(in answer to a redditor confused about the seed-oil hypothesis)
You shouldn't believe things at all. There are lots of different ideas, any of which might be true.
You should develop the ability to hold them all in your head at the same time, and shift the weight of your belief according to the evidence you see.
Think less about modern religion, with its "one truth and you burn if you get it wrong", and more about the ancient Romans, patiently trying out all the gods and rituals they found to see which ones actually worked.
Epidemiological studies are evidence, but not terribly good evidence.
Randomized double-blind placebo controlled trials are good evidence, but you need to work out what it is they actually tested!
Self-experiments are gold dust, but watch out, it's easy to fool yourself.
Anecdotes from other people are evidence, but watch out for motivated thinking, and remember that not everyone works the same way, which is one of the reasons medical "science" is so very hard.
Evolutionary reasoning, and biochemical theory are good evidence, but it's hard to work out the implications of either for all sorts of reasons.
This is all very difficult and murky. If it was straightforward we would already know the answers.
But whatever you currently believe, what you should first do is look for evidence against it, until you aren't so sure any more!
