I'd like to second the gluten thing, as an unexpected behavioral change that changed my life immeasurably.
I was told to cut wheat down to 10% max of my diet, under very strange circumstances (not going to go into it here; it was weird and made no sense, also, this person wasn't a doctor or anything and didn't tell me how it might affect me to change my diet this way).
I tried it for two weeks. I went to the store, got gluten-free equivalents of regular foods, and this was long before gluten-free was a fad so it was hard to find stuff.
Within two days, literally, I felt 10 years younger. My digestion went from kinda bad, always feeling bloated, often unpleasant times in the bathroom, to, well, a really good experience. My skin went from being kind of sallow and puffy to healthy and glowing.
I used to strain my back and neck regularly, and for no reason. I'd spend maybe a month's worth of days limping around with back/neck strain. That ended, entirely.
My mood improved to an amazing degree, which was also really surprising.
And last, I used to get sick 4-5 times a year, and without fail if I didn't get enough sleep, even two nights in a row. Now, I get sick maybe once a year, much more in line with what I see in other people.
I think I too had a low-grade gluten intolerance (probably not celiac, though I've never been tested); getting rid of it was probably the best health decision I've ever made, for impact on quality of life.
Anyway, just wanted to share my story. :)