The Primal Blueprint calls for lots of consuming of meat. This makes sense from the stand point of losing weight.
But there is a dark side: the acceleration of growth of cancer tumors.
This is the big idea in the seminal research project on diet and disease published under the name "The China Study".
The science behind it is of the highest level: a vast research project spanning 20 years, funded by the US government, FDA, and academic grants, vetted by the best universities (performed at Cornell).
The author compares the diets of Americans and their resulting diseases with the diets of the Chinese and their resulting diseases.
In short, the big discovery is that the proteins in meat accelerate the growth of cancer tumor cells. The instances of colon cancer and other types of cancers were much higher in the west than in China. At the time of the study (the 1970s and 80s), the Chinese diet was mostly vegetarian, due to economic restrictions.
To illustrate the point vividly, in one experiment, a group of rats were given an equal dose of dioxin, one of the most potent carcinogens known. Then the group was split in two, and one group given a high protein diet (around 20%, which is typical of Western diet) and the other group was given a low protein diet (less than 5%). The result was striking: all of the rats in the group with high protein diet developed liver tumors, and none of the low protein group developed tumors. He details this experiment in his paper, if you'd like to see the details. The conclusion: high protein diet acts as a switch, accelerating the growth of tumor cells. The worst culprit turned out to be red meat and dairy, which were highest in the kinds of proteins which fed tumors. Proteins from fish and poultry were not so bad, and protein from vegetables were mostly harmless, with regard to feeding particular kinds of tumors.
There is a well documented linkage between red meat consumption and colon cancer.

Just wondering if those who promote the consumption of lots of meat were aware of this study, and if they had an opinion about it.