If you've strolled the dairy aisle in your local supermarket lately, you've probably noticed the labels on certain milk packages proclaiming it "hormone-free" or "rBGH-free." You might have reached the conclusion that hormones in milk are dangerous and to be avoided at all costs — or maybe you've decided that it's just alarmist advertising. The answer is probably somewhere in between: Yes, there are hormones in milk. But it's not entirely clear if or how much these hormones affect us.
The uproar about hormones in milk started with the introduction of rBGH (recombinant bovine growth hormone), a man-made version of the naturally occurring bovine growth hormone. The U.S. Food and Drug Administration approved it in 1993. Injections of rBGH raise the level of the hormone IGF-1 (insulin-like growth factor 1) in a cow's blood, leading to increased milk production. (Injections of rBGH are also known to cause udder infections in cows, which lead to antibiotic injections, which lead to the growth of antibiotic-resistant bacteria — which is a story we don't have room for on this page.)
No matter what the source of the bovine growth hormone, it does make its way into a cow's milk — but the levels aren't higher in milk from rBGH-treated cows. The growth hormone doesn't affect humans, anyway, because it's destroyed so quickly in the digestion process. The greater concern with rBGH has been its effect on the IGF-1 levels in milk. IGF-1, which humans also naturally produce, has been shown to promote tumor growth in rats and has been linked to colorectal, breast and prostate cancers [sources: American Cancer Society, Qin et al. ].
One study found that people who drink milk have been found to have 10 percent higher levels of IGF-1 in their blood. But is this dangerous? The jury is out. The same study found that the amount of IGF-1 a person would absorb from drinking rBGH-treated milk every day would equal only 0.09 percent of what they would naturally create in a day [source: Heaney et al. ]. And people who drink soy milk report about the same increase because of the naturally occurring hormones in soybeans [source: American Cancer Society].
Nevertheless, there was so much hubbub about rBGH that by 2007, it was used in only 17 percent of dairy cows in the United States [source: USDA]. The greater concern now is about cows that are milked when they're pregnant — and cows on commercial dairy farms are milked 300 days a year and are almost always pregnant [source: Holstein Association]. In late pregnancy, a Harvard University study found, a cow's blood contains 33 times the normal amount of estrogen [source: Ireland]. Estrogen, unlike BGH, is absorbed intact by humans. But, again, how much goes through and how much is unsafe? One study said that an adult woman produces up to 630,000 nanograms (ng) of her own estrogen every day and there are about 68 ng of cow estrogen in three servings of milk, so milk is completely safe [source: Macrina et al.]. Another study determined that we absorb so much estrogen from milk that it could drastically decrease testosterone production in men and lead to early puberty in children [sources: Davasaanmuuet al., Maruyama et al.].
Clearly, there's more research to be done. One way to avoid the issue altogether is to consume only fat-free dairy products, because the hormones are carried in fat. And hormone injections harm cows (remember those udder infections?), so if you're an animal lover you might do best to support the farms with therBGH-free labels.