This is the source of the new wave of "milk cancer theory". Casein is found in mammalian milk, but in varying amounts and types. Professor Campbell's experimental method is to use two groups of rats, through the carcinogen aflatoxin in their bodies to produce liver tumors, and then a group with only plant protein feed fed, the other group to eat only milk casein feed. In the end, it was found that the condition of the rats in the vegetable protein group remained unchanged, while the condition of the rats in the casein group worsened significantly. At the same time, they compared the tumor rates of rats fed diets containing 20% and 5% casein, respectively, and concluded that casein, which makes up 87% of the milk protein, "significantly promotes the development of hepatocellular carcinomas in an animal model".

What are we to make of this study? Prof. Chen Junshi discusses his views: First, the findings of the rat study cannot be directly extrapolated to humans. Second, this study was conducted using casein as the only source of protein for the experimental rats, a dietary structure that is virtually impossible to find in a human's daily diet. "Normally, more than 90% of the milk we drink is water, and all the proteins add up to only 3%; a glass of milk of 250 milliliters, how much casein can there be in it? And people need at least 60 grams of protein a day" - obviously, by "a glass of milk a day" can only meet a small part of the protein needs. Once again, the experiment was first triggered by aflatoxin, and the large amount of casein enhanced the carcinogenic effect of aflatoxin, rather than directly triggering cancer. The most critical thing is that Campbell's study is casein, "did not say that milk will cause cancer", directly to the conclusion that "milk promotes cancer" is misleading.