Something that is good for us can also be bad for us. It may sound illogical, but it's why one week we can read that something is a universal panacea only to become a powerful carcinogen the next.
Take oily fish. It's full of good things such as long-chain omega-3 fatty acids, but also contains pollutants. How good or bad it is depends on who you are and how much you eat. (Check out the NHS Choices website for information on oily fish).
And what about the claims for 'superfoods' such as blueberries and goji berries? Read the smallprint of the research and inevitably it will say something like, 'at some unspecified point in the future the chemical in this superfood may provide benefit to some patients'.