Hi Thank you for asking HCM I have gone through your query. It is most likely due to your body constitution. You may put on weight after 23 to 25 years after your linear growth stops. Avoid taking weight gainers as it can cause lot of side effects like hypertension. Get a stool routine to rule out malabsorption syndromes. In case of malabsorption syndrome antibiotic therapy will be helpful to normalize. Also do worm eradication yearly once by taking anti helminthics like Albendazole. Take a balanced diet with good quality proteins. Good quality proteins like that is from fish , milk , egg or meat is good for weigh gaining. Doing body building exercises under a instructor also helps. You can take protein supplements with out steroids. If you want to gain ore weight or mass then take more carbohydrates also. Hope this may help you. Let me know if anything not clear. Thanks.