You must get your blood
sugar levels checked.If all blood parameters are normal,then to gain weight,you must take calorie dense food ,rich in protein at regular intervals.
Also do not do excess exercise.Walk only for 30 mins everyday.Take good sleep of 6-8 hours.
Next Steps
Calorie dense food means things like nuts,high fat foods,paneer,full fat milk,curd etc.If non vegetarian,you can also take eggs & chicken.
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Health Tips
If you are facing any stress you must find remedy for it too.