Dairy Consumption and Hormone-Dependent Cancers — a meta-analysis of meta-analyses
We found an interesting pattern of the rapid growth of milk consumption and an increase in the incidence of hormone-dependent cancers in the past decades in Hong Kong (Yiu, 2019a, 2019b). Yet, it is not easy to prove the causality scientifically. First, it is well known that a simple correlation does not imply causation. Second, the increase in cancer incidence can also be the result of people’s awareness and more frequent body checks. Third, there are numerous other factors not controlled and the growth of the two series may simply be the consequence of a non-stationary spurious result. Fourth, the result can be strongly affected by the research methodology and data collection procedures.
In the medical field, there have been many scientific studies on the relationship between the two and more recently there have also been meta-analyses on these studies. This year there is even a meta-analysis of the meta-analyses. Figure 1 shows, among others, the 4 “radar plots depicting the nature of association between various dairy products consumption and risk of hormone-dependent cancers. The numbers on the circles of each plot represent the total number of meta-analyses from included pooled analyses/meta-analyses and systematic reviews reporting a specific cancer association. The black, red and green lines represent number of meta-analyses reporting no association with, increased risk or decreased risk of hormone-dependent cancers, respectively.” (Jeyaraman et. al., 2019)
Amongst the four hormone-dependent cancers, prostate cancer seems to have much more red lines, i.e. more meta-analyses that are showing an increased risk of prostate cancer by consuming more dairy products, and less green and black lines, i.e. less meta-analyses that are showing a decreased or no association of the risk of prostate cancer by consuming more dairy products. In other words, it is of stronger scientific evidence supporting the hypothesis that prostate cancer is associated with dairy consumption.
The exact numbers of studies confirming and rejecting the hypothesis are as follows:
“Twenty-eight meta-analyses explored associations between various dairy products consumption and risk of prostate cancer. Thirteen meta-analyses showed non-significant association between ‘all-dairy products’, milk, skim milk, yogurt, cheese, butter or ice cream consumption and risk of prostate cancer. Two meta-analyses showed decreased risk of prostate cancer with higher whole milk and cheese consumption. Thirteen meta-analyses showed increased risk of prostate cancer with higher consumption of ‘all-dairy products’, milk, low-fat milk, cheese or dairy calcium.” (Jeyaraman et. al., 2019)
In fact, one of the objectives of Jeyaraman et al. (2019) paper is to address the quality of prospective study design and its impacts on the reliability of the study. Meta-analysis is nowadays a popular research approach to combine most of the previous study results to consolidate the findings to minimize research bias. Yet, as the data of previous studies are mostly not available for re-running any analyses, and there is no way to ascertain whether the data in previous studies are trustworthy or not. As garbage-in-garbage-out, the results of a meta-analysis can still be biased. We may have to wait for another decade’s research before we can ascertain (with higher confidence) the real cause of hormone-dependent cancers. Hopefully, we can devise new scientific methods to have faster and better results. However, we have to appreciate that scientific methods can never prove the truth, but just to find a not-yet-be-refuted-hypothesis.
Jeyaraman MM, Abou-Setta AM, Grant L, et al (2019) Dairy product consumption and development of cancer: an overview of reviews, BMJ Open 2019;9:e023625. doi: 10.1136/bmjopen-2018–023625
Yiu, C.Y. (2019a) Incidence Rates and Death Rates of Various Cancers in Hong Kong, Medium, July 17. https://medium.com/@edwardyiu/edc-and-the-food-cancer-nexus-a31967fc7902
Yiu, C.Y. (2019b) Meat Consumption Growth in Hong Kong is Alarming, Medium, May 19. https://medium.com/@edwardyiu/meat-consumption-growth-in-hong-kong-is-alarming-872e46bf40ca