I'm 21 female, I started out about 80kg (176lb) and within six months reached a low point of 53kg (117lbs). My BMI went from about 30 (so right at the brink of 'official' obesity) to 20. It took only six months because I was so dedicated.
I have now regained about half of it, sitting at 68-70kg (152lbs ish).
I went on a diet because I thought it would fix everything. I was 20, had very few friends (NOT because of my weight but because I was shy and had been screwed over so much by 'friends' in the past that I thought why bother anymore), never even held a guy's hand let alone been kissed, and thought if I looked thinner, suddenly I'd be bubbly with a gang of gal pals and a guy who loved me.
Well, I lost weight, but absolutely none of that happened. But here's what did change:
I was depressed, grumpy, miserable, numb, neurotic and couldn't stop thinking about suicide because I was so terrified of regaining the weight and what family/relatives/friends would think. It sounds embarassing but I honestly thought killing myself would be better than a future lifetime of worrying about if putting whatever in my mouth would make me blow up. I couldn't imagine the humiliation and shame if it put weight back on. I didn't want to see anyone in case they secretly thought I looked 3 pounds heavier.
I was cold, all the time, and it completely changed my mood. At my lowest, it was during the middle of winter in New Zealand and even indoors I would bundle up in excessive layers, a hood on my head for 'head warmth', practically sitting on top of the heater, clutching and chugging mugfull after mugfull of hot water. You'd think being cold is just an annoyance, but it was so horrible. I couldn't even focus on a movie without thinking about how cold I was.
I could never get a decent night's sleep because I wasn't used to feeling my hips, kneecaps, shoulders etc pressing against the mattress. It hurt, and my mattress isn't even that hard.
I was never satisfied with the mirror. When I was fatter, I just accepted it, and was far less critical because I was going to look chubby anyway, so why bother obsessing over my hair/outfit/size etc. But then when I didn't really have a fat shape to blame, I sought hatred elsewhere, things as simple as how the hem of my T-shirt fell (was it wonky, frumpy, too long etc).
It's been 13 months and counting without a period.
Those are the main ones. I've gained about 33lbs back and thank god. I can eat much more food now and completely enjoy it without really worrying, I'm not half as critical when I look in the mirror, I sleep like a baby, I smile so much more and have much more enthusiasm to get out and do stuff. I still don't have a boyfriend or a group of friends, but I don't really care about that anymore.