I am underweight, what is the fastest, healthiest way to gain weight?

See a doctor: You should see a doctor first to find out why you are not gaining weight. Some people has thyroid problem and it can cause problem to gain weigh. There's also other causes can get the same problem.

How do you FEEL?: Your physician will check you for thyroid disease and if you're lost weight recently, other causes. The number isn't key; many elite track-and-field atheletes are "underweight". I bet you're simply a slim guy who wants to add muscle. Eat sensibly when you're hungry and hit the resistance training daily, different body parts on different days. Consider creatine. Best wishes.

Dr Altman agreed