Renewed debate over milk and ovarian cancer risk
of milk products and ovarian cancer risk, finds a new meta-analysis
of epidemiological studies, but the case is far from cut and dried.
A possible correlation was first reported in 1989, but research conducted since then has yielded contradictory results. This led researchers at the Karolinksa Institute in Sweden to analyze the results of three prospective cohort studies and 18 case-control studies, in an effort to gain better understanding of the relationship.
Ovarian cancer is the seventh most common form of cancer in women, affecting two percent of women worldwide at some point in their lifetime.
The investigations, published in the International Journal of Cancer, involved three prospective cohort studies and 18-case control studies, all of which offered data on the association between intakes of milk, yogurt, cheese or lactose and incidence or mortality from epithelial ovarian cancer.
The prospective cohort studies suggested that high intakes of milk may increase the risk of ovarian cancer, and a 10g per day increase in lactose intake (equivalent to one glass of milk) led to a 13 percent increase in risk.
In the case control studies, on the other hand, no evidence was found for the role of lactose in increasing the risk. While there was a positively associated between whole milk and ovarian cancer risk, the association with low fat milk was negative.
When the researchers looked at all 21 studies together regardless of methodology, they found that yogurt was associated with a "non significant" increased risk of cancer, but that there was no risk with cheese.
Since the two studies included in the meta-analysis that did so indicated that the milk and lactose association occurred only with serous ovarian cancer, the researchers recommend that "future studies should consider specific subtypes of ovarian cancer, and the interrelationship between intakes of dairy foods and lactose, genetic polymorphisms, and ovarian cancer risk."
One of these was a 13-year, observational study of 64,084 women carried out by the same scientists at the Karolinska Institute and published in the American Journal of Clinical Nutrition. In this instance, they wrote that "continued research is warranted to further elucidate the association and mechanisms."
As to the differences between the results for cohort and case control studies, these may be due to changes in dietary practice following diagnosis of cancer, selection bias or recall bias. Moreover the two types of study do not assess dietary intake at the same time point: for cohort studies the assessment could take place at any time before the illness occurs - in some cases even decades before; in case control studies, on the other hand, diet is assessed at or close to the time of diagnosis.
