There are many ways that dairy, especially in its lower-fat formulations, does a body good: It reduces the risk of high blood pressure, shores up bone density and helps prevent diabetes. So Americans—who have upped their dairy consumption over the past three decades to nearly two cups a day—should be congratulated, right? According to some nutritional experts, the answer is maybe not. Turns out that today’s milk-based products, particularly those derived from cows, pack a surprising hormonal punch, stronger than the one our ancestors experienced.
Here’s what dairy* products—milk, yogurt and cheese—naturally contain: estrogens (sex hormones that can spur tissue growth), a sugar component (which may act like a reproductive hormone on the ovaries) and a protein ingredient (which seems to stimulate the production of growth hormones in our bodies). These are all a boon if you’re a calf or a human toddler. But once you’ve reached adulthood, growth and sex hormones have the potential to trigger and speed the development of some cancers, notably of the breast. So far, the link between dairy and cancer is driven more by preliminary research and theory than by conclusive evidence. And none of this research has definitively tested the difference in hormonal load between organic and nonorganic milk. Nonetheless, if you regularly drink nonorganic milk—and perhaps have been trying to drink more—consider some experts’ concerns before you pour another glass.
[*Both butter and ice cream contain, in small amounts, the kinds of hormones that milk, cheese and yogurt do. However, the USDA considers butter a fat and sweetened ice cream a treat rather than a dairy product.]
The Hormonal Stew
Most American women have become leery of estrogen because of its well-reported connection to breast cancer, at least when it enters the body through hormone therapy (HT). While the amount of estrogen in a glass of milk is just thousandths of that found in an HT pill, a 2009 study funded by the National Cancer Institute in Frederick, Maryland, found that several kinds of nonorganic cow’s milk—skim, low fat, whole and buttermilk—contain 11 types of estrogen, some of which have been linked to the development of estrogen-positive breast-cancer tumors. (Note: Cow’s milk is the dairy form nutrition experts are most likely to study—as opposed to, say, milk from sheep or goats—but all milk-derived foods contain hormone and hormone-like ingredients.)
According to studies, dairy products are the main source of the estrogen we consume in our food, contributing 60 to 70 percent of what’s found in our diets, says estrogen researcher Timothy Veenstra, a laboratory director at the National Cancer Institute. To be sure, estrogens from dairy make up a relatively small amount of our total estrogen load when compared with all other contributors. Most of the estrogen in premenopausal women’s bodies is produced by their ovaries; in postmenopausal women not on HT, the estrogen comes largely from fat tissue and other organs. Still, dairy products affect the total amount of estrogen circulating in women’s bodies.
Adding to the risk is the fact that cow’s milk is rich in lactose, a milk sugar that includes a molecule of galactose. Galactose may set in motion processes that stimulate the growth of ovarian cells and follicles, adding to the lifetime stress on the ovaries, “which could, according to one theory, damage the organs and promote cancer,” says Jeanine Genkinger, PhD, assistant professor of epidemiology at Columbia University’s Mailman School of Public Health.
Also, some experts hypothesize that a milk protein called casein spurs our bodies to make more of the growth hormone IGF-1. “We know that in both pre- and postmenopausal women, those with higher levels of IGF-1 have a higher risk of breast cancer,” says Michael Pollak, MD, a professor of oncology at McGill University in Montreal and one of the world’s foremost experts on IGF-1. “We also learned in the Harvard Nurses’ Health Study that if you drink two glasses of milk a day compared with no glasses a day, your IGF-1 levels go up.”