are milked only from June to October.

Epidemiological evidence linking cow’s milk to cancers

In a 2001 paper, Ganmaa and colleagues suggested that cow’s milk may be responsible for male reproductive disorders. They focussed particularly on data from Japan, where milk consumption increased almost twenty-fold over fifty years after World War II. Over that same period, the incidence of prostate cancer in Japan increased at the fastest rate in the world, rising from almost zero per 100,000 to 7 per 100,000 by 1997.

Another study by Ganmaa and colleagues, published in 2002, compared diet and rates of prostate and testicular cancer across 42 counties. Among food items examined, cheese was most closely correlated with the incidence of testicular cancer at ages 20-39, while milk was most closely correlated with the incidence of prostatic cancer. Testicular cancer rates were highest in countries like Switzerland and Denmark, where cheese is a national food, and lowest in countries like Algeria, where dairy products contribute little to diets.

In 2003, Ganmaa and colleagues published two further papers examining correlations between reproductive cancers and rising consumption of dairy products in post-war Japan. Firstly, although breast and ovary cancers remain rarer in Japan than in other industrialized countries, the former doubled between 1947 and 1997, while the latter quadrupled. Secondly, whereas death from testicular cancer peaked in men aged 30-50 before 1945, the peak was in the 20s for those born thereafter. Over a 50-year period following the war, the death rate from prostate cancer steadily increased by a factor of 25. However, milk was not the only animal product for which consumption increased markedly. Consumption of meat and eggs also increased, albeit to a lesser extent.

Taking a different tack, in 2013, Kroenke and colleagues reported on a study of breast cancer survivors. Drawing on the Life After Cancer Epidemiology project, they examined almost two thousand women diagnosed with early-stage invasive breast cancer in 1997-2000. Women consuming foods with substantial amounts of high-fat cow’s milk were found to have a greater risk of dying from breast cancer than those who ingested small quantities or none at all. No increased risk was seen with low-fat dairy products. (An excellent blog piece by Conner Middelmann-Whitney covers this study in Psychology Today, posted March 22, 2013).

Experimental evidence

Of course, survey findings depend on correlations and need not reflect causal relationships (see my blog post on the “stork-and-baby problem”, July 12, 2013). But some studies have yielded actual experimental evidence linking hormones in milk to reproductive disorders. One, reported by Ganmaa and colleagues in 2006, examined effects of cow’s milk on wombs of sexually immature rats or young adult individuals whose ovaries had been surgically removed. Groups of rats drank either commercially available low-fat milk or artificial milk (negative control), sometimes with added oestrogen (positive control). Rats in the low-fat milk group had significantly greater womb weights than control rats. Promotion of womb growth with artificial milk plus oestrogen exceeded that with low-fat milk in immature rats, but was almost the same in young rats lacking ovaries. The second study, published by Maruyama and colleagues in 2010, involved human subjects. Concentrations of sex hormones were examined after subjects (7 men, 5 women, 6 prepubertal children) had drunk commercial milk following standard protocols. In all subjects, concentrations of oestrone (the main oestrogen) and progesterone increased significantly in both serum and urine. Contrastingly, testosterone levels decreased significantly in men.

What action to take?

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 oestrogen levels in goat's milk were significantly lower than in any cow's milk product tested.


Farlow, D.W., Xu, X. & Veenstra, T.D. (2012) Comparison of estrone and 17β-estradiol levels in commercial goat and cow milk. J. Dairy Sci. 95:1699-1708. [Erratum in: J. Dairy Sci. (2012) 95:4732.]

Ganmaa, D., Li, X.M., Qin, L.Q., Wang,P.-Y., Takeda,M. & Sato,A. (2003) The experience of Japan as a clue to the etiology of testicular and prostatic cancers. Med. Hypoth. 60:724-730. 

Ganmaa, D., Li, X.M., Wang, J., Wang,J., Qin,L.-Q., Wang,P.-Y. & Sato,A. (2002) Incidence and mortality of testicular and prostatic cancers in relation to world dietary practices. Int. J. Cancer 98:262-267.

Ganmaa, D. & Sato, A. (2005) The possible role of female sex hormones in milk from pregnant cows in the development of breast, ovarian and corpus uteri cancers. Med. Hypoth. 65:1028-1037.

Ganmaa, D., Tezuka, H,  Enkhmaa, D, Hoshi, K. & Sato, A. (2006) Commercial cows' milk has uterotrophic activity on the uteri of young ovariectomized rats and immature rats. Int. J. Cancer 118:2363-2365

Ganmaa, D., Wang, P.Y., Qin, L.Q., Hoshi, K. & Sato, A. (2001) Is milk responsible for male reproductive disorders? Med. Hypoth. 57:510-514.

Hinde, K., Carpenter, A.J., Clay, J.S. & Bradford, B.J. (2014) Holsteins favor heifers, not bulls: Biased milk production programmed during pregnancy as a function of fetal sex. PLoS ONE 9:2 e86169:1-7.

Ireland, C. (2006) Hormones in milk can be dangerous. http://www.news.harvard.edu/gazette/2006/12.07/11-dairy.html

Kroenke, C.H., Kwan, M.L., Sweeney, C., Castillo, A. & Caan, B.J. (2013) High- and low-fat dairy intake, recurrence, and mortality after breast cancer diagnosis. J. Natl. Cancer Inst. 105:616-623.

Li, X.M., Ganmaa, D. & Sato, A. (2003) The experience of Japan as a clue to the etiology of breast and ovarian cancers: relationship between death from both malignancies and dietary practices. Med. Hypoth. 60:268-275.

Maruyama,K., Oshima,T. & Ohyama,K. (2010) Exposure to exogenous estrogen through intake of commercial milk produced from pregnant cows. Pediatr. Int. 52:33-38.

Raloff, J. (2009) Scientists find a soup of suspects while probing milk's link to cancer: Latest studies focus on estrogens, androgens and IGF-1. Science News 175(7):5-6.

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