Lactose-intolerant people - who consume low amounts of milk and dairy products - have a reduced risk of lung, breast and ovarian cancers.
Previous studies have shown that there are large differences in breast and ovarian cancer incidence across different areas of the globe. Experts know, for instance, that the highest incidence of breast and ovarian cancer is in North America, Western Europe and Scandinavia, while incidence is lowest in East and Central Africa.
Twin studies and studies of immigrants have suggested that these differences in incidence are due to environmental factors rather than genetic or ethnic differences.
The high consumption of dairy products in North America and Western Europe has been assumed by some researchers to contribute to these regions' higher cancer incidence.
However, studies examining this link - such as a recent review by the World Cancer Research Fund and the American Institute of Cancer Research - have been inconclusive.
The researchers behind the new study, from Lund University and Region Skåne in Sweden, took a slightly different approach in addressing the suggested link between dairy products and cancer incidence.
They investigated whether low consumption of milk and other dairy products protects lactose-intolerant people against breast, ovarian and lung cancers.
Jianguang Ji, associate professor at Lund University and researcher at the Center for Primary Care Research in Malmö, describes the results of the study:
"Using nationwide data from two Swedish registers (the Inpatient Register and the Outpatient Register), we identified 22,788 individuals with lactose intolerance and examined their risk of suffering from lung, breast and ovarian cancers. The risks of lung cancer (standardised incidence ratio [SIR] = 0.55), breast cancer (SIR = 0.79) and ovarian cancer (SIR = 0.55) were significantly lower in people with lactose intolerance compared to people without lactose intolerance, irrespective of country of birth and gender.
By contrast, the risks in their siblings and parents were the same as in the general population. This suggests that the lower cancer risk in people with lactose intolerance may be due to their diet."
In other words, the team found that lactose-intolerant people - who consume low amounts of milk and dairy products - have a reduced risk of lung, breast and ovarian cancers. Because the cancer risk was not reduced in relatives of people with lactose intolerance, the results suggest that this protection against cancer is related to diet.
Study cannot prove cause and effect
Jianguang Ji points out, however, that "it would be wrong to conclude that milk is a risk factor for these cancers." For instance, factors such as lower calorie intake because of low milk consumption or protective factors in plant-based milk drinks could contribute to the negative association between lactose intolerance and cancer risk.
"We must interpret these results with caution because the association we found is insufficient to conclude a causative effect," says Jianguang Ji. "Further studies are needed to identify factors that explain the study's results."
Indeed, the relationship between milk and cancer appears to be a complex one. In 2012, Medical News Today reported on a previous study from Lund University that suggested drinking milk may protect against the progression of colon cancer.
The Lund researchers behind that study found that a milk protein called lactoferricin4-14 reduces the growth rate of colon cancer cells and increases DNA repair.