What are the positive and negative health implications of being underweight?

None if not losing w: If you are otherwise healthy and free from any medical condition and are not loosing weight and your weight is stable than you can keep eating and enjoying your balanced diet.If you want to gain weight you can increase your caloric intake as tolerated.I am presuming you had a check up including blood tests etc a and all is normal.If not get yourself checked and examined and tested.Good luck.

Number isn't key: A light-framed track-and-field athlete, for whom aerobic fitness is a plus and muscle mass largely a minus, is likely to be "underweight" according to the tables yet in excellent health. A sedentary person who is starving themselves because of an alcohol or drug addiction is in terrible health despite being "slim". It's the person, the big picture, not the numbers.