My brother tried being a member in a gym near our area for a month. He was keen on building muscles and gaining weight as he starts being conscious about his looks. For a 5 foot eight inches tall weighing 54 kilos, he is indeed underweight.
My other brother who's older than him used to frequent the gym. I always saw him eating foods that are rich in protein like soya, raw egg, bananas and potatoes. I suggested that he do the same but so far his only drinking soya and he tried a cup of weight gainer in the gym. I gave him a warning not to just take any kind of liquid drink as it may have a side effect. I introduced him to proteinpulver which he can use as a source of protein. It is one of the many protein powders available in the market that is a dietary supplement and used for body building or if it's not your goal, simply use it as a protein shaker to help the body stabilize your blood sugar rest of the day. Just like fiber supplement, you can mix it on your drinks or even in food that you eat!
I know many of us are doubtful with a foreign powder mixing to our food or drinks as I have the same reaction. But this particular protein powder is made up of one or more of these ingredients -whey, egg, soy and rice which are processed into powder thus it won't affect the taste of anything edible you want to mix it with. Just by having this conversation with my brother, I'm kind curious of trying a protein powder myself especially Proteinpulver comes now in different flavors.