Here's another study. On rats. After administering a carcinogen (aflatoxin), researchers fed rats a high-protein diet. The rats' cancer grew. Researchers switched to a low-protein diet, the rat's cancer regressed. Researchers switched back to the high-protein diet. The rats' cancer came back, with a vengeance.
High Protein Intake Promotes the Growth of Hepatic Preneoplastic Foci in Fischer #344 Rats: Evidence that Early Remodeled Foci Retain the Potential for Future Growth, The Journal of Nutrition, 1991
"Switching from the high protein diet (20% casein) to a low protein diet (5% casein) resulted in marked remodeling (regression) of the growing lesions to a response level similar to that in animals that did not receive the initial promotional stimulus of high protein feeding. However, refeeding the high protein diet caused significant reappearance of these lesions."The protein used was casein, milk protein. Interestingly, when the rats were switched to a low-protein diet, to keep their caloric intake the same, they were given more sugar and corn starch. The addition of proportionately more carbohydrate in the form of sugar and corn starch (more than what the high-protein-fed rats were eating) did not lessen the positive effect of the low-protein diet.
Casein is about 87% protein. So, a 5% casein diet provides about 4.35% protein. A 20% casein diet provides about 17.4% protein. Americans consume on average about 15.4% protein.
Update: Out of curiosity, I checked to see how much casein is in some typical dairy foods. You can see what I found out here.