If you are thin, you have never been fat. On the one hand, it may run in the family. If one of your parents is like this, there is really no way to change it. If your parents are normal and only you are particularly thin, it is recommended to see a doctor, see a gastroenterology department, or check your stomach. Hyperthyroidism should be ruled out. . .
PS: You said the problem was eating less. . . . . . How can you gain weight? Does this mean no appetite? Don't eat snacks if you have no appetite and want to be fat. Snacks are very bad for the digestive system. Take more exercise. The water I told you can stimulate your appetite, hehe.