I'm about 17 years old and around age 11 or 12, I lost a severe amount of weight. I weighed less than 50kg for about a year and a half, a lot of my bones were showing through my skin, especially my rib cage, and it still does even though my weight fluctuates throughout the 50kg range every so often.The reason for my question is that I'm really unhappy with this and I'm pretty sure it's 100% my diet since I'm a pretty athletic guy despite my physique, so I want to know the absolute fastest way to gain weight fast. Fat, muscle, I don't mind, preferably muscle though. If anyone can tell me specifically what to eat or have any diets that you think I should try, it would be really appreciated.