HPH NOW, Nov 10, 2006, What's the Connection? Workshop Presents Information about Milk, Hormones, and Human Health

Walter Willett

American adults should consider drinking no more than one or two servings of milk a day-less than the current U.S. government recommendation of three servings-suggested Walter Willett, chair of the Department of Nutrition, at a workshop on milk, hormones, and human health on October 26 at the Harvard Club of Boston.

The evidence that high milk consumption is related to the risk of fatal prostate cancer is quite strong, said Willett. For ovarian cancer the evidence suggests about a 20 to 30 percent increased risk with the equivalent of three servings of milk a day, said Willett.

He observed that the major justification for drinking milk has been preventing fractures, but epidemiological evidence is quite strong that high dairy consumption as an adult does not reduce the risk of fractures, he asserted.

Willett made his remarks as part of a summary of discussions held over a three-day conference, which was organized by the Harvard Center for Cancer Prevention. The speakers included academicians, a dairy journalist, a representative of a pharmaceutical company, and a researcher at a reference lab.

When it comes to children, who are also urged by the government to drink three glasses of milk a day, Willett said that the story gets more complicated.

``We don't really know all the long-term consequences,'' he said. ``We know that calcium is an essential nutrient, and we have to get it someplace. But it doesn't have to be dairy. We know that kids do grow up in other parts of the world without consuming dairy products, and they have healthy bones.''

The statistics on prostate cancer had come from a discussion earlier in the workshop with Edward Giovannucci, professor of nutrition and epidemiology at HSPH, who said that most studies show that a higher consumption of dairy products is associated with an increased risk of an aggressive form of prostate cancer.

He said that the mechanism is not clearly known, but that milk may increase the body's production of IGF-1, a hormone that could fuel the growth of cancer cells.

Michael Pollak, a professor in the Departments of Oncology and Medicine at McGill University and director of its Cancer Prevention Program, said that the conference showed that milk consumption has its pros and cons. (For example, attendee Alicja Wolk from the Karolinska Institute presented data on an inverse correlation between high-fat dairy foods such as cheese, as well as conjugated linoleic acid intake, and risk of colorectal cancer.)

Pollak added that much remains to be learned about milk consumption's benefits and risks. His laboratory studies the influence of various hormones, including insulin, insulin-like growth factors, and numerous steroid hormones, on cancer risk.

He said that the fact that milk consumption raises the levels of the growth hormone IGF-1 is an important topic for future research.

Pollak also noted that it has been difficult for scientists to measure the levels of hormones in milk and that much must be learned about milk's effects, particularly in terms of when in life it is consumed. ``It is a huge factor that we have not recognized,'' he said.

—ML