I would say remains of Descartes etc. The (outdated) philosophy, that everything can be broken down to single factors and then correctly calculated ... It took science some time to realize that the world is too (infinite) complicated to be calculated.
It surely helps though, to know the probable impact of single factors. But I believe we still generalizing way too much. Fat is bad, Sugar is bad, Cholesterine is bad (no wait good), Alcohol, smoking etc. ...
Which, I believe is stupid, when humans are complex and moderation and balance is the key. (even smoking can be beneficial I believe, for certain people at certain times, not as a addiction, obviously)
So seeing those single factors from studies help us as guidelines, but shouldn't be taken literally.