If I eat all natural unprocessed foods and no additives, perservatives,or chemicals would my body naturally...?

I actually did this for a month while vacationing. My friend is a vegetarian, and it's just cheaper to eat that way, so I had her diet and I was insanely slim (muscles of course) and felt soo much better, physically and psycologically. I had more energy and was very happy. Just coming home and going back to my regular diet, (although still healthy), I gained a few pounds back. I say stick to it. :)