your as per your body mass index
you are under weight.but as far as your wt is not decreasing you must not worry about this.
The best way to add weight is to increase your intake of complex carbohydrates, particularly whole grain ones. Foods like whole wheat bread, muffins, pasta, crackers, and bagels are good to include. Also, legumes and fruits would be wise choices.
In order to gain weight, you will have to eat more calories. You will need to include regular exercise and strength training into your lifestyle in order to prevent gaining too much weight as fat. And, as I mentioned, those extra calories should come mainly from additional carbohydrate.
To start, set up a realistic goal weight for yourself. You may need to resign yourself to a less than (what you may consider) an ideal weight. Make good nutrition your priority, and weight gain the second.
Some more tips
Drink 6-8 glasses of distilled water a day.
Eat frequent but small meals.
Eat lots of raw fruits and vegetables (green leafy vegetables are great)
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