I have got the same problem. I want to gain some weight, but it seems like it is impossible. Everytime I had gained let's say 2 kilos, I've lost them really fast. I was talking to a friend and he said something like my body knows what it needs and even when I try hard to gain weight it is much harder for it to keep the weight because it isn't natural for it. I feel like he could be right. What do you think?

Anyway, I am tall and thin. Some more kilos would made me look and feel better :)

Well the best way for to gain weight wiuld be to build muscle mass. Start taking protein shakes (whey protein) would be better for you so that you dont look bulky but toned and sexy. Im sure thats what most girls want. Start using free weights eating more carbs and your ptotein shakes. But dont forget to at least do 5 min of moderate to intense cardio so you stay in great shape. Later and good luck!

Otherwise than to eat. My best advise is to talk to a nutritionist. Good luck!

Same boat here, when I was 15 I weight 55kgs and I looked healthy. But after having babies it's like my metabolism has kicked into high gear and no matter what I eat or drink (protein shakes) my meight never goes above 51kgs. At one point I weighed 47kgs. I want to get back to 55-57. But it is EXTREMLY hard to do.

You should go to a nutritionist. A friend of mine tried to gain weight fot years especially after pregnancy ( yep she was that skinny). The only way was through the doctor. She had had some kind of food allergy that made her body eliminate everything ad soon as she ate it.