Can I gain 40 pounds? .. I am underweight ? :(

My BMI is just 17.4 and I want to gain 40 pounds .

How is it possible ? I eat really well but I sweat a lot and it takes most of my strength .

I consulted a doctor and he said its heredity thing . What can I do to gain weight ?

Comment below ... poll is just given so that other menmbers, who might have the same problem can refer to this answer.
You can gain weight very quickly
You cannot gain weight
You can gain weight in some time
Anything else
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Can I gain 40 pounds? .. I am underweight ? :(
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