I've had many people reference the China study and specific individual studies in this discussion that "prove" dairy is harmful.
The first step would be to reiterate that science never "proves" anything, it only disproves hypothesis and through averages makes assumptions on best practices within a highly specific context.
That being said, there is a hierarchy in research ranging from observation and opinion to meta-analysis and systematic review. Near the bottom of the hierarchy are epidemiological studies or uncontrolled longitudinal studies. The latter are what make up the China study. Almost entirely correlative data that have been invalidated at higher levels of scrutiny.
The distinction between correlation and causation for instance is neglected throughout the china study. The article Why correlation does not imply causation? does a good job at breaking this down. And the research (although abundant) is cherry picked to support a preexisting agenda or philosophy. These studies are not representative of the large body of research literature on the subject that suggest the high biological value of dairy products.
To further elaborate on my point that correlation does not equate to causation lets look at another correlation in exercise physiology. Cortisol is the hormone that is most strongly correlated to hypertrophy. However, we know that cortisol is catabolic in nature with regard to muscle tissue and has no net positive effect on anabolism post workout.
The correlation is in the consistent presence of cortisol post workout, but the hormonal signalling responsible for muscle anabolism is entirely different. Strenuous exercise is catabolic in nature and causes a stress response which stimulates production of cortisol. This however is followed by a hormonal cascade that induces hypertrophy through various metabolic machinery that is entirely unrelated to cortisol.
So although cortisol is highly correlated we know with certainty that it is not anabolic in nature and does not induce muscular hypertrophy. In fact it's quite the opposite.
The correlative data referenced in the china study was largely uncontrolled or misinterpreted altogether. Another major issue with uncontrolled longitudinal studies is the inaccuracy of participant reporting. Food journals are no longer a viable method of data collection in peer reviewed literature due to the inaccuracy of reporting (up to 80%).
Confounding variables like lifestyle that are impossible to account for even when using statistical regression models, and are known for their massive gaps in accuracy.
For instance meat eaters have a proclivity to smoke, consume a hyper-caloric diet, and maintain low levels of physical activity (1). All of these over time are known mechanisms that predispose you to cancer and various other metabolic disorders. So can you conclude that observed diseases are caused by dairy consumption, or is it because of lifestyle choices? I think you can see that in this light the research becomes much less convincing.
Lastly, I'd ask the question what is the specific mechanism when dairy is consumed that leads to cancer and other health issues? This is a major point of contention, since there hasn't been any research outside of correlative epidemiological studies to support this. However in more rigorous studies (RCT, Systematic Review, Meta-Analysis etc) the biological value of dairy has been demonstrated quite convincingly (2). And they have concluded that dairy has a net neutral association to all cause mortality. This means it neither increases nor decreases risk of all cause mortality to a degree of statistical significance (3).
I believe that research (this includes observation and personal experience) is not about being right in ones own assertions. It's about validating or invalidating hypothesis based on quality empirical evidence when it's available. The China Study is one of many cases where an author compiles a large body of data and manipulates the findings to fit a very clear preexisting agenda. This level of confirmation bias should be considered when reviewing literature.
1. Wang, Y., and M. A. Beydoun. "Meat Consumption Is Associated with Obesity and Central Obesity among US Adults." International Journal of Obesity 33, no. 6 (2009): 621-28. doi:10.1038/ijo.2009.45.
2. Rozenberg, Serge, Jean-Jacques Body, Olivier Bruyère, Pierre Bergmann, Maria Luisa Brandi, Cyrus Cooper, Jean-Pierre Devogelaer, Evelien Gielen, Stefan Goemaere, Jean-Marc Kaufman, René Rizzoli, and Jean-Yves Reginster. "Effects of Dairy Products Consumption on Health: Benefits and Beliefs—A Commentary from the Belgian Bone Club and the European Society for Clinical and Economic Aspects of Osteoporosis, Osteoarthritis and Musculoskeletal Diseases." Calcified Tissue International 98, no. 1 (2015): 1-17. doi:10.1007/s00223-015-0062-x.
3. Soedamah-Muthu, Sabita S., Eric L. Ding, Wael K. Al-Delaimy, Frank B. Hu, Marielle F. Engberink, Walter C. Willett, and Johanna M. Geleijnse. "Milk and Dairy Consumption and Incidence of Cardiovascular Diseases and All-cause Mortality: Dose-response Meta-analysis of Prospective Cohort Studies." The American Journal of Clinical Nutrition 93, no. 1 (2010): 158-71. doi:10.3945/ajcn.2010.29866.