I Am Underweight. What Should I Do To Gain Weight?
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The first thing to do MUST be to see an internist (medical doctor).
Have a physical examination and investigations (basically blood test) to rule out any cause that can be responsible - like thyroid, sugar and so on.
If nothing is found - which is highly probable - then see a nutrition specialist (Dietician). Several sittings may be required to be well acquainted.
Weight is like bank account. The account grows when you withdraw less (Cut down unnecessary physical work) and you deposit more (eat more).
So for short term, eat AS MUCH AS POSSIBLE / AS OFTEN AS POSSIBLE . HIGH CALORIE food / Sweets and snacks in between. Fatty foods add lot of calorie - one gram of fat (oil) has 8 calorie compared to Carbohydrate having only 4 calories. Sweets have lot of ghee and sugar.
Supplement with a multi-vitamin pill once a day
THIS HOWEVER IS BAD IN THE LONG TERM / ESPECIALLY WITH INCREASING AGE / THEY INCREASE CHOLESTEROL and so on. Beware. For a short time and in young age, to indulge like this may be excused...
Don't worry. You WILL gain. A time may come, when you may be overweight and start looking for ways to reduce!
Take care
Wishing speedy recovery
God bless
Good luck