It is as difficult for thin people to gain weight as for fat people to lose weight. I have been a fitness instructor for 8 years and have helped some friends gain weight successfully (we have become stronger). The method is simple, just stick to it.

Do more aerobic exercise at first, such as climbing mountains.

run quickly

Swimming, using these exercises can not only improve your cardiopulmonary function, but also improve your digestive function and increase your appetite. When you eat more than before, reduce your aerobic exercise intensity appropriately, so that after a period of time, your body will change.