About weight gain
Weight gain is the increase of fat storage in the body, thereby increasing your body weight. Weight gain can also be due to an increase in muscle mass if you do weight lifting.
How the process of weight gaining works?
You gain weight by consuming more calories over your maintenance. Your maintenance is your current body weight and the amount of calories you need to eat to hold that weight. Your body will store any excess of calories over maintenance as fat, therefore gaining weight.
Think of a balloon as your body and the air that goes in it as food. The more air that goes in the bigger the balloon, the less air the smaller the balloon. Your body size changes and adapts to what you put into it.
How to gain weight?
Find your current maintenance level and consume a few more calories (100-500) over this everyday. Your body will adapt accordingly and store this excess as fat or a little muscle if you are weight lifting. Once your bodyweight matches the calories you are eating the weight gain will stop and this will now be your new maintenance. To continue gaining weight from here you will now need to eat a little more calories over this new maintenance.
Having gained weight, is it sustainable?
Yes, provided you keep eating the same amount of calories daily that it took you to reach your goal weight.
Risks of gaining too much weight
Too much body fat puts a lot of stress on your internal organs. Being over weight leads to diabetes and heart disease as well as many other health defects.