This is actually my very favorite kind of reasoning to oppose to conspiracy theorists. I don’t use food though, because people rarely think of death as a consequence of tampering with food: I use air travel.

“So you say I shouldn’t trust anyone, I see… So you don’t fly anywhere?” “What has that anything to do with it?” “Do you realize how many different people are involved in running checks just to keep you alive at any given moment of a flight and even in the airport?”

Usually these people drive everywhere and are borderline against the existence of public transport, so using trains or subway lines doesn’t work.