The research further determined that only when mice were given 1,000 times more estrogen than average did it have any impact on reproductive health.
The study was published in the Journal of Dairy Science.
There has been concern that consuming cow’s milk containing elevated amounts of estrogen could affect blood levels of the hormone in humans, leading to an increased risk of some cancers.
Bovine milk is known to contain naturally-occurring estrogens. To complicate matters, estrogen levels in milk rise when a cow is pregnant. Cows are typically milked until 60 days before their expected calving, meaning milk from cows in their third trimester of pregnancy can contain up to 20 times more estrogen than milk from cows that are not pregnant.
"The aim of our study was to evaluate whether the consumption of milk with known doses of estrogens (both naturally presented and added in concentrations 100 and 1,000 times higher) could affect blood hormone levels and reproductive organs in mice," said senior co-author Tomaz Snoj, DVM, PhD, Veterinary Faculty at the Institute for Preclinical Sciences at the University of Ljubljana.
The study examined how different concentrations of estrogens in milk affected plasma levels of natural estrone (E1) and 17β-estradiol (E2); uterine weight in females; and testosterone levels, testes weight, and seminal vesicle weight in males.
The results showed that consumption of milk from a pregnant cow did not raise plasma levels of E1 and E2 in mice. It also did not affect the weight of the sex organs examined in either male or female mice. The same results were found for the milk containing an additional 10 ng/mL of E1 and E2; however, investigators did find that when the concentration was raised to 100 ng/mL, effects were seen in the mice.
Snoj said that it is likely that plasma E1 and E2 did not increase in mice drinking pregnant cow's milk because the estrogens in the milk were at low enough levels to be metabolized during first liver passage and did not reach systemic circulation.
More research needed
The authors note, however, that tests were done on mature mice and more research is needed to examine the effect estrogen from milk has on the development of the reproductive system before and during puberty.
"Our results suggest that estrogens in milk, even when derived from cows in the third trimester of pregnancy, do not pose a risk to reproductive health," said senior co-author Gregor Majdic, DVM, PhD, Vice Dean, Center for Animal Genomics, Veterinary Faculty, University of Ljubljana.
He said that as estrogen at concentrations 100 times higher than potentially present in milk did not cause any physiological effects in the present study, meaning that naturally-occurring hormones in milk are found in far too low concentrations to have any biological effect on consumers.