Ganmaa Davaasambuu is a physician (Mongolia), a Ph.D. in environmental health (Japan), a fellow (Radcliffe Institute for Advanced Study), and a working scientist (Harvard School of Public Health).
The link between cancer and dietary hormones – estrogen in particular – has been a source of great concern among scientists, said Ganmaa, but it has not been widely studied or discussed.
The potential for risk is large. Natural estrogens are up to 100,000 times more potent than their environmental counterparts, such as the estrogen-like compounds in pesticides.
“Among the routes of human exposure to estrogens, we are mostly concerned about cow’s milk, which contains considerable amounts of female sex hormones,” Ganmaa told her audience. Dairy, she added, accounts for 60 percent to 80 percent of estrogens consumed.
Part of the problem seems to be milk from modern dairy farms, where cows are milked about 300 days a year. For much of that time, the cows are pregnant. The later in pregnancy a cow is, the more hormones appear in her milk.
Milk from a cow in the late stage of pregnancy contains up to 33 times as much of a signature estrogen compound (estrone sulfate) than milk from a non-pregnant cow.
In a study of modern milk in Japan, Ganmaa found that it contained 10 times more progesterone, another hormone, than raw milk from Mongolia.
In traditional herding societies like Mongolia, cows are milked for human consumption only five months a year, said Ganmaa, and, if pregnant, only in the early stages. Consequently, levels of hormones in the milk are much lower.
“The milk we drink today is quite unlike the milk our ancestors were drinking” without apparent harm for 2,000 years, she said. “The milk we drink today may not be nature’s perfect food.”
Earlier studies bear out Ganmaa’s hypothesis that eating dairy heightens the risk of some cancers.
One study compared diet and cancer rates in 42 counties. It showed that milk and cheese consumption are strongly correlated to the incidence of testicular cancer among men ages 20 to 39. Rates were highest in places like Switzerland and Denmark, where cheese is a national food, and lowest in Algeria and other countries where dairy is not so widely consumed.
Cancer rates linked to dairy can change quickly, said Ganmaa. In the past 50 years in Japan, she said, rising rates of dairy consumption are linked with rising death rates from prostate cancer – from near zero per 100,000 five decades ago to 7 per 100,000 today.
Butter, meat, eggs, milk, and cheese are implicated in higher rates of hormone-dependent cancers in general, she said. Breast cancer has been linked particularly to consumption of milk and cheese.
In another study, rats fed milk show a higher incidence of cancer and develop a higher number of tumors than those who drank water, said Ganmaa.