Well, here's an easy one that I've even got some empirical evidence for: refined sugars being added to common foods where you simply don't expect sugars to be.

I know that when I'm here in Israel, I have an easy time controlling my eating (to the point that skipping meals sometimes becomes my default), but when I'm in the States, I have a very hard time controlling my eating. I've noticed that when I even partially cut refined sugars from my diet, I get through the day with a much clearer mind, particularly in the realm of executive/self-disciplining functions. It's to the point that I'm noticeably more productive at work without refined sugar.

There are lots of differences in diet between Israel and the USA, but the single biggest background factor is that in Israel, sweets are sweets and not-sweets are not sweetened. Whereas in the US, everything but the very rawest raw ingredients (ie: including sliced bread) has some added refined sugars.

With a large background level of "derp drug" in your basic foodstuffs, it's probably quite easy to suffer blood-sugar problems, get cravings, and lose a degree of focus and self-control. It's certainly what I experience when I'm there.