Why can't a gain weight ? how to gain weight with a high metabolism

If healthy, then...: Eat more healthy calories (snacks and meals) to gain weight. Choosing foods with protein and "good" fats and oils. Avoid excess amounts of simple starches and sugars. To increase muscle mass, one has to work out against resistance (with weights). This exercise will increase daily calorie needs, so one must eat more of the healthy, balanced diet he should already be on.

Eat and exercise.: You need to focus your exercise regimen more on weight training than aerobics. As well, make sure you eats lots of food, and focus on complex carbohydrates and lean protein sources.