I don't know of any definative proof, which I think is what makes issues like this so dangerous. Scientific proof is a high bar when dealing with something this complex, pit that against powerful food lobbies and truths can stay buried for a long time.

For me, it's been an observation of many cues, then making a personal decision about how I eat from that. One interesting place to start is corn: Most corn that you eat isn't the sweet corn you buy in the store and cook on the grill. It is very hard and must be broken down with lye before being eaten.