"Proof" is hard to achieve. There is still a lot of bad science. Such as getting grants to find ways of keeping cows from passing flatus, because methane contributes to
global warming. The anal orifices never stop to think that
the cow feed is inevitably going to turn to CO2, no matter
what is done with it.
I remember a top British scientist sneering that American studies on rats getting lung cancer from tobacco smoke
"proved conclusively that rats shouldn't smoke." Very funny.
But, who's laughing now? At least a half dozen of my physician friends died of lung cancer caused by cigarette smoking.