I used to eat a ton of sugar, until one day a couple months ago it started making me nauseous, so I quit eating it and had to readjust my diet to eating first thing when I wake up, and not letting myself go too long without eating or else I'll crave the quick pick-me-up of sugar and pay for it afterwards by feeling sick and having no energy at all the next day, bad skin etc. I can eat some sugar now (and naturally occurring sugar in fruits etc. never was a problem) but I have to be really careful of it or it completely throws me off again. Sometimes I'm sad about it, especially with stuff like ice cream but if I eat just a tiny bit it's okay. A little though can make me want more, so I guess I'm kind of a sugaraholic, where it's better not even to have a little.
Long story short, it's like what some other ppl said already - you have to listen to your body and do what makes you feel good. I don't know why I suddenly had a bad reaction to sugar after years of depending on it for quick energy, but I did start feeling a lot better after readjusting how I ate. Fad diets that don't address your personal food needs aren't much help though imo - unless they happen to align with what you need.