Actually, fast metabolism is not always the reason why people don't gain weight. It can sometimes be in your genes, but it can also be affected by your environment. Consult your doctor for more concrete advice. However, here are some ways you can do to gain weight in a healthier way.
Have meals with the right balance of proteins, carbohydrates, and the right kinds of fat (such as unsaturated and monounsaturated fats, olive oil, canola oil, pistachios, almonds and walnuts). Heinemann suggests the following ratio: 60%-70% carbohydrates, 10%-15% protein, and a small amount of fat.
Eat foods higher in calories, vitamins, and minerals, as opposed to higher in fat or sugar.
Pack more nutritious calories in each serving. For example, you may add grated cooked eggs to mashed potatoes, ground chicken to soups and gravies, cheese in casseroles, eggs, and soups, and nonfat dried milk in soups, shakes, milk, and mashed potatoes.
If you get too full too fast, try having more high-calorie foods or slices of foods as opposed to consuming the whole thing (raisins versus grapes, granola and Grape Nuts versus corn flakes, mango slices versus the whole mango).
Limit drinking beverages to a half-hour before and after a meal.
Drink mixed juices (apple/berry, peach/orange/banana as opposed to one juice beverages) for a higher calorie intake.
Try a small amount of alcohol (4 ounces of wine, 6 ounces of beer, or a half-ounce of liquor with juice) before a meal, as it could stimulate appetite. Moore warns, however, that this recommendation must be cleared with your doctor, especially if you are on any medication. Too much alcohol can be detrimental to health, and could lessen your resolve for eating healthy.
With moderation, you may add in good fat sources to meals such as nuts, avocado, olives, and fatty fish (salmon and mackerel).
Snack in between meals. Nuts, dried fruits, and yogurt are good options, but it's also important to find nutritious foods that you will enjoy.
Have a nutritious snack before bedtime, such as a peanut butter sandwich.
Hope these helps.
Best of luck,
Dr. Michael Cohen