Your body needs energy to get thro the day. I am borderline underweight (yes, my psych meds seem to have done the opposite to me!) I used to be able to eat like a horse, and on some days still do. But most of the time I just have no appetite. I am starting to monitor how my appetite ties in with my episodes, but I am assuming that the more depressed I am, the less I eat. I have now ended up buying myself a protein shake, and if I cannot stomach my typical 10:30 AM sandwich (My first meal of the day) I know I have to force myself to have a protein shake.
I am already battling with energy, and survive on my vitamins and energy drinks.
It's not a healthy place to be in.

You have enough to worry about with mental illness - please ensure you look after your physical self