I have the most reliable source. For me, that would be me. You don't need some double blind study. If what you're trying to achieve is mental acuity, it's self-evident, really. Just test different kinds of food and you will notice.

BTW nostromo, I did write "healthy food" in my other comment ;-)

> I have a hard time believing diet has a statistically significant impact on cognitive performance - excluding caffeine or other stimulants of course

Oh for me it's just the opposite:

Caffeine makes me jumpier, but I'm not at all sure it improves my cognitive performance. Do you have any evidence caffeine has positive impact on cognitive performance?

I'm pretty sure eating healthy improves the clarity of my thinking. When I was young, I'd eaten a bar of chocolate for lunch a couple of times when short of time during a go tournament. This has invariably been a terrible idea and worsened my performance.