High intakes of whole milk and lactose might be associated with an increased risk of ovarian cancer, suggests a review of 21 published studies.
However, since the association was a weak one, the new report is no reason to stop drinking milk in moderation, the researchers note.
This new meta-analysis appears in the Aug.
5 online issue of the International Journal of Cancer.
"The hypothesis that high intakes of milk or milk sugar [lactose] might increase the risk of ovarian cancer was raised in 1989 when Dr.
[David] Cramer reported that countries in which milk consumption was high had a higher occurrence of ovarian cancer," explained study author Susanna Larsson, a researcher at the Karolinska Institute in Stockholm, Sweden.
"Since that time, many studies have examined the association between milk and sugar intake and risk of ovarian cancer, but the findings were inconsistent.
" But three large, well-designed studies in which dietary intake was assessed among cancer-free women who were followed up over time to see who developed ovarian cancer showed an increased risk of ovarian cancer among women with high intakes of milk and lactose, Larsson noted.
If women took in a daily increase of 10 grams of lactose, about the amount in one glass of milk, their risk of ovarian cancer increased by 13 percent, Larsson found.
In the 18 case-control studies, in which those who had ovarian cancer were compared with healthy controls, no evidence of an association between dairy food and lactose intake and ovarian cancer was found, except for whole milk.
Those in the highest whole milk consumption group had a 27 percent higher risk of ovarian cancer.
But low-fat milk consumption was negatively associated with ovarian cancer in the case-control studies, she found.
Exactly how the dairy foods may boost risk isn't known, but lactose produces galactose and glucose, and galactose has been thought to increase the risk by direct toxicity to the ovarian germ cells.
But the bottom line is that "it is still uncertain whether the potential increased risk of ovarian cancer associated with high milk/milk sugar intake is similar for all women, or if the risk is restricted to some women who are genetically susceptible to milk," according to Larsson.
"Although milk consumption may increase the risk of ovarian cancer, this cancer is relatively uncommon," Larsson added.
"In contrast, there is strong evidence that milk consumption (and a high intake of calcium, which is found in milk) may reduce the risk of colorectal cancer, which is a much more common cancer than ovarian cancer.
Consumption of low-fat milk might also lower the risk for other diseases, such as type 2 diabetes and cardiovascular disease.
" This year, about 22,220 new cases of ovarian cancer will be diagnosed in the United States, according to the American Cancer Society, and about 16,000 women will die from the disease.
Larsson is not recommending women stop drinking milk.
"However, in the future when we know more about possible interactions between milk consumption and genetic susceptibility, it might be that some women should not drink milk because of a high risk of developing ovarian cancer.
" Another expert, Dr.
Jonathan Berek, executive vice chairman and a professor of obstetrics and gynecology at the University of California, Los Angeles Jonsson Comprehensive Cancer Center, agreed women should not stop drinking milk but should practice moderation.
He said that although the association between milk drinking and ovarian cancer has been made, "it's a weak positive epidemiological factor.
" Even if it proves to be true, it does not play as strong a role as other factors -- such as the use of oral contraceptives -- do, he added.
"We know that women who take the birth control pill for at least five years cut their risk [of ovarian cancer] in half.
If you take the birth control pill and also have a couple of kids your risk may be as low as one-third that of the general population," Berek said.
"I don't think you can prove [the link] using these kinds of studies," Berek noted.
"What is needed are more refined molecular genetic studies.
"