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'.
That's fine, but all this means is that by now many of us have a healthy scepticism about the next claim to come along. We start to suspect that quite apart from making the people who eat them hugely dull, foods that are 'good for us' may not be quite as good for us as we've been lead to believe.
So when beetroot hit the headlines a few years ago with claims about improved athletic performance and lower blood pressure, many probably dismissed them. I certainly did.
Then I met Dr Andrew Webb.