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There are various causes of low weight gain. Try eating food rich in carbohydrates like bananas, cheese, and also high protein like eggs, milk, chicken (if you are a non-vegetarian only). Consume well washed fresh fruits and vegetables everyday. Also exercise daily. There are Appetizer medication also available for which you can consult with your Doctor. Check your blood for low Haemoglobin
and stool for parasites if you feel weak and pale to look at.