You would have to go quite a long way back in time to find a scientific theory being completely overturned. Usually science is modified rather than overturned. But the thing is that the ability of science to self correct is one of the good things about science. If you think that you can prove a scientific theory is wrong all you have to do is show sufficient evidence to prove your case to your peers.
It is important to distinguish between 'hard' science and the 'softer' sciences, one of the examples given was healthy foodstuffs. Anyone can call themselves a 'nutritionist' and hence these 'nutritionists' can make any sort of claim and not have to back it up with evidence.