First, practice. Most thin people have poor absorption, poor appetite and limited daily intake and absorption. Without exercise consumption, there is no strong appetite. According to my personal experience, I suggest doing anaerobic exercise twice a week, and 30 minutes each time is enough to ensure strength. Then eat, four meals a day, morning, noon and evening, and midnight snack can't be too late, otherwise you will feel uncomfortable going to bed the next morning after eating, and you can't eat breakfast. You can eat six meals if possible. Eating protein and carbohydrates between meals is considered as two small meals, and it doesn't need too much, provided that it doesn't affect the intake of the next meal. For example, if you eat a lot at 3 pm, you can't have dinner, so you can't. Eat fruit every day. If you want to try protein powder, eat it half an hour after exercise. Then go to bed, which is very important. Make sure to rest for more than 8 hours every day, and sleep at night should not exceed 10. If the biological clock does not match, adjust it slowly until it matches. Finally, quit. If you can't do this, don't bother. It's useless. It will dissipate if you practice more.