Drinking lots of milk might slightly raise a woman's risk of ovarian cancer, a new study suggests.
But even if the risk is real -- which is not yet proved -- the study authors say there's no reason for women to avoid dairy foods.
Susanna C. Larsson, MSc, a researcher at Sweden's Karolinska Institute, led a team that analyzed 21 published studies of milk products and ovarian cancer. Their report appears in the current issue of the International Journal of Cancer.
"In my opinion it is better for women to drink milk than not to drink milk," Larsson tells WebMD. "Ovarian cancer is relatively uncommon. Colorectal cancer is much more common, and milk apparently decreases colon cancer risk. So I don't think women should be worried about it."
That's also the opinion of Carmen Rodriguez, MD, MPH, senior epidemiologist at the American Cancer Society.
"This is very well conducted analysis," Rodriguez tells WebMD. "Whether the results here find a real dairy-related increase in ovarian cancer risk or not, we don't know. But if there is a risk it is small."
Not surprisingly, the findings also fail to faze Stephanie Smith, RD, a spokesperson for the National Dairy Council, the trade group that represents the dairy industry. The National Dairy Council is a WebMD sponsor.
"Women should still aim for three servings of milk and dairy each day," Smith tells WebMD. "Some early research suggests that there are components in dairy foods -- such as calcium, vitamin D, and conjugated linoleic acid -- that may be helpful against cancer.
The Suspect: Milk Sugar
How might milk cause cancer? Larsson notes that milk contains a sugar called lactose. In the stomach, this milk sugar breaks down into regular sugar (glucose) and another sugar called galactose. It's possible that galactose might be directly toxic to young ovary cells. It may also increase hormones that stimulate ovarian cells to grow, potentially leading to cancer.
Larsson says the studies supporting a link between milk and ovarian cancer support this theory. The studies show no ovarian cancer link for cheese, which has very little lactose. Similarly, the studies find only a modest effect for yogurt, which has less lactose than milk.
"There is a very good hypothesis that lactose will increase the risk of ovarian cancer," Rodriguez says. "Even if this is confirmed, we are not talking about a big risk."
Larsson says there may be some women whose genetic makeup puts them at particular risk from lactose. But more studies will be needed to find out if this is true -- and to learn how to identify such women.
Two Study Designs, Two Different Results
Taken together, 18 of the 21 studies analyzed by Larsson and colleagues provide no evidence of a link between milk and ovarian cancer. But these studies are suspect, Larsson says, because they are what researchers call "case-control studies."
Case-control studies look at a person with a disease (in this case, ovarian cancer), and compare them to similar people without disease. But look-back studies of people's diets are notoriously unreliable. And such studies are subject to recall bias -- that is, people with a disease are highly motivated to recall things that might affect their illness.
The three other studies -- one conducted by Larsson's team -- are what researchers call cohort studies. Cohort studies rely on information collected or people followed over time, so recall is not an issue. These three studies all find a link between ovarian cancer and milk consumption: for women who drink at least one glass of milk per day, a 13 percent increase in risk.
That may seem like a lot. But according to the American Cancer Society, a U.S. woman has only a 1.8 percent chance of getting ovarian cancer and a 1.05 percent chance of dying from the disease.
SOURCES: Larsson, S.C. International Journal of Cancer, Aug. 5, 2005; online edition. Susanna C. Larsson, MSc, Karolinska Institute, Stockholm, Sweden. Carmen Rodriguez MD, MPH, senior epidemiologist, American Cancer Society, Atlanta. Stephanie Smith, RD, nutritionist, National Dairy Council.