> how do you propose to measure long term mortality rates in a direct feeding study?
Simple, lock lots of people in a building, feed them the same thing for 50 years and then measure :) That's impossible, of course, which is why I try to take all these studies, no matter what the type, no matter which side of the argument they are on, with a monstrous grain of salt. When trying to find the perfect diet, I think it's better to look into the diet of our ancestors and to refer to the works of Dr. Weston Price and Vilhjalmur Stefansson. What they did might not be considered technically "science" because it's not peer-reviewed, doesn't have confidence intervals, mortality rates and whatnot but I consider it much more valuable as they spent years on the field studying people's diets and their health, not sitting in a comfy chair measuring blood cholesterol levels.
Inuit lived practically on very fatty meat only. One can only imagine their LDL. Why weren't they dropping dead of cancer, diabetes and heart attacks at 30 years of age? Dr. Price says that in all his travels he didn't come upon a group that thrived on plant food only. If I remember correctly there was a community in the Swiss Alps that lived exclusively on wholegrain rye bread, cheese + some meat once a week and they were very healthy. That's was the diet closest to vegan that he described.
Of course, these days with all the nutrition science advancements, with the addition of supplements, it's may be possible to thrive as vegan but I'm not willing to be the guinea pig.
And yes, I'm well aware that feeding everyone animal protein is not sustainable in any way but that doesn't have much to do with the discussion whether animal protein is bad for your health or not. Given the problems with modern agriculture, it's questionable whether the Earth can sustain 9 billion vegans in the long term either.
I try to take into account the cruelty of industrial farming when choosing my food, at least to certain extent. However, you need to note that the very fact of my existence means hundreds/thousands of critters are being killed. From ecological point of view, the best option would be for me to kill myself, not become a vegan. I just have to draw the line somewhere.