It depends on what I eat and how much of it I eat. Most things generally avoided (grains, legumes) don't do anything to me and dairy is no problem either. Sugar doesn't do much other than give me lots of energy for half an hour or so.
HOWEVER, soda (and I'm assuming most artificial sweeteners i.e. aspartame, etc) will make me feel like crap within half an hour. I had gone without it for about a year and then bought one a few weeks ago because I was dead tired and had a 3-hour drive ahead of me so I wanted caffeine... bad choice, lol.