Question: how to gain weight fast when you have a fast metabolism?
Answer: Eat More. Eat more calories than your body burns. ... 
Eat More Meals. Small meals are easier to eat than big ones. ... 
Eat Caloric Dense Foods. Food high in carbs and/or fats has more calories per serving. ... 
Eat More Protein. ... 
Go Liquid. ... 
Track Calories. ... 
Lift Heavy. ... 
Be Consistent.