Hormones in Milk
Many dairy farmers inject their cows with rbST (also called rbGH), a genetically engineered replica of bovine growth hormone (BGH), which the animals also produce naturally, to control milk production. Treatment typically boosts cows' milk output by up to 15 percent, and about one third of U.S. dairy cows are now injected with rbST. But because milk from different farms is mixed together in tanker trucks, it's reasonable to assume that almost every carton of milk contains some BGHtreated product, unless it's labeled organic rbST or rbGHfree.
The Potential Danger
Ever since the FDA approved rbST in 1993, there have been allegations that drinking the milk from treated cows increases human health risks ranging from breast cancer to reproductive abnormalities. With these concerns in mind, the consumer advocacy group Food & Water Watch has pressured Starbucks to switch to hormonefree milk, with positive results. But hundreds of studies over the years have settled most of those issues. "AntiBGH activists have lost every round of scientific, peerreviewed research," says Phillips.
Most recently, however, a study published in The Journal of Reproductive Medicine found that women who drank milk were three times more likely than vegans to have twins. One theory: Insulinlike growth factor (IGF), a protein produced by all mammals that has been linked with twinning in cows, is having the same effect in women. "When animals receive BGH replica, their IGF levels go up," says study author Gary Steinman, M.D., an obstetrician at Long Island Jewish Medical Center in New Hyde Park, NY. He suspects that elevated levels of IGF from hormonetreated cows are passed to people in milk. No research offers slamdunk proof that rbST milk makes women have more twins. But twinning rates in U.S. women are higher since rbST was introduced, Steinman says, and use of assisted reproductive technologies doesn't fully account for the increase. "The rate has gone up twice as fast here in the U.S. as in Britain, where there's been a moratorium on synthetic BGH," he says.
Should You Worry?
It's unlikely that rbST or IGF in milk has anything to do with your odds of having twins, according to Baumrucker. In fact, IGF from food can't be absorbed directly by the body, he says. What can happen is that other proteins in milk boost the body's production of its own IGF. But even if milk protein indirectly raises IGF, Baumrucker adds, the elevation would be well within the normal range.
Steinman admits he hasn't connected the dots between IGF in milk from BGH cows and twins. And if a connection does exist, you would only need to be concerned about it if you're trying to get pregnant and don't want twins, in which case he suggests switching to soy or organic milk as a precaution.