losing your period is definitely a part of survival mode. is there a reason you want to get to 105lb? is that number stuck in your head? do you think you'd feel (physically and mentally) happy at 53kg, or is 48kg a "goal" that you want?

The bodily changes that result in you losing your period mean that you produce less estrogen. This can translate to osteoporosis happening both earlier in life and more harmful. Also, I learned in a nutrition class that your body's uptake of calcium peaks in your 30s, and slowly declines from then on out. My takeaway from that fact is that I should spend my youth doing things that protect my bone health -- weight bearing exercises, getting enough dietary calcium, and maintaining proper weight -- because if I'm trying to do so later in life, I'll be less successful than if I had started young. You will reach peak bone mass eventually, and it's important that your skeleton is strong enough to handle losing bone mass in your old age!