hello.110 pound for 6 ft is certainly underweight even if you do not have any problems watsoever. its always easy to put up weight rather than loose weight. i feel you should gain your weght till around 140-150 pounds atleast. being underweight or thin and lean may be sometimes associated with problems like weaker abdominal muscles which might sometimes predispose to hernias. have regular balanced diet and start aerobic exercises to built up muscles or activities like swimming, badminton etc that will consume your energy,increase your diet, make you more fitter , healthier and attractive afterall muscular body has its own medical advantages and it adds to external physical appearance as well. all the best.