What is the most appropriate method of gaining weight (muscle mass)?

Answer 1

The recommended way to do this is to eat healthy foods in amounts that are more than your body needs. Eating unhealthy foods would also add to your weight but might harm you in the long run.
If you want muscle-mass more than fat-gain, you'll also need to get plenty of exercise. Try to get plenty of aerobic exercise (but not that overly intense exercise may damage your joints). It isn't essential to join a gym; you can do sit-ups, pushups, dumbbell-lifting, jumping-jacks, and many other basic exercises at home. Walk as much as possible. Bicycling and swimming are good too. Even for people who are not trying to lose weight, being active helps your digestion, your circulation, and other body processes, and will help improve your mood.
Healthy foods to gain weight:
red meat; fish (especially oily fish such as sardines)
seeds and nuts; peanut butter, almond butter, tahini
dairy and eggs
sweet fruits such as mango, banana and papaya
avocado
granola; dried fruit; protein bars
whole wheat bread; whole grain breads, crackers and foods; legumes
olive oil on salads or wherever appropriate
carbohydrate-veggies such as potatoes and yams
1) Note that red meat, dairy, and eggs should be eaten in moderation. If your body's cholesterol is low, you can increase the amount somewhat.
2) Try to eat several (4 to 6) times a day. Have snacks and drink water between meals, not during meals.
3) Try to avoid junk foods except as a treat here and there.
4) Don't overdo it. Gaining weight faster than about 1 pound per week should be done under supervision.See also:

Could you describe a balanced diet?

Different types of fat - which are healthy?

Some healthy snacks