How can I gain weight with a fast metabolism?

First you are going to have to increase your calorie intake by at least five hundred. Another, unhealthy option, is to eat high carbohydrate, high fat, junk food. It may not be the healthiest, but it will do the trick. Next increase your protein intake by at least one gram of protein per every pound you weigh. So around 85 grams of protein. Another tip is to eat only three very large meals all day. When you eat a ton of food spaced throughout the day your metabolism actually increases. Another thing you should try would be to eat a whole meal about three hours before you go to bed. People who try to lose weight have to avoid eating during that time period. Hope I helped and good luck!!