Diets high in animal protein are associated with an increased diabetes risk. Our findings also suggest a similar association for total protein itself instead of only animal sources. Consumption of energy from protein at the expense of energy from either carbohydrates or fat may similarly increase diabetes risk. This finding indicates that accounting for protein content in dietary recommendations for diabetes prevention may be useful."
I knew from reading Song et al recently that the quartiles for vegetable protein actually represent quite small amounts. In the graph above, the upper quartile of vegetable protein is eating 33g/day, while the lower quartile of animal protein is eating 35g/day. So if you want to compare similar amounts of these proteins, you need to compare upper quartile vege with lower quartile animal, and they have exactly the same association with diabetes. And total protein (meat and vege combined) actually had a stronger association with diabetes than animal protein (1.67 in model 3, vs 1.58 for animal protein).
In real life, most people were eating both sorts of protein. Across the animal protein quartiles in Table 1, vegetable protein stayed very constant (people ate much the same amount of wheat). Unfortunately, there is no baseline data that tells us how much animal protein the quartiles of vegetable protein ate.
But let's take a common-sense approach to this data. Model 3, which I cited earlier, isn't adjusted for BMI and waist circumference. The highest protein quartile reports eating fewer total calories than the others, but has significantly greater BMI and waist circumference. And when these are adjusted for (Model 4, Table 2), voila, the association between protein and diabetes disappears from the quartile calculations; only the per 10g association remains. And this, though small, is greater for total protein (1.16) than for animal protein (1.13).