PIP: Researchers do not understand what causes ovarian cancer. Some studies find galactose to be toxic to oocytes which renders ovaries susceptible to cancer. 1 hypothesis is that high levels of lactose consumption and retained ability to digest lactose (lactase persistence) lead to high levels of galactose exposure which increases the ovarian cancer risk. Denmark, Sweden, and Switzerland have the highest risk of ovarian cancer in the world. They also have the highest levels of lactase persistence and among the highest levels of milk supply for the population. Conversely, Hong Kong, Japan, Shanghai, and Singapore have low ovarian cancer risk, low consumption of milk, and low levels of lactase persistence. yet other researchers using the same data did not find per capita availability of milk to be significant independent of nondairy animal fat. Lactase persistence did have an independent association, however, but data was not standardized across populations. In a case control study, researchers took the dietary history of 11 dairy products to measure lactose consumption. A significant increase in ovarian cancer risk only existed with frequent intake of yogurt and cottage cheese. Researchers hypothesized that the prehydrolyzed lactose in both foods was the main contributor. Using the case control data, others found an increased risk for total lactose intake in women who never used oral contraceptives (OCs). The strongest predictor to date is the lactose/transferase ratio, yet those who found this association did not define this index based on an earlier study or examination. 1 problem with these studies is that dietary fat confounds the association. 2 studies showed that low fat milk was associated with decreased risk, even though both milks have the same lactose content. Another problem is that the subsets of exposure or subgroups of the population are unpredictable. Further research in nutritional epidemiology of ovarian cancer is needed.