Bladder cancer is a difficult condition to treat. It can hit anyone at any age but is more likely to afflict men than women, and smokers more than non-smokers. It is certainly costly for individuals. For health-system managers, tasked with trying to save as many years of life as possible with finite resources, it has the notorious title of the most expensive malignancy to treat from diagnosis to death. Identifying preventative measures, especially cheap ones, can therefore bring benefits beyond reducing bladder cancer rates, as they may free up resources for treatments of other diseases. Over the years, whether dairy products are preventative of bladder cancer has been debated. However, recently, one study that pooled evidence from many other studies found that yogurt consumption is associated with lower bladder cancer risk. A second recent analysis, also combining data from many previous studies, concludes that consuming more milk is linked to lower bladder cancer rates.
Dairy, especially low-fat dairy, such as milk and various fermented milk products, is thought to protect against a number of diseases, including several cancers. For example, milk and yogurt consumption is associated with lower rates of cardiovascular disease around the world, and yogurt fermented with certain types of bacteria may act as a therapeutic against arthritis and additional autoimmune diseases. When it comes to cancer, eating and drinking fermented milk products has been linked to reduced odds of developing colorectal and esophageal cancers, as well as premenopausal breast cancers.
Researchers seeking to establish broad tendencies between diet and disease are faced with a complex array of available data, collected as part of studies that were designed differently. For them, an especially important task is deciding exactly which studies are of sufficient quality and comparability to be included in a larger assessment. One of the two recent papers about bladder cancer risk and dairy, published in the European Journal of Clinical Nutrition, included 13 cohort studies from countries spread across Europe and North America. All of these were coordinated under the same umbrella international consortium, called BLEND (BLadder cancer Epidemiology and Nutritional Determinants). Crucially, to avoid recall bias, which can occur in nutrition studies when study participants are asked to remember what they ate in the past, these were all prospective studies. The second recent paper started with a literature search and then excluded 180 of 202 studies initially identified because they did not meet strict criteria, such as providing sufficient data. This paper was published in the journal, Nutrition and Cancer.
In the analysis of the BLEND data, the research team of Anke Wesselius and Merab Acham, of Maastricht University in The Netherlands, first converted all of the food-intake data from different cohort studies into weekly consumption totals, from which daily averages could be calculated. They were able to separately analyze liquid milk intake from other forms of dairy, including cheese, ice cream, cream and yogurt. Of the 3,590 bladder cancer cases in the 13 BLEND studies, three-quarters occurred in men, and 71% occurred in smokers. These studies included 593,637 people who did not develop bladder cancer. Comparing dairy consumption patterns of people who had developed bladder cancer with those who did not revealed only one statistically significant finding: bladder-cancer-free individuals were more likely to be yogurt-eaters. Indeed, when the researchers looked within the at-risk groups—comparing with men separately to among women, and with smokers separately to among non-smokers—the same pattern was evident: individuals who ate yogurt were less likely to develop the disease.
The BLEND analysis did not, however, pick up a dose-dependent response. In other words, the data did not suggest that the more yogurt you eat, the less likely you are to get bladder cancer. Instead, the study found that participants who ate more than an average of 25 grams of yogurt per day—that is, roughly a couple of servings per week—had a 15% lower chance of developing bladder cancer compared with participants who did not eat yogurt. Among the studies pooled together under the BLEND consortium, almost all had individually identified this association. Moreover, a double-blind randomized control trial conducted in Japan pointed to a particular bacterial strain in yogurt, called Lactobacillus casei, providing a protective effect.
The second recent paper combined and compared studies from four continents. Bringing together all of the data from 18 studies led the research group that authored this paper to conclude that consuming milk specifically—as opposed to dairy products in general—is associated with a 26% reduction in bladder cancer risk. Curiously, when the researchers, based out of Sun Yat-Sen University, in HuiZhou, China, compared different world regions, they found a stronger protective effect in Asian studies than in studies conducted in Europe and North America. This finding could be linked to the fact that people in Asia tend to consume less dairy overall, and hence, among those who do include milk in their diets, the results are statistically cleaner.
There is no straightforward conclusion as to why milk and yogurt could reduce the risk of developing bladder cancer. Like many cancers, bladder cancer’s development involves many complex steps. One idea is that vitamin B2 (riboflavin), found in high levels in both milk and yogurt, is the component conferring protection. It is excreted in urine, and so logically comes into contact with the bladder epithelium. Another proposal as to why yogurt appears to be protective is that fermentation, while reducing sugar content, increases the amount of antioxidative phenolic compounds in yogurt. These compounds quench free radicals that promote cellular mutations.
Whatever the mechanisms responsible for the apparent protective effects, these findings are heartening, especially to individuals most at risk. As well as being hugely expensive to treat, bladder cancer is the ninth most common cancer globally. As such, doctors may wish to consider recommending a yogurt every few days, and a splash of milk in one’s tea. — By Anna Petherick, University of Oxford & the International Milk Genomics Consortium