I can understand that your BMI of 15.7 is lower than the lowest limit of normal of 1,5 meaning you are underweight.
You need to get your BMI of minimum 18.5.
I think you should consult your Physician first to have clinical evaluation and investigations to find whether there is any factor/disease responsible for low BMI.
If all the factors are normal, then you have to consult a Dietitian to get a diet chart and follow this to get corrected the BMI.
This is possible and not that difficult, barring the genetic factor.
Hope this helps you.
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