Hi and thank you so much for this query. I am so sorry to hear about your worries with being underweight. Your BMI of 21 shows that your weight is normal for your height. Persons may make you think that you need extra kilograms on but that isn t true. feel comfortable with your weight, continue to eat healthy and stay physically fit. That s what matters and not some extra kilograms that are not warranted and not justified. I hope this helps. I wish you well. Feel free to ask more questions and request for more information if you feel so.