I think one of the most astounding demonstrations ever done was Caldwell Esselstyn's first human diet intervention for patients with severe heart disease.
He's a great guy, olympic gold medalist, bronze star surgeon during the Vietnam war, renowned surgeon at the world's most prestigious heart clinic, I could go on for paragraphs. What he noticed after tens of thousands of surgeries is when you do open heart surgery, the same mechanism that caused your arteries to clog in the first place, cause them to clog again in an average of 10 years. Eventually, you can't keep having open heart surgeries, so you are sent to home hospice care, same as terminal cancer patients.
He took 21 of them for an experiment of only dietary intervention. They couldn't exercise because they were not ambulatory (couldn't get out of bed). They were the worst of the worst with a long history of heart attacks, strokes, and genetic pre-disposition to heart disease. All were on heavy meds.
One died of a heart attack from arrhythmia caused by damaged nerves from multiple heart attacks, at the very beginning of the trial (none of them were expected to live more than 6 months). He put the others on a diet of no animal or refined foods, just beans, vegetables, fruit and whole grains.
5 of them said doc, that's a fate worse than death, we need pizza and burgers. Those 5 died shortly after. The remaining 15 stuck to the program and were able to get out of bed, play golf, run marathons, and live full lives. 13 are still alive 20 years later, 2 died of old age. One guy had another minor heart attack 2.5 years after the program started because he decided moderation was okay and had a little birthday cake during social occasions, etc. That scared him back on the program and he had no further problems.