The investigation found notable differences between case-control studies (where diet was analyzed after cancer diagnosis) and cohort studies (where diet was analyzed prior to cancer diagnosis). Case-control studies showed low-fat milk consumption negatively associated, and whole milk consumption positively associated, with the risk of ovarian cancer.
In contrast, prospective cohort studies indicated that high intakes of milk may increase the risk of ovarian cancer. They also revealed a 13% boost in ovarian cancer risk with a daily increase of 10 grams of lactose (the approximate amount in one glass of milk).
When case-control and cohort studies were considered in combination, yogurt consumption was associated with a non-significant increase in cancer risk, while cheese was not associated with risk.
Study lead author Suzanna C. Larsson and her team explain that the differences between the findings of case-control studies and those of cohort studies could be explained by a number of factors: selection bias, recall bias, or changes in dietary practices after cancer diagnosis. They might also be due to the time interval between diet assessment and illness, since cohort studies may record dietary practices decades before illness occurs, while case-control studies assess diet around the time of diagnosis.
“In conclusion,” they write, “prospective cohort studies, but not case-control studies, support the hypothesis that high intakes of dairy foods and lactose may increase the risk of ovarian cancer.”
International Journal of Cancer – August 5, 2005;Online.