In the book, "The China Study" by T. Colin Campbell, here's what the author found:
He started with small-animal studies because the experiment called for injecting aflatoxin into the subjects (mice).
Aflatoxin is a very potent carcinogen.
Aflatoxin was injected into 100 mice and then the mice were evenly divided into 2 groups.
One group was fed a low protein diet and the other group was fed a high protein diet.
After a time, the mice were examined and it was found that the low protein diet inhibited the initiation of cancer - no early stage cancer was found.
But all the mice who were fed a high protein diet showed evidence of cancer initiation (i.e. early stage cancer).
In subsequent studies, they found they could actually switch diets and reverse the outcomes. But only in the early stage of cancer.
The protein used was Casein from cow's milk. 82% of cow's milk is Casein.
Protein didn't cause cancer, it promoted cancer. It's carcinogens that cause cancer.