The Different Hormones in Cow’s Milk

Cow’s milk is naturally produced to support the growth and development of a calf. It is widely marketed as a dietary source of calcium and protein, among other nutrients. However, recent research has drawn some concern in the potential health implications associated with the hormones present in cow’s milk, especially estrogen and its metabolites. Estrogen is a hormone that plays a role in the growth and development of the female reproductive system and secondary sex characteristics, but is also implicated in the development of hormone-dependent cancers. While humans have been drinking milk without apparent harm for millennia, modern milk is different due to commercial dairy practices – this is aside from the possible use of steroids, antibiotics and other growth hormones for cows in a medical/veterinary setting. Cow’s milk is produced almost year-round, as dairy cattle are milked throughout their pregnancy. This results in significant changes in the composition of the milk due to the pregnant mother producing a variety of different hormones to support the growth and development of the fetus throughout the gestation period.


The Cow Milking Process

Dairy cows are usually bred around 15 months of age through artificial insemination, and will have their first calf at about two years of age. After a dairy cow has given birth, the calf is quickly separated from its mother in order to prevent the common mother/calf bonding that develops. After separation, young dairy calves are typically fed a commercial milk replacer; a cost-effective alternative to whole milk based on dried milk powder, or sometimes fed pasteurized milk from other cows in the herd. Cows are typically milked throughout pregnancy for 10 months out of the year, aside from a two-month “dry” period that takes place to prepare the birth for the next calf, and this lactation cycle repeats for an average 4 or 5 times before they are now longer able to produce milk and sent to the slaughterhouse for meat production.1


About the Hormones In Cow’s Milk

  • Prolactin – released during the cow’s lactation period, with the highest concentration during the first days after parturition.2
  • Insulin-like Growth Factor-1 (IGF-1) – often elevated due to use of growth hormone injections to increase milk yield, but also naturally exists at higher concentrations in milk colostrum.3,4  Milk IGF-1 is not destroyed by pasteurization and will be present in shelf milk.5  It plays an important role in cellular growth and metabolism, and can promote tumour development by stimulating cell proliferation.High levels of IGF-1 are associated with an increased risk of developing tumours of the prostate, breast, endometrium, pancreas, and colon.7
  • Glucocorticoids – steroid hormones that may be present due to the widespread use of dexamethasone, a prescription drug (corticosteroid) used for the treatment of various inflammatory diseases in veterinary medicine.8  Stressful conditions, such as transport, diet changes, changes in environment, and other stressful factors may also contribute to the levels of these hormones in milk.
  • Androgens – steroid hormones, like testosterone, whose concentrations are increased during pregnancy.9
  • Progesterone – mainly concentrated in the fat fraction of milk,10  progesterone helps regulate and maintain pregnancy, and plays a role in the development of mammary glands in breast tissue.
  • Estrogens – an important hormone in female growth and reproduction


Estrogens and its Metabolites in Cow’s Milk

While a variety of hormones have been detected in cow’s milk, there is a lack of research showing that these hormones are actually absorbed and detectable in the human body. There is one exception, however, and it is the presence of estrogen and its metabolites.

Estrogens are unavoidable hormones in non-vegetarian human nutrition. It is naturally occurring in animals including beef, veal, and pork.11,12,13  In a typical western diet, about 60-80% of estrogen comes from milk and dairy products.14  Estrogen is the primary female sex hormone that is responsible for the growth and development of the female reproductive system and secondary sex characteristics. Estrogen induces growth of breast tissue, widens the pelvis and increases the amount of body fat in hips, thighs and buttocks.   Estrogen also induces growth of the uterus, proliferation of the endometrium, and menses.

The major estrogens detected in cow’s milk are estrone, 17 B-oestradiol, and estriol. In the third trimester of pregnancy, the amount of the total concentration of these hormones (1639 ng/L) is more than 27 times higher than in cows during the first trimester of pregnancy (60 ng/L). The bioavailability of 17B-estradiol and estrogen are relatively low; however, estrogen sulphate (estrone), the main conjugate in milk, is known to have a high level of bioactivity.16


Hormone1st Trimester2nd Trimester3rd Trimester
Concentrations (ng/L) of the sum of free and conjugated estrogens in bovine milk collected at different trimester of pregnancy from lactating cows.17
Estrone (E1)7.9 +/- 0.7452 +/- 661266 +/- 38
17-a-Estradiol (aE2)33 +/- 784 +/- 4322 +/- 35
17-b-Estradiol (bE2)18.6 +/- 0.251.4 +/- 2.751.2 +/- 1.5


Because of the presence and bioavailability of estrogen and its metabolites, cow’s milk is likely to have some biological effect in animals and humans. Hormones are potent compounds and even at low doses, can exert significant biological effects. Population studies assessing the risk of cow’s milk for different types of cancer remain equivocal; however, there is evidence suggesting that high milk and dairy consumption play a role in the development of prostate and testicular cancer.18 Furthermore, in a recent review researchers suggested that special concern should be made for populations consuming dairy foods that may be more sensitive to the potential hormonal impact of estrogen, including those during the perinatal and pubertal periods.19


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