Sister was in diet and she is 158 cm . 47.5 kg. 18 year and very inactive person so she take 1400 calory to maintain weight but its period is absent?

Amenorrhea; diet: Her body mass index is now 19 (within preferred range of 18-25) based on numbers u gave. Irregular or absent periods can result from weight loss, weight gain, strong physical exertion (athletes/ballerinas), physical or emotional stress. If she maintains her current weight, her periods may return. If she is sexually active, a home pregnancy test can r/o pregnancy. If no change, consult gyn.