We know that type 2 diabetes is associated with processed food.
We know that type 2 diabetes is associated with the top section of the gut. How do we know this? A treatment for obesity is to do surgery that bypasses that part of the gut. The intention was just to reduce the total size of the gut. But a miracle occurs: if the patient has type 2 diabetes (raised blood sugar), then they are instantly cured! No need to wait for them to lose weight.
So what is the role of the top section of the gut in a normal human primitive diet?
Normally primitive food will come in with the cell walls intact. So it is obvious that the first job, the job done by the top section of the gut, will be to deal with those cell walls. And it will be no surprise if it expects to see cell walls, and uses them to self regulate, and fails to function correctly without them.
And, of course, the characteristic property of processed food is that the cell walls have already been destroyed by industrial processes.
This ain't rocket science. (Whatever happened to "ain't").