I was overweight (technically) in my twenties.
First half of twenties diet was vegetarian with LOTS of dairy. Also lots of junk but more than 5 fresh fruits and veggies a day- I started this specifically as I noticed myself looking and feeling tired all the time.
I went vegan age 33 and conceived at 34. I was underweight then right up until this year - this was due to high stress in pregnancy, bad relationship, then massive upheaval and moving house, all whilst breastfeeding - and in the end I was 8 stone (I'm 5ft 9) and because my diet had become so based around whole foods, I can't tolerate junk too much, my gut punishes me for it. So where was I going to put weight on? I'm sure I got 1,000 calories a day but it was all in raw and lightly cooked vegetables! No way to put on weight.
So this year, (age 40) I added in junk and decided to eat whatever I wanted. I did that and three weeks later I was up to size 10 again, where I remain, doing the same thing.
Now I feel a bit sluggish again because of all the crap I'm eating but it's better than being underweight. Ideally I think I should simply be eating bigger portions of the good stuff.
So for me I lose and gain very easily. So I would probably be one of those yo yo dieters they referred to in Hollywood where I'm fat, then thin, then fat, then thin.
I think if I had not become accustomed to such a healthful whole food diet I wouldn't have lost the weight in the first place, and if I'd have begun eating junk the way I have again, I would have ballooned.
Now I still eat to maintain my weight, and I look better, with an arse, boobs, and a filled out body. So I'm happy. I also still look young for my age and I do put that down to being vegan and my vegan diet containing lots of whole foods.