It is best to go to the hospital to check whether it is pathological or physiological.

If it is pathological, my suggestion is to look at the pulse condition of Chinese medicine and adjust it to see where the problem lies. Or check your endocrine.

If you don't want to go to the hospital, consciously add meals. For example, if allowed, eat your favorite food (preferably a little higher in calories). Don't eat too much every time, pay attention to rest, basically follow the principle of "work at sunrise and rest at sunset", don't stay up late, and reduce smoking (nicotine will affect digestion and absorption). Effective in 3-5 months.

Look at the information and you will know that you are men, because you don't know whether you smoke or not, your living habits, your age and working environment, and whether your stomach can withstand fatigue fighting, so I can only say so much first.

If you want to get fat regardless, add a meal half an hour before going to bed, and then go to bed. (strongly discouraged)

If it is physiological, the family inheritance is thin, but there are no other problems, don't forcibly gain weight, otherwise it will be counterproductive and cause diseases of the endocrine system.