Recently, Novak Djokovic has laid on an impressive (potentially record-breaking, even) win streak in professional men's tennis. This comes after his change to a gluten-free diet last year.
Here is an article discussing it:
The Diet that Shook Up Tennis?
I thought it was interesting that they emphasize the idea that this might be, well, a placebo effect in their quote:
Levitsky said a gluten-free diet might have benefits for those with mild allergies, or even no allergy at all. "The other part of the story is, if you believe in a cause of your disorder, it becomes the cause," he said. "We see this in many different studies. If you believe it, you change your behavior in the direction of being cured." (from the article)
So, it might be the fact that his diet wasn't agreeing with him and causing him actual, physical problems. Or it might be the idea that he made a change, changing made him feel like he was better, and so he improved. Perhaps. He was No. 3 in the world in a very, very competitive sport before his diet change, so it's not like he sucked.
It's certainly possible that their is a psychological aspect to this - almost certainly, even. But it's interesting they chose to emphasize that aspect over the possibility that dropping a potential allergen from his diet actually improved his physical well-being, allowing him to do better overall.
With your own diet, it can easily be a mix of the two. You exert more control over your diet in almost any respect and you'll see improvement; consistency is key. But it's also important what you put in your body. Novak Djokovic is certainly seeing that!